Package segfaults when built with -O2 but not with -O0

[Paul Howarth]

One of my packages, pptp, suffers occasional segfaults as reported in
http://bugzilla.redhat.com/749455. However, whilst investigating this,
it seems to be the case that simply rebuilding the package using no
optimization (-O0) as opposed to the default -O2 is enough to stop this
happening.

This raises two questions (at least!):

1. Is it reasonable for me to flout the packaging guidelines by
rebuilding with -O0 until this is resolved?

2. How to determine what the actual problem is, e.g. a problem with the
way the code is written leading to unsafe optimizations, or a gcc bug?

Paul.

[Andrew Haley]

On 11/18/2011 11:31 AM, Paul Howarth wrote:
>  One of my packages, pptp, suffers occasional segfaults as reported
> in
>  http://bugzilla.redhat.com/749455. However, whilst investigating this,
>  it seems to be the case that simply rebuilding the package using no
>  optimization (-O0) as opposed to the default -O2 is enough to stop this
>  happening.
>  
>  This raises two questions (at least!):
>  
>  1. Is it reasonable for me to flout the packaging guidelines by
>  rebuilding with -O0 until this is resolved?
>  
>  2. How to determine what the actual problem is, e.g. a problem with the
>  way the code is written leading to unsafe optimizations, or a gcc bug? 
You're just going to have to debug it.

To start with, build with warnings, and look at them all.  If none of
that works and you're stuck, I'll have a look.

This is unlikely to be a gcc bug.

Does the upstream package segfault?

Andrew.

[Paul Howarth]

On Fri, 18 Nov 2011 15:28:27 +0000
Andrew Haley <aph(a)redhat.com&gt; wrote:

>  On 11/18/2011 11:31 AM, Paul Howarth wrote:
>  > One of my packages, pptp, suffers occasional segfaults as reported
>  > in http://bugzilla.redhat.com/749455. However, whilst investigating
>  > this, it seems to be the case that simply rebuilding the package
>  > using no optimization (-O0) as opposed to the default -O2 is enough
>  > to stop this happening.
>  > 
>  > This raises two questions (at least!):
>  > 
>  > 1. Is it reasonable for me to flout the packaging guidelines by
>  > rebuilding with -O0 until this is resolved?
>  > 
>  > 2. How to determine what the actual problem is, e.g. a problem with
>  > the way the code is written leading to unsafe optimizations, or a
>  > gcc bug?
>  
>  You're just going to have to debug it.
>  
>  To start with, build with warnings, and look at them all.  If none of
>  that works and you're stuck, I'll have a look. 
It already builds with -Wall and there are no warnings:

http://koji.fedoraproject.org/koji/getfile?taskID=3507432&name=build.log

>  This is unlikely to be a gcc bug.
>  
>  Does the upstream package segfault? 
Upstream's Makefile uses -O0 and doesn't appear to segfault (probably
as a result).

Paul.

[Susi Lehtola]

On Fri, 18 Nov 2011 16:32:23 +0000
Paul Howarth <paul(a)city-fan.org&gt; wrote:
>  On Fri, 18 Nov 2011 15:28:27 +0000
>  Andrew Haley <aph(a)redhat.com&gt; wrote:
>  
>  > On 11/18/2011 11:31 AM, Paul Howarth wrote:
>  > > One of my packages, pptp, suffers occasional segfaults as reported
>  > > in http://bugzilla.redhat.com/749455. However, whilst
>  > > investigating this, it seems to be the case that simply
>  > > rebuilding the package using no optimization (-O0) as opposed to
>  > > the default -O2 is enough to stop this happening. 
(clip)

>  > This is unlikely to be a gcc bug.
>  > 
>  > Does the upstream package segfault?
>  
>  Upstream's Makefile uses -O0 and doesn't appear to segfault (probably
>  as a result).
>  
>  Paul. 
.. but https://bugzilla.redhat.com/show_bug.cgi?id=749455 it's shown
that the upstream makefile uses "-Wall -O -Wuninitialized -g".
According to the gcc man page, -O is equal to -O1.
-- 
Jussi Lehtola
Fedora Project Contributor
jussilehtola(a)fedoraproject.org

[Paul Howarth]

On Fri, 18 Nov 2011 18:43:30 +0200
Jussi Lehtola <jussilehtola(a)fedoraproject.org&gt; wrote:

>  On Fri, 18 Nov 2011 16:32:23 +0000
>  Paul Howarth <paul(a)city-fan.org&gt; wrote:
>  > On Fri, 18 Nov 2011 15:28:27 +0000
>  > Andrew Haley <aph(a)redhat.com&gt; wrote:
>  > 
>  > > On 11/18/2011 11:31 AM, Paul Howarth wrote:
>  > > > One of my packages, pptp, suffers occasional segfaults as
>  > > > reported in http://bugzilla.redhat.com/749455. However, whilst
>  > > > investigating this, it seems to be the case that simply
>  > > > rebuilding the package using no optimization (-O0) as opposed to
>  > > > the default -O2 is enough to stop this happening.
>  
>  (clip)
>  
>  > > This is unlikely to be a gcc bug.
>  > > 
>  > > Does the upstream package segfault?
>  > 
>  > Upstream's Makefile uses -O0 and doesn't appear to segfault
>  > (probably as a result).
>  > 
>  > Paul.
>  
>  .. but https://bugzilla.redhat.com/show_bug.cgi?id=749455 it's shown
>  that the upstream makefile uses "-Wall -O -Wuninitialized -g".
>  According to the gcc man page, -O is equal to -O1. 
Ah, I could have sworn it explicitly said "-O0".

More importantly, the bug reporter now tells me that the -O0 build has
now segfaulted - it just took longer to happen that way. So I'll go
back to more conventional debugging now. Thanks anyway.

Paul.

[Ralf Corsepius]

On 11/18/2011 05:32 PM, Paul Howarth wrote:
>  On Fri, 18 Nov 2011 15:28:27 +0000
>  Andrew Haley<aph(a)redhat.com>  wrote:
>
> > On 11/18/2011 11:31 AM, Paul Howarth wrote:
> >> One of my packages, pptp, suffers occasional segfaults as reported
> >> in http://bugzilla.redhat.com/749455. However, whilst investigating
> >> this, it seems to be the case that simply rebuilding the package
> >> using no optimization (-O0) as opposed to the default -O2 is enough
> >> to stop this happening.
> >>
> >> This raises two questions (at least!):
> >>
> >> 1. Is it reasonable for me to flout the packaging guidelines by
> >> rebuilding with -O0 until this is resolved?
> >>
> >> 2. How to determine what the actual problem is, e.g. a problem with
> >> the way the code is written leading to unsafe optimizations, or a
> >> gcc bug?
> >
> > You're just going to have to debug it.
> >
> > To start with, build with warnings, and look at them all.  If none of
> > that works and you're stuck, I'll have a look.
>
>  It already builds with -Wall and there are no warnings: 
This doesn't mean
much.

Adding a couple of more agressive options, this is what happens to pptp:

orckit_quirks.c:65:2: warning: missing initializer 
[-Wmissing-field-initializers]
orckit_quirks.c:65:2: warning: (near initialization for 
'fixed_packet.header.pptp_type') [-Wmissing-field-initializers]
pptp.c:147:21: warning: unused parameter 'sig' [-Wunused-parameter]
pptp.c:153:19: warning: unused parameter 'sig' [-Wunused-parameter]
pptp.c:459:33: warning: dereferencing type-punned pointer might break 
strict-aliasing rules [-Wstrict-aliasing]
pptp.c:492:65: warning: unused parameter 'argc' [-Wunused-parameter]
pptp_callmgr.c:105:48: warning: unused parameter 'envp' [-Wunused-parameter]
pptp_callmgr.c:202:48: warning: dereferencing type-punned pointer might 
break strict-aliasing rules [-Wstrict-aliasing]
pptp_callmgr.c:215:17: warning: ignoring return value of 'read', 
declared with attribute warn_unused_result [-Wunused-result]
pptp_callmgr.c:216:17: warning: ignoring return value of 'read', 
declared with attribute warn_unused_result [-Wunused-result]
pptp_callmgr.c:325:29: warning: dereferencing type-punned pointer might 
break strict-aliasing rules [-Wstrict-aliasing]
pptp_callmgr.c:330:28: warning: dereferencing type-punned pointer might 
break strict-aliasing rules [-Wstrict-aliasing]
pptp_callmgr.c:358:25: warning: dereferencing type-punned pointer might 
break strict-aliasing rules [-Wstrict-aliasing]
pptp_callmgr.c:368:44: warning: unused parameter 'inetaddr' 
[-Wunused-parameter]
pptp_callmgr.c:39:29: warning: unused parameter 'sig' [-Wunused-parameter]
pptp_callmgr.c:44:29: warning: unused parameter 'sig' [-Wunused-parameter]
pptp_callmgr.c:71:18: warning: ignoring return value of 'write', 
declared with attribute warn_unused_result [-Wunused-result]
pptp_ctrl.c:1062:13: warning: function declaration isn't a prototype 
[-Wstrict-prototypes]
pptp_ctrl.c:177:1: warning: function declaration isn't a prototype 
[-Wstrict-prototypes]
pptp_ctrl.c:206:31: warning: comparison between signed and unsigned 
integer expressions [-Wsign-compare]
pptp_ctrl.c:245:13: warning: comparison between signed and unsigned 
integer expressions [-Wsign-compare]
pptp_ctrl.c:535:14: warning: comparison between signed and unsigned 
integer expressions [-Wsign-compare]
pptp_gre.c:237:27: warning: comparison between signed and unsigned 
integer expressions [-Wsign-compare]
pptp_gre.c:241:30: warning: comparison between signed and unsigned 
integer expressions [-Wsign-compare]
pptp_gre.c:249:18: warning: comparison between signed and unsigned 
integer expressions [-Wsign-compare]
pptp_gre.c:260:23: warning: comparison between signed and unsigned 
integer expressions [-Wsign-compare]
pptp_gre.c:441:26: warning: comparison between signed and unsigned 
integer expressions [-Wsign-compare]
pptp_gre.c:446:23: warning: comparison between signed and unsigned 
integer expressions [-Wsign-compare]
pptp_gre.c:493:27: warning: comparison between signed and unsigned 
integer expressions [-Wsign-compare]
pptp_gre.c:527:19: warning: comparison between signed and unsigned 
integer expressions [-Wsign-compare]
pptp_gre.c:74:10: warning: function declaration isn't a prototype 
[-Wstrict-prototypes]
pptp_gre.c:85:19: warning: nested extern declaration of 'localbind' 
[-Wnested-externs]
pptp_gre.c:92:29: warning: dereferencing type-punned pointer might break 
strict-aliasing rules [-Wstrict-aliasing]
pptp_gre.c:99:28: warning: dereferencing type-punned pointer might break 
strict-aliasing rules [-Wstrict-aliasing]
pptp_quirks.c:33:5: warning: function declaration isn't a prototype 
[-Wstrict-prototypes]
pptp_quirks.h:56:1: warning: function declaration isn't a prototype 
[-Wstrict-prototypes]
pqueue.c:220:11: warning: function declaration isn't a prototype 
[-Wstrict-prototypes]
pqueue.h:27:1: warning: function declaration isn't a prototype 
[-Wstrict-prototypes]
routing.c:117:8: warning: ignoring return value of 'fgets', declared 
with attribute warn_unused_result [-Wunused-result]
routing.c:125:6: warning: function declaration isn't a prototype 
[-Wstrict-prototypes]
routing.c:155:6: warning: function declaration isn't a prototype 
[-Wstrict-prototypes]
routing.h:2:1: warning: function declaration isn't a prototype 
[-Wstrict-prototypes]
routing.h:3:1: warning: function declaration isn't a prototype 
[-Wstrict-prototypes]
test.c:145:14: warning: comparison between signed and unsigned integer 
expressions [-Wsign-compare]
test.c:174:27: warning: function declaration isn't a prototype 
[-Wstrict-prototypes]
test.c:55:16: warning: comparison between signed and unsigned integer 
expressions [-Wsign-compare]
test.c:99:14: warning: comparison between signed and unsigned integer 
expressions [-Wsign-compare]
test.h:5:8: warning: function declaration isn't a prototype 
[-Wstrict-prototypes]
util.c:121:8: warning: ignoring return value of 'write', declared with 
attribute warn_unused_result [-Wunused-result]
util.c:136:5: warning: function declaration isn't a prototype 
[-Wstrict-prototypes]
util.c:142:5: warning: function declaration isn't a prototype 
[-Wstrict-prototypes]
util.c:145:7: warning: ignoring return value of 'read', declared with 
attribute warn_unused_result [-Wunused-result]
util.c:149:6: warning: function declaration isn't a prototype 
[-Wstrict-prototypes]
util.c:90:5: warning: function declaration isn't a prototype 
[-Wstrict-prototypes]
util.h:38:1: warning: function declaration isn't a prototype 
[-Wstrict-prototypes]
util.h:47:1: warning: function declaration isn't a prototype 
[-Wstrict-prototypes]
util.h:50:1: warning: function declaration isn't a prototype 
[-Wstrict-prototypes]
util.h:52:1: warning: function declaration isn't a prototype 
[-Wstrict-prototypes]
vector.c:38:9: warning: function declaration isn't a prototype 
[-Wstrict-prototypes]
vector.h:15:1: warning: function declaration isn't a prototype 
[-Wstrict-prototypes]

I can't spot anything obvious, but each of these are potential candiates 
for issues and should be inspected.

> > This is unlikely to be a gcc bug.
> >
> > Does the upstream package segfault?
>
>  Upstream's Makefile uses -O0 and doesn't appear to segfault (probably
>  as a result). 
Does upstream have an explanation why they are doing so?

If being hostile, one would be tempted to consider this to be an attempt 
to play down known issues of sloppy programming :-)

Ralf

[Paul Howarth]

On Fri, 18 Nov 2011 18:53:03 +0100
Ralf Corsepius <rc040203(a)freenet.de&gt; wrote:

>  On 11/18/2011 05:32 PM, Paul Howarth wrote:
>  > On Fri, 18 Nov 2011 15:28:27 +0000
>  > Andrew Haley&lt;aph(a)redhat.com&gt;  wrote:
>  >
>  >> On 11/18/2011 11:31 AM, Paul Howarth wrote:
>  >>> One of my packages, pptp, suffers occasional segfaults as reported
>  >>> in http://bugzilla.redhat.com/749455. However, whilst
>  >>> investigating this, it seems to be the case that simply
>  >>> rebuilding the package using no optimization (-O0) as opposed to
>  >>> the default -O2 is enough to stop this happening.
>  >>>
>  >>> This raises two questions (at least!):
>  >>>
>  >>> 1. Is it reasonable for me to flout the packaging guidelines by
>  >>> rebuilding with -O0 until this is resolved?
>  >>>
>  >>> 2. How to determine what the actual problem is, e.g. a problem
>  >>> with the way the code is written leading to unsafe optimizations,
>  >>> or a gcc bug?
>  >>
>  >> You're just going to have to debug it.
>  >>
>  >> To start with, build with warnings, and look at them all.  If none
>  >> of that works and you're stuck, I'll have a look.
>  >
>  > It already builds with -Wall and there are no warnings:
>  This doesn't mean much.
>  
>  Adding a couple of more agressive options, this is what happens to
>  pptp:
>  
>  orckit_quirks.c:65:2: warning: missing initializer 
>  [-Wmissing-field-initializers]
>  orckit_quirks.c:65:2: warning: (near initialization for 
>  'fixed_packet.header.pptp_type') [-Wmissing-field-initializers]
>  pptp.c:147:21: warning: unused parameter 'sig' [-Wunused-parameter]
>  pptp.c:153:19: warning: unused parameter 'sig' [-Wunused-parameter]
>  pptp.c:459:33: warning: dereferencing type-punned pointer might break 
>  strict-aliasing rules [-Wstrict-aliasing]
>  pptp.c:492:65: warning: unused parameter 'argc' [-Wunused-parameter]
>  pptp_callmgr.c:105:48: warning: unused parameter
>  'envp' [-Wunused-parameter] pptp_callmgr.c:202:48: warning:
>  dereferencing type-punned pointer might break strict-aliasing rules
>  [-Wstrict-aliasing] pptp_callmgr.c:215:17: warning: ignoring return
>  value of 'read', declared with attribute warn_unused_result
>  [-Wunused-result] pptp_callmgr.c:216:17: warning: ignoring return
>  value of 'read', declared with attribute warn_unused_result
>  [-Wunused-result] pptp_callmgr.c:325:29: warning: dereferencing
>  type-punned pointer might break strict-aliasing rules
>  [-Wstrict-aliasing] pptp_callmgr.c:330:28: warning: dereferencing
>  type-punned pointer might break strict-aliasing rules
>  [-Wstrict-aliasing] pptp_callmgr.c:358:25: warning: dereferencing
>  type-punned pointer might break strict-aliasing rules
>  [-Wstrict-aliasing] pptp_callmgr.c:368:44: warning: unused parameter
>  'inetaddr' [-Wunused-parameter]
>  pptp_callmgr.c:39:29: warning: unused parameter
>  'sig' [-Wunused-parameter] pptp_callmgr.c:44:29: warning: unused
>  parameter 'sig' [-Wunused-parameter] pptp_callmgr.c:71:18: warning:
>  ignoring return value of 'write', declared with attribute
>  warn_unused_result [-Wunused-result] pptp_ctrl.c:1062:13: warning:
>  function declaration isn't a prototype [-Wstrict-prototypes]
>  pptp_ctrl.c:177:1: warning: function declaration isn't a prototype 
>  [-Wstrict-prototypes]
>  pptp_ctrl.c:206:31: warning: comparison between signed and unsigned 
>  integer expressions [-Wsign-compare]
>  pptp_ctrl.c:245:13: warning: comparison between signed and unsigned 
>  integer expressions [-Wsign-compare]
>  pptp_ctrl.c:535:14: warning: comparison between signed and unsigned 
>  integer expressions [-Wsign-compare]
>  pptp_gre.c:237:27: warning: comparison between signed and unsigned 
>  integer expressions [-Wsign-compare]
>  pptp_gre.c:241:30: warning: comparison between signed and unsigned 
>  integer expressions [-Wsign-compare]
>  pptp_gre.c:249:18: warning: comparison between signed and unsigned 
>  integer expressions [-Wsign-compare]
>  pptp_gre.c:260:23: warning: comparison between signed and unsigned 
>  integer expressions [-Wsign-compare]
>  pptp_gre.c:441:26: warning: comparison between signed and unsigned 
>  integer expressions [-Wsign-compare]
>  pptp_gre.c:446:23: warning: comparison between signed and unsigned 
>  integer expressions [-Wsign-compare]
>  pptp_gre.c:493:27: warning: comparison between signed and unsigned 
>  integer expressions [-Wsign-compare]
>  pptp_gre.c:527:19: warning: comparison between signed and unsigned 
>  integer expressions [-Wsign-compare]
>  pptp_gre.c:74:10: warning: function declaration isn't a prototype 
>  [-Wstrict-prototypes]
>  pptp_gre.c:85:19: warning: nested extern declaration of 'localbind' 
>  [-Wnested-externs]
>  pptp_gre.c:92:29: warning: dereferencing type-punned pointer might
>  break strict-aliasing rules [-Wstrict-aliasing]
>  pptp_gre.c:99:28: warning: dereferencing type-punned pointer might
>  break strict-aliasing rules [-Wstrict-aliasing]
>  pptp_quirks.c:33:5: warning: function declaration isn't a prototype 
>  [-Wstrict-prototypes]
>  pptp_quirks.h:56:1: warning: function declaration isn't a prototype 
>  [-Wstrict-prototypes]
>  pqueue.c:220:11: warning: function declaration isn't a prototype 
>  [-Wstrict-prototypes]
>  pqueue.h:27:1: warning: function declaration isn't a prototype 
>  [-Wstrict-prototypes]
>  routing.c:117:8: warning: ignoring return value of 'fgets', declared 
>  with attribute warn_unused_result [-Wunused-result]
>  routing.c:125:6: warning: function declaration isn't a prototype 
>  [-Wstrict-prototypes]
>  routing.c:155:6: warning: function declaration isn't a prototype 
>  [-Wstrict-prototypes]
>  routing.h:2:1: warning: function declaration isn't a prototype 
>  [-Wstrict-prototypes]
>  routing.h:3:1: warning: function declaration isn't a prototype 
>  [-Wstrict-prototypes]
>  test.c:145:14: warning: comparison between signed and unsigned
>  integer expressions [-Wsign-compare]
>  test.c:174:27: warning: function declaration isn't a prototype 
>  [-Wstrict-prototypes]
>  test.c:55:16: warning: comparison between signed and unsigned integer 
>  expressions [-Wsign-compare]
>  test.c:99:14: warning: comparison between signed and unsigned integer 
>  expressions [-Wsign-compare]
>  test.h:5:8: warning: function declaration isn't a prototype 
>  [-Wstrict-prototypes]
>  util.c:121:8: warning: ignoring return value of 'write', declared
>  with attribute warn_unused_result [-Wunused-result]
>  util.c:136:5: warning: function declaration isn't a prototype 
>  [-Wstrict-prototypes]
>  util.c:142:5: warning: function declaration isn't a prototype 
>  [-Wstrict-prototypes]
>  util.c:145:7: warning: ignoring return value of 'read', declared with 
>  attribute warn_unused_result [-Wunused-result]
>  util.c:149:6: warning: function declaration isn't a prototype 
>  [-Wstrict-prototypes]
>  util.c:90:5: warning: function declaration isn't a prototype 
>  [-Wstrict-prototypes]
>  util.h:38:1: warning: function declaration isn't a prototype 
>  [-Wstrict-prototypes]
>  util.h:47:1: warning: function declaration isn't a prototype 
>  [-Wstrict-prototypes]
>  util.h:50:1: warning: function declaration isn't a prototype 
>  [-Wstrict-prototypes]
>  util.h:52:1: warning: function declaration isn't a prototype 
>  [-Wstrict-prototypes]
>  vector.c:38:9: warning: function declaration isn't a prototype 
>  [-Wstrict-prototypes]
>  vector.h:15:1: warning: function declaration isn't a prototype 
>  [-Wstrict-prototypes]
>  
>  I can't spot anything obvious, but each of these are potential
>  candiates for issues and should be inspected. 
Hmm, thanks for that. I naively assumed that -Wall was "all warnings"!
I'll look into those.

>  >> This is unlikely to be a gcc bug.
>  >>
>  >> Does the upstream package segfault?
>  >
>  > Upstream's Makefile uses -O0 and doesn't appear to segfault
>  > (probably as a result).
>  Does upstream have an explanation why they are doing so?
>  
>  If being hostile, one would be tempted to consider this to be an
>  attempt to play down known issues of sloppy programming :-) 
Actually the comment that went with the original commit (a long time
ago) was that it was to make debugging easier.

Paul.

[Kevin Kofler]

Paul Howarth wrote:
>  Hmm, thanks for that. I naively assumed that -Wall was "all
> warnings"!
>  I'll look into those. 
Indeed, -Wall is not really all. :-) -Wall -Wextra is closer to all, but 
there are still some things those won't warn about, e.g. -Wwrite-strings 
catches places which use a string literal as a potentially writable char * 
instead of a const char *.

        Kevin Kofler

[Andrew Haley]

On 11/18/2011 05:53 PM, Ralf Corsepius wrote:
>  pptp.c:459:33: warning: dereferencing type-punned pointer might break
>
>  strict-aliasing rules [-Wstrict-aliasing] 
Bingo!  Bugs like this must be fixed.

Andrew.

[Tom Lane]

Andrew Haley <aph(a)redhat.com&gt; writes:
>  On 11/18/2011 05:53 PM, Ralf Corsepius wrote:
> > pptp.c:459:33: warning: dereferencing type-punned pointer might break 
> > strict-aliasing rules [-Wstrict-aliasing] 
>  Bingo!  Bugs like this must be fixed. 
Sometimes that's easier said than done.  -fno-strict-aliasing might be
your friend.

-fwrapv is another good tool for keeping the compiler from breaking
traditional understandings of C semantics.

			regards, tom lane

[Andrew Haley]

On 11/18/2011 11:32 PM, Tom Lane wrote:
>  Andrew Haley <aph(a)redhat.com&gt; writes:
> > > On 11/18/2011 05:53 PM, Ralf Corsepius wrote:
> >> >> pptp.c:459:33: warning: dereferencing type-punned pointer might break 
> >> >> strict-aliasing rules [-Wstrict-aliasing]
> > > Bingo!  Bugs like this must be fixed.
>  Sometimes that's easier said than done.  -fno-strict-aliasing might be
>  your friend. 
It's always easier said than done!  Still, one could argue
that -fno-strict-aliasing is a fix, kinda sorta.  My point is
that you've got to do *something*; this warning must not be
ignored.

Andrew.

[Ralf Corsepius]

On 11/19/2011 01:23 PM, Andrew Haley wrote:
>  On 11/18/2011 11:32 PM, Tom Lane wrote:
> > Andrew Haley<aph(a)redhat.com>  writes:
> >>> On 11/18/2011 05:53 PM, Ralf Corsepius wrote:
> >>>>> pptp.c:459:33: warning: dereferencing type-punned pointer might
> break
> >>>>> strict-aliasing rules [-Wstrict-aliasing]
> >>> Bingo!  Bugs like this must be fixed.
> > Sometimes that's easier said than done.  -fno-strict-aliasing might be
> > your friend.
>
>  It's always easier said than done!  Still, one could argue
>  that -fno-strict-aliasing is a fix, kinda sorta. 
It's a work-around to evaluate
whether aliasing actucally is the cause 
of this BZ.

>  My point is
>  that you've got to do *something*; this warning must not be
>  ignored. 
The code in question is a classic of the GCC-aliasing cases:
sockaddr vs. sockaddr_{in,un,..} casts.

The standard work-around to the GCC warnings would be using unions, 
containing fields of these types. Whether these spots actually are 
affected by aliasing issues, is a different matter.

[I have seen cases of this sort, where aliasing actually had malicious 
effects, but I've also seen cases it didn't.]

Ralf

[Paul Howarth]

Ralf,

On 11/18/2011 05:53 PM, Ralf Corsepius wrote:
>  On 11/18/2011 05:32 PM, Paul Howarth wrote:
> > It already builds with -Wall and there are no warnings:
>  This doesn't mean much.
>
>  Adding a couple of more agressive options, this is what happens to pptp:
>
>  orckit_quirks.c:65:2: warning: missing initializer
>  [-Wmissing-field-initializers]
>  orckit_quirks.c:65:2: warning: (near initialization for
>  'fixed_packet.header.pptp_type') [-Wmissing-field-initializers]
>  pptp.c:147:21: warning: unused parameter 'sig' [-Wunused-parameter]
>  pptp.c:153:19: warning: unused parameter 'sig' [-Wunused-parameter]
>  pptp.c:459:33: warning: dereferencing type-punned pointer might break
>  strict-aliasing rules [-Wstrict-aliasing]
>  pptp.c:492:65: warning: unused parameter 'argc' [-Wunused-parameter]
>  pptp_callmgr.c:105:48: warning: unused parameter 'envp' [-Wunused-parameter]
>  pptp_callmgr.c:202:48: warning: dereferencing type-punned pointer might
>  break strict-aliasing rules [-Wstrict-aliasing]
>  pptp_callmgr.c:215:17: warning: ignoring return value of 'read',
>  declared with attribute warn_unused_result [-Wunused-result]
>  pptp_callmgr.c:216:17: warning: ignoring return value of 'read',
>  declared with attribute warn_unused_result [-Wunused-result]
>  pptp_callmgr.c:325:29: warning: dereferencing type-punned pointer might
>  break strict-aliasing rules [-Wstrict-aliasing]
>  pptp_callmgr.c:330:28: warning: dereferencing type-punned pointer might
>  break strict-aliasing rules [-Wstrict-aliasing]
>  pptp_callmgr.c:358:25: warning: dereferencing type-punned pointer might
>  break strict-aliasing rules [-Wstrict-aliasing]
>  pptp_callmgr.c:368:44: warning: unused parameter 'inetaddr'
>  [-Wunused-parameter]
>  pptp_callmgr.c:39:29: warning: unused parameter 'sig' [-Wunused-parameter]
>  pptp_callmgr.c:44:29: warning: unused parameter 'sig' [-Wunused-parameter]
>  pptp_callmgr.c:71:18: warning: ignoring return value of 'write',
>  declared with attribute warn_unused_result [-Wunused-result]
>  pptp_ctrl.c:1062:13: warning: function declaration isn't a prototype
>  [-Wstrict-prototypes]
>  pptp_ctrl.c:177:1: warning: function declaration isn't a prototype
>  [-Wstrict-prototypes]
>  pptp_ctrl.c:206:31: warning: comparison between signed and unsigned
>  integer expressions [-Wsign-compare]
>  pptp_ctrl.c:245:13: warning: comparison between signed and unsigned
>  integer expressions [-Wsign-compare]
>  pptp_ctrl.c:535:14: warning: comparison between signed and unsigned
>  integer expressions [-Wsign-compare]
>  pptp_gre.c:237:27: warning: comparison between signed and unsigned
>  integer expressions [-Wsign-compare]
>  pptp_gre.c:241:30: warning: comparison between signed and unsigned
>  integer expressions [-Wsign-compare]
>  pptp_gre.c:249:18: warning: comparison between signed and unsigned
>  integer expressions [-Wsign-compare]
>  pptp_gre.c:260:23: warning: comparison between signed and unsigned
>  integer expressions [-Wsign-compare]
>  pptp_gre.c:441:26: warning: comparison between signed and unsigned
>  integer expressions [-Wsign-compare]
>  pptp_gre.c:446:23: warning: comparison between signed and unsigned
>  integer expressions [-Wsign-compare]
>  pptp_gre.c:493:27: warning: comparison between signed and unsigned
>  integer expressions [-Wsign-compare]
>  pptp_gre.c:527:19: warning: comparison between signed and unsigned
>  integer expressions [-Wsign-compare]
>  pptp_gre.c:74:10: warning: function declaration isn't a prototype
>  [-Wstrict-prototypes]
>  pptp_gre.c:85:19: warning: nested extern declaration of 'localbind'
>  [-Wnested-externs]
>  pptp_gre.c:92:29: warning: dereferencing type-punned pointer might break
>  strict-aliasing rules [-Wstrict-aliasing]
>  pptp_gre.c:99:28: warning: dereferencing type-punned pointer might break
>  strict-aliasing rules [-Wstrict-aliasing]
>  pptp_quirks.c:33:5: warning: function declaration isn't a prototype
>  [-Wstrict-prototypes]
>  pptp_quirks.h:56:1: warning: function declaration isn't a prototype
>  [-Wstrict-prototypes]
>  pqueue.c:220:11: warning: function declaration isn't a prototype
>  [-Wstrict-prototypes]
>  pqueue.h:27:1: warning: function declaration isn't a prototype
>  [-Wstrict-prototypes]
>  routing.c:117:8: warning: ignoring return value of 'fgets', declared
>  with attribute warn_unused_result [-Wunused-result]
>  routing.c:125:6: warning: function declaration isn't a prototype
>  [-Wstrict-prototypes]
>  routing.c:155:6: warning: function declaration isn't a prototype
>  [-Wstrict-prototypes]
>  routing.h:2:1: warning: function declaration isn't a prototype
>  [-Wstrict-prototypes]
>  routing.h:3:1: warning: function declaration isn't a prototype
>  [-Wstrict-prototypes]
>  test.c:145:14: warning: comparison between signed and unsigned integer
>  expressions [-Wsign-compare]
>  test.c:174:27: warning: function declaration isn't a prototype
>  [-Wstrict-prototypes]
>  test.c:55:16: warning: comparison between signed and unsigned integer
>  expressions [-Wsign-compare]
>  test.c:99:14: warning: comparison between signed and unsigned integer
>  expressions [-Wsign-compare]
>  test.h:5:8: warning: function declaration isn't a prototype
>  [-Wstrict-prototypes]
>  util.c:121:8: warning: ignoring return value of 'write', declared with
>  attribute warn_unused_result [-Wunused-result]
>  util.c:136:5: warning: function declaration isn't a prototype
>  [-Wstrict-prototypes]
>  util.c:142:5: warning: function declaration isn't a prototype
>  [-Wstrict-prototypes]
>  util.c:145:7: warning: ignoring return value of 'read', declared with
>  attribute warn_unused_result [-Wunused-result]
>  util.c:149:6: warning: function declaration isn't a prototype
>  [-Wstrict-prototypes]
>  util.c:90:5: warning: function declaration isn't a prototype
>  [-Wstrict-prototypes]
>  util.h:38:1: warning: function declaration isn't a prototype
>  [-Wstrict-prototypes]
>  util.h:47:1: warning: function declaration isn't a prototype
>  [-Wstrict-prototypes]
>  util.h:50:1: warning: function declaration isn't a prototype
>  [-Wstrict-prototypes]
>  util.h:52:1: warning: function declaration isn't a prototype
>  [-Wstrict-prototypes]
>  vector.c:38:9: warning: function declaration isn't a prototype
>  [-Wstrict-prototypes]
>  vector.h:15:1: warning: function declaration isn't a prototype
>  [-Wstrict-prototypes]
>
>  I can't spot anything obvious, but each of these are potential candiates
>  for issues and should be inspected. 
What were the "couple of more agressive options" you used, and which 
distro? I've tried "-Wall -Wextra" and still can't provoke the strict 
aliasing warning locally.

Paul.

[Ralf Corsepius]

On 12/01/2011 11:23 AM, Paul Howarth wrote:
>  Ralf,
>
>  On 11/18/2011 05:53 PM, Ralf Corsepius wrote:
> > On 11/18/2011 05:32 PM, Paul Howarth wrote:
> >> It already builds with -Wall and there are no warnings:
> > This doesn't mean much.
> >
> > Adding a couple of more agressive options, this is what happens to pptp:
> > 
>  What were the "couple of more agressive options" you used,
This is what I did (rebased against today's git):

diff --git a/pptp.spec b/pptp.spec
index 289dd08..78e36ca 100644
--- a/pptp.spec
+++ b/pptp.spec
@@ -86,7 +86,7 @@ tunnels.
  perl -pi -e 's/install -o root -m 555 pptp/install -m 755 pptp/;' Makefile

  %build
-make %{?_smp_mflags} CFLAGS="-Wall %{optflags}" IP=/sbin/ip
+make %{?_smp_mflags} CFLAGS="-Wall %{optflags} -Wextra 
-Wstrict-aliasing -Wnested-externs -Wstrict-prototypes" IP=/sbin/ip

  %install
  rm -rf %{buildroot}


>  and which
>  distro? 
Local rawhide mock.

The result I posted was extracted from a "fedpkg mockbuild"'s build.log 
in a local mock on fedora-16/x86_64.


>  I've tried "-Wall -Wextra" and still can't provoke
> the strict
>  aliasing warning locally. 
-Wstrict-aliasing is responsible for this warning.

Today's version looks like this:

# grep warning: results_pptp/1.7.2/13.fc17/build.log  | grep punned
pptp_gre.c:92:29: warning: dereferencing type-punned pointer might break 
strict-aliasing rules [-Wstrict-aliasing]
pptp_gre.c:99:28: warning: dereferencing type-punned pointer might break 
strict-aliasing rules [-Wstrict-aliasing]
pptp_callmgr.c:202:48: warning: dereferencing type-punned pointer might 
break strict-aliasing rules [-Wstrict-aliasing]
pptp_callmgr.c:325:29: warning: dereferencing type-punned pointer might 
break strict-aliasing rules [-Wstrict-aliasing]
pptp_callmgr.c:330:28: warning: dereferencing type-punned pointer might 
break strict-aliasing rules [-Wstrict-aliasing]
pptp_callmgr.c:358:25: warning: dereferencing type-punned pointer might 
break strict-aliasing rules [-Wstrict-aliasing]
pptp.c:459:33: warning: dereferencing type-punned pointer might break 
strict-aliasing rules [-Wstrict-aliasing]

Ralf

[Paul Howarth]

On Thu, 01 Dec 2011 15:45:48 +0100
Ralf Corsepius <rc040203(a)freenet.de&gt; wrote:

>  On 12/01/2011 11:23 AM, Paul Howarth wrote:
>  > Ralf,
>  >
>  > On 11/18/2011 05:53 PM, Ralf Corsepius wrote:
>  >> On 11/18/2011 05:32 PM, Paul Howarth wrote:
>  >>> It already builds with -Wall and there are no warnings:
>  >> This doesn't mean much.
>  >>
>  >> Adding a couple of more agressive options, this is what happens to
>  >> pptp:
>  >>
>  
>  > What were the "couple of more agressive options" you used,
>  
>  This is what I did (rebased against today's git):
>  
>  diff --git a/pptp.spec b/pptp.spec
>  index 289dd08..78e36ca 100644
>  --- a/pptp.spec
>  +++ b/pptp.spec
>  @@ -86,7 +86,7 @@ tunnels.
>    perl -pi -e 's/install -o root -m 555 pptp/install -m 755 pptp/;'
>  Makefile
>  
>    %build
>  -make %{?_smp_mflags} CFLAGS="-Wall %{optflags}" IP=/sbin/ip
>  +make %{?_smp_mflags} CFLAGS="-Wall %{optflags} -Wextra 
>  -Wstrict-aliasing -Wnested-externs -Wstrict-prototypes" IP=/sbin/ip
>  
>    %install
>    rm -rf %{buildroot}
>  
>  
>  > and which
>  > distro?
>  
>  Local rawhide mock.
>  
>  The result I posted was extracted from a "fedpkg mockbuild"'s
>  build.log in a local mock on fedora-16/x86_64.
>  
>  
>  > I've tried "-Wall -Wextra" and still can't provoke the strict
>  > aliasing warning locally.
>  -Wstrict-aliasing is responsible for this warning.
>  
>  Today's version looks like this:
>  
>  # grep warning: results_pptp/1.7.2/13.fc17/build.log  | grep punned
>  pptp_gre.c:92:29: warning: dereferencing type-punned pointer might
>  break strict-aliasing rules [-Wstrict-aliasing]
>  pptp_gre.c:99:28: warning: dereferencing type-punned pointer might
>  break strict-aliasing rules [-Wstrict-aliasing]
>  pptp_callmgr.c:202:48: warning: dereferencing type-punned pointer
>  might break strict-aliasing rules [-Wstrict-aliasing]
>  pptp_callmgr.c:325:29: warning: dereferencing type-punned pointer
>  might break strict-aliasing rules [-Wstrict-aliasing]
>  pptp_callmgr.c:330:28: warning: dereferencing type-punned pointer
>  might break strict-aliasing rules [-Wstrict-aliasing]
>  pptp_callmgr.c:358:25: warning: dereferencing type-punned pointer
>  might break strict-aliasing rules [-Wstrict-aliasing]
>  pptp.c:459:33: warning: dereferencing type-punned pointer might break 
>  strict-aliasing rules [-Wstrict-aliasing] 
This is weird: I can't reproduce these either locally in mock or in
koji:

http://koji.fedoraproject.org/koji/taskinfo?taskID=3555838

Paul.

[Kevin Kofler]

Paul Howarth wrote:
>  This is weird: I can't reproduce these either locally in mock or
> in
>  koji:
>  
>  http://koji.fedoraproject.org/koji/taskinfo?taskID=3555838 
-Wstrict-aliasing only works if -fstrict-aliasing is enabled, so if you're 
using -fno-strict-aliasing (or an optimization level below -O2 –
-fstrict-aliasing is only enabled by default at -O2, -O3 or -Os), you won't 
get the warnings.

-fno-strict-aliasing is a workaround for production builds, but it also 
sweeps the warnings under the carpet.

        Kevin Kofler

[Paul Howarth]

On Thu, 01 Dec 2011 21:19:14 +0100
Kevin Kofler <kevin.kofler(a)chello.at&gt; wrote:

>  Paul Howarth wrote:
>  > This is weird: I can't reproduce these either locally in mock or in
>  > koji:
>  > 
>  > http://koji.fedoraproject.org/koji/taskinfo?taskID=3555838
>  
>  -Wstrict-aliasing only works if -fstrict-aliasing is enabled, so if
>  you're using -fno-strict-aliasing (or an optimization level below -O2
>  – -fstrict-aliasing is only enabled by default at -O2, -O3 or -Os),
>  you won't get the warnings.
>  
>  -fno-strict-aliasing is a workaround for production builds, but it
>  also sweeps the warnings under the carpet. 
Understood... but I'm using -O2 and not -fno-strict-aliasing:

gcc -Wall -O2 -g -pipe -Wall -Wp,-D_FORTIFY_SOURCE=2 -fexceptions
-fstack-protector --param=ssp-buffer-size=4  -m64 -mtune=generic
-Wextra -Wstrict-aliasing -Wnested-externs -Wstrict-prototypes   -c -o
pptp_gre.o pptp_gre.c

Paul.

[Ralf Corsepius]

On 12/01/2011 07:11 PM, Paul Howarth wrote:
>  On Thu, 01 Dec 2011 15:45:48 +0100
>  Ralf Corsepius<rc040203(a)freenet.de>  wrote:
>
> > On 12/01/2011 11:23 AM, Paul Howarth wrote:
> >> Ralf,
> >>
> >> On 11/18/2011 05:53 PM, Ralf Corsepius wrote:
> >>> On 11/18/2011 05:32 PM, Paul Howarth wrote:
> >>>> It already builds with -Wall and there are no warnings:
> >>> This doesn't mean much.
> >>>
> >>> Adding a couple of more agressive options, this is what happens to
> >>> pptp:
> >>>
> >
> >> What were the "couple of more agressive options" you used,
> >
> > This is what I did (rebased against today's git):
> >
> > diff --git a/pptp.spec b/pptp.spec
> > index 289dd08..78e36ca 100644
> > --- a/pptp.spec
> > +++ b/pptp.spec
> > @@ -86,7 +86,7 @@ tunnels.
> >    perl -pi -e 's/install -o root -m 555 pptp/install -m 755 pptp/;'
> > Makefile
> >
> >    %build
> > -make %{?_smp_mflags} CFLAGS="-Wall %{optflags}" IP=/sbin/ip
> > +make %{?_smp_mflags} CFLAGS="-Wall %{optflags} -Wextra
> > -Wstrict-aliasing -Wnested-externs -Wstrict-prototypes" IP=/sbin/ip
> >
> >    %install
> >    rm -rf %{buildroot}
> >
> >
> >> and which
> >> distro?
> >
> > Local rawhide mock.
> >
> > The result I posted was extracted from a "fedpkg mockbuild"'s
> > build.log in a local mock on fedora-16/x86_64.
> >
> >
> >> I've tried "-Wall -Wextra" and still can't provoke the strict
> >> aliasing warning locally.
> > -Wstrict-aliasing is responsible for this warning.
> >
> > Today's version looks like this:
> >
> > # grep warning: results_pptp/1.7.2/13.fc17/build.log  | grep punned
> > pptp_gre.c:92:29: warning: dereferencing type-punned pointer might
> > break strict-aliasing rules [-Wstrict-aliasing]
> > pptp_gre.c:99:28: warning: dereferencing type-punned pointer might
> > break strict-aliasing rules [-Wstrict-aliasing]
> > pptp_callmgr.c:202:48: warning: dereferencing type-punned pointer
> > might break strict-aliasing rules [-Wstrict-aliasing]
> > pptp_callmgr.c:325:29: warning: dereferencing type-punned pointer
> > might break strict-aliasing rules [-Wstrict-aliasing]
> > pptp_callmgr.c:330:28: warning: dereferencing type-punned pointer
> > might break strict-aliasing rules [-Wstrict-aliasing]
> > pptp_callmgr.c:358:25: warning: dereferencing type-punned pointer
> > might break strict-aliasing rules [-Wstrict-aliasing]
> > pptp.c:459:33: warning: dereferencing type-punned pointer might break
> > strict-aliasing rules [-Wstrict-aliasing]
>
>  This is weird: I can't reproduce these either locally in mock or in
>  koji: 
Neither can I ;)

>  http://koji.fedoraproject.org/koji/taskinfo?taskID=3555838 
... mock appends build.log's from subsequent mock runs ...

... the warnings above stem from older builds.

I had not cleaned up the build.log when "git merging" your recent 
changes into my git checkout.

Ralf

[Paul Howarth]

On 12/02/2011 04:12 AM, Ralf Corsepius wrote:
>  On 12/01/2011 07:11 PM, Paul Howarth wrote:
> > On Thu, 01 Dec 2011 15:45:48 +0100
> > Ralf Corsepius<rc040203(a)freenet.de> wrote:
> >
> >> On 12/01/2011 11:23 AM, Paul Howarth wrote:
> >>> Ralf,
> >>>
> >>> On 11/18/2011 05:53 PM, Ralf Corsepius wrote:
> >>>> On 11/18/2011 05:32 PM, Paul Howarth wrote:
> >>>>> It already builds with -Wall and there are no warnings:
> >>>> This doesn't mean much.
> >>>>
> >>>> Adding a couple of more agressive options, this is what happens to
> >>>> pptp:
> >>>>
> >>
> >>> What were the "couple of more agressive options" you used,
> >>
> >> This is what I did (rebased against today's git):
> >>
> >> diff --git a/pptp.spec b/pptp.spec
> >> index 289dd08..78e36ca 100644
> >> --- a/pptp.spec
> >> +++ b/pptp.spec
> >> @@ -86,7 +86,7 @@ tunnels.
> >> perl -pi -e 's/install -o root -m 555 pptp/install -m 755 pptp/;'
> >> Makefile
> >>
> >> %build
> >> -make %{?_smp_mflags} CFLAGS="-Wall %{optflags}" IP=/sbin/ip
> >> +make %{?_smp_mflags} CFLAGS="-Wall %{optflags} -Wextra
> >> -Wstrict-aliasing -Wnested-externs -Wstrict-prototypes" IP=/sbin/ip
> >>
> >> %install
> >> rm -rf %{buildroot}
> >>
> >>
> >>> and which
> >>> distro?
> >>
> >> Local rawhide mock.
> >>
> >> The result I posted was extracted from a "fedpkg mockbuild"'s
> >> build.log in a local mock on fedora-16/x86_64.
> >>
> >>
> >>> I've tried "-Wall -Wextra" and still can't provoke the
> strict
> >>> aliasing warning locally.
> >> -Wstrict-aliasing is responsible for this warning.
> >>
> >> Today's version looks like this:
> >>
> >> # grep warning: results_pptp/1.7.2/13.fc17/build.log | grep punned
> >> pptp_gre.c:92:29: warning: dereferencing type-punned pointer might
> >> break strict-aliasing rules [-Wstrict-aliasing]
> >> pptp_gre.c:99:28: warning: dereferencing type-punned pointer might
> >> break strict-aliasing rules [-Wstrict-aliasing]
> >> pptp_callmgr.c:202:48: warning: dereferencing type-punned pointer
> >> might break strict-aliasing rules [-Wstrict-aliasing]
> >> pptp_callmgr.c:325:29: warning: dereferencing type-punned pointer
> >> might break strict-aliasing rules [-Wstrict-aliasing]
> >> pptp_callmgr.c:330:28: warning: dereferencing type-punned pointer
> >> might break strict-aliasing rules [-Wstrict-aliasing]
> >> pptp_callmgr.c:358:25: warning: dereferencing type-punned pointer
> >> might break strict-aliasing rules [-Wstrict-aliasing]
> >> pptp.c:459:33: warning: dereferencing type-punned pointer might break
> >> strict-aliasing rules [-Wstrict-aliasing]
> >
> > This is weird: I can't reproduce these either locally in mock or in
> > koji:
>  Neither can I ;)
>
> > http://koji.fedoraproject.org/koji/taskinfo?taskID=3555838
>
>  ... mock appends build.log's from subsequent mock runs ...
>
>  ... the warnings above stem from older builds.
>
>  I had not cleaned up the build.log when "git merging" your recent
>  changes into my git checkout. 
Hmm, that sort of explains it - except that even if I don't apply any of 
the last few patches since the F14 version, I *still* can't provoke a 
strict-aliasing warning.

Paul.

[Paul Howarth]

On 12/02/2011 02:08 PM, Paul Howarth wrote:
>  On 12/02/2011 04:12 AM, Ralf Corsepius wrote:
> > On 12/01/2011 07:11 PM, Paul Howarth wrote:
> >> On Thu, 01 Dec 2011 15:45:48 +0100
> >> Ralf Corsepius<rc040203(a)freenet.de>  wrote:
> >>
> >>> On 12/01/2011 11:23 AM, Paul Howarth wrote:
> >>>> Ralf,
> >>>>
> >>>> On 11/18/2011 05:53 PM, Ralf Corsepius wrote:
> >>>>> On 11/18/2011 05:32 PM, Paul Howarth wrote:
> >>>>>> It already builds with -Wall and there are no warnings:
> >>>>> This doesn't mean much.
> >>>>>
> >>>>> Adding a couple of more agressive options, this is what happens to
> >>>>> pptp:
> >>>>>
> >>>
> >>>> What were the "couple of more agressive options" you used,
> >>>
> >>> This is what I did (rebased against today's git):
> >>>
> >>> diff --git a/pptp.spec b/pptp.spec
> >>> index 289dd08..78e36ca 100644
> >>> --- a/pptp.spec
> >>> +++ b/pptp.spec
> >>> @@ -86,7 +86,7 @@ tunnels.
> >>> perl -pi -e 's/install -o root -m 555 pptp/install -m 755 pptp/;'
> >>> Makefile
> >>>
> >>> %build
> >>> -make %{?_smp_mflags} CFLAGS="-Wall %{optflags}" IP=/sbin/ip
> >>> +make %{?_smp_mflags} CFLAGS="-Wall %{optflags} -Wextra
> >>> -Wstrict-aliasing -Wnested-externs -Wstrict-prototypes" IP=/sbin/ip
> >>>
> >>> %install
> >>> rm -rf %{buildroot}
> >>>
> >>>
> >>>> and which
> >>>> distro?
> >>>
> >>> Local rawhide mock.
> >>>
> >>> The result I posted was extracted from a "fedpkg mockbuild"'s
> >>> build.log in a local mock on fedora-16/x86_64.
> >>>
> >>>
> >>>> I've tried "-Wall -Wextra" and still can't provoke the
> strict
> >>>> aliasing warning locally.
> >>> -Wstrict-aliasing is responsible for this warning.
> >>>
> >>> Today's version looks like this:
> >>>
> >>> # grep warning: results_pptp/1.7.2/13.fc17/build.log | grep punned
> >>> pptp_gre.c:92:29: warning: dereferencing type-punned pointer might
> >>> break strict-aliasing rules [-Wstrict-aliasing]
> >>> pptp_gre.c:99:28: warning: dereferencing type-punned pointer might
> >>> break strict-aliasing rules [-Wstrict-aliasing]
> >>> pptp_callmgr.c:202:48: warning: dereferencing type-punned pointer
> >>> might break strict-aliasing rules [-Wstrict-aliasing]
> >>> pptp_callmgr.c:325:29: warning: dereferencing type-punned pointer
> >>> might break strict-aliasing rules [-Wstrict-aliasing]
> >>> pptp_callmgr.c:330:28: warning: dereferencing type-punned pointer
> >>> might break strict-aliasing rules [-Wstrict-aliasing]
> >>> pptp_callmgr.c:358:25: warning: dereferencing type-punned pointer
> >>> might break strict-aliasing rules [-Wstrict-aliasing]
> >>> pptp.c:459:33: warning: dereferencing type-punned pointer might break
> >>> strict-aliasing rules [-Wstrict-aliasing]
> >>
> >> This is weird: I can't reproduce these either locally in mock or in
> >> koji:
> > Neither can I ;)
> >
> >> http://koji.fedoraproject.org/koji/taskinfo?taskID=3555838
> >
> > ... mock appends build.log's from subsequent mock runs ...
> >
> > ... the warnings above stem from older builds.
> >
> > I had not cleaned up the build.log when "git merging" your recent
> > changes into my git checkout.
>
>  Hmm, that sort of explains it - except that even if I don't apply any of
>  the last few patches since the F14 version, I *still* can't provoke a
>  strict-aliasing warning. 
Got it: I had to use -Wstrict-aliasing=2

Paul.

[Gregory Maxwell]

On Fri, Nov 18, 2011 at 6:31 AM, Paul Howarth <paul(a)city-fan.org&gt; wrote:
>  2. How to determine what the actual problem is, e.g. a problem with
> the
>  way the code is written leading to unsafe optimizations, or a gcc bug? 
[Obviously Andrew's look at warnings advice is good but also…]

See if you can reproduce it when compiled with -O2 -fno-strict-aliasing
if not, then the package violates C rules for pointer aliasing and
-Wstrict-aliasing=n can help you find the bug.

Otherwise, reproduce the crash while running in valgrind and it's pretty
likely that you'll see the cause there. (e.g. some use-uninitilized which
turns out to be harmless in O0)

Between these two that covers a pretty significant portion of bugs that
vanish at O0.