[fedora-arm] e2fsprogs alignment

[Gordan Bobic]

On 07/09/2012 03:24 PM, Eric Sandeen wrote:
> On 7/8/12 1:07 AM, Jon Masters wrote:
>> On 07/07/2012 08:42 PM, Gordan Bobic wrote:
>>> Can anyone confirm whether this has been fixed since the bug was filed?
>>> Or whether there is still a risk of trashing the file system on ARMv5/ARMv6?
>>>
>>> https://bugzilla.redhat.com/show_bug.cgi?id=680090
>>
>> I believe Eric said this was now likely fixed upstream. Have you asked
>> him to confirm? Let me copy him, and I'm sure he'll say it's all fine :)
>
> nope, sorry - well, not exactly fine.  ;)
>
> We fixed on-disk structure alignment errors for XFS - this is a different
> problem, with in-memory alignment...
>
> These memory access problem on arm may well still exist.
>
> It's "only" a warning about alignment though, right - I don't think this
> results in any corruption etc, does it (anyway, filefrag is read-only).

What concerns me is that the buffers are allocated as char[4096] all 
over the place in e2fsprogs code files. That is inherently unsafe on 
everything except x86 and ARMv7+. I wouldn't want to assume that 
filefrag is the only affected program from the e2fsprogs suite, 
especially since the scope for trashing the file system is pretty high.

> And as the comments in this bug indicate, I'm really not quite sure what
> the accepted method of fixing is.

The options I can see are:

1) Detect sizeof(int) and declare the arrays as a suitable multiple of 
that. int _should_ be word sized on most things AFAIK.

2) Check the required alignment for the arch at build time, and apply it 
with "__attribute__ aligned" in every relevant instance. There's a LOT 
of those, but that's what you get for using non-portable techniques. :(

3) Add a new flag to gcc to align all arrays to a particular boundary. 
Unlikely to happen. FWIW, ICC has such an option, IIRC as it helps with 
performance, too, if things aren't straddling cache lines.

> I imagine there are several applications
> that exhibit similar behavior?

Not many, if you are referring to other packages. I filed bugs for most 
of them, but I filed against the available ARM releases (since rawhide 
wasn't up to running on ARM), so they all got closed without resolution 
pretty quickly. I gave up on bothering to file bugs after that.

Gordan