[Bug 1254033] New: Chapter 9.2 advice on using /proc/self/fd to close file descriptors before execve is dangerously wrong

[bugzilla at redhat.com]

https://bugzilla.redhat.com/show_bug.cgi?id=1254033

            Bug ID: 1254033
           Summary: Chapter 9.2 advice on using /proc/self/fd to close
                    file descriptors before execve is dangerously wrong
           Product: Fedora Documentation
           Version: devel
         Component: defensive-coding-guide
          Assignee: fweimer at redhat.com
          Reporter: sstewartgallus00 at mylangara.bc.ca
        QA Contact: docs-qa at lists.fedoraproject.org
                CC: fweimer at redhat.com



Description of problem:

The documentation advises to open /proc/self/fd after forking and close the
file descriptor entries found in there before calling execve. However, in a
multi-threaded environment many memory allocators may deadlock after fork
(opendir, and readdir may allocate memory.) As well, most commonly programmers
implement this by looping over /proc/self/fd while closing file descriptors
which is unsafe because it is like looping over a list while removing entries
from it at the same time. Instead, one should set the close on execute bit on
these file descriptors when looping over the directory entries (probably using
the FIONCLEX ioctl for speed.)

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