[libvirt] [PATCH 3/3] virCommand: use procfs to learn opened FDs

[Eric Blake]

On 7/14/19 12:23 AM, Florian Weimer wrote:
> * Eric Blake:
> 
>> Does anyone know if glibc guarantees that opendir/readdir in between
>> multi-threaded fork() and exec() is safe, even though POSIX does not
>> guarantee that safety in general?
> 
> glibc supports malloc after multi-threaded fork as an extension (or as
> a bug, because it makes malloc not async-signal-safe).

It's not a bug for glibc to provide guarantees above what POSIX
requires, but IS a bug for applications to depend on those guarantees
without realizing they are non-portable.

> 
> If you do not want to use opendir/readdir, issuing getdents64 directly
> and parsing the buffer is your best option right now.  (Lowering the
> RLIMIT_NOFILE limit does not enable probing for stray descriptors,
> unfortunately.)  But opendir/readdir after fork should be fine,
> really.

Thanks for checking; I'm okay with the patch that started this thread
going in libvirt if we tweak it to also include a big fat comment
stating that use of opendir/readdir is not safe in general, but should
be safe in this specific use (because glibc adds async-signal safety to
those functions that was not required by POSIX), since the patch is only
using opendir on Linux.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

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