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

[Daniel P. Berrangé <berrange at redhat dot com>]

On Sun, Jul 14, 2019 at 07:23:10AM +0200, 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).
> 
> Lots of programs depend on this.  OpenJDK even calls malloc after
> vfork, which is not officially supported (but of course we can't break
> OpenJDK).

Yep, libvirt already relies glibc semantics that let malloc/free  work
after fork(), for other code we've had since essentially forever.

> > I know that one approach to avoid
> > having to close all fds is religiously using O_CLOEXEC everywhere (so
> > that the race window of having an fd that would leak is not possible),
> > but that's also an expensive audit, compared to just ensuring that
> > things are closed after fork().  Are there any other ideas out there
> > that we should be aware of (and no, pthread_atfork is not a sane idea)?
> > (various BSD systems have the closefrom() syscall which is more
> > efficient than lots of close() calls; and Linux may be adding something
> > similar https://lwn.net/Articles/789023/), Is there any saner way to
> > close all unneeded fds that were not properly marked O_CLOEXEC by an
> > application linking against a multithreaded lib that must perform fork/exec?
> 
> I tried to add getdents64 (which got committed, but may yet move from
> <unistd.h> to <dirent.h>, to match musl) and <sys/direntries.h> (which
> did not) in glibc 2.30.  Those interfaces are async-signal-safe
> (except on some MIPS variants, where getdents64 has complex
> emulation).
> 
> 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.

Ok, lets just keep it simple & use opendif/readdir.

If we ever hit problems, we can just disable the code & go back to the
slower code we have right now.

Hopefully the kernel folks will finally merge one of the recent
closefrom()-like  proposals and make life much easier.

Regards,
Daniel
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