Re: [PATCH trivial 1/2] close_all_open_fd(): move to oslib-posix.c

[Daniel P . Berrangé]

On Fri, Jan 26, 2024 at 01:45:39PM +0300, Michael Tokarev wrote:
> 26.01.2024 12:06, Daniel P. Berrangé wrote:
> > On Fri, Jan 26, 2024 at 08:44:13AM +0100, Laurent Vivier wrote:
> > > Le 25/01/2024 à 23:29, Michael Tokarev a écrit :
> 
> 
> > > I think the way using sysconf(_SC_OPEN_MAX) is more portable, simpler and
> > > cleaner than the one using /proc/self/fd.
> > 
> > A fallback that uses _SC_OPEN_MAX is good for portability, but it is
> > should not be considered a replacement for iterating over /proc/self/fd,
> > rather an additional fallback for non-Linux, or when /proc is not mounted.
> > It is not uncommon for _SC_OPEN_MAX to be *exceedingly* high
> > 
> >    $ podman run -it quay.io/centos/centos:stream9
> >    [root@4a440d62935c /]# ulimit -n
> >    524288
> > 
> > Iterating over 1/2 a million FDs is a serious performance penalty that
> > we don't want to have, so _SC_OPEN_MAX should always be the last resort.
> 
> From yesterday conversation in IRC which started this:
> 
>  <mmlb> open files          (-n) 1073741816
> 
> (it is a docker container)
> They weren't able to start qemu.. :)
> 
> Sanity of such setting is questionable, but ok.
> 
> Not only linux implement close_range(2) syscall, it is also
> available on some *BSDs.
> 
> And the most important point is, - we should aim at using O_CLOEXEC
> everywhere, without this need to close each FD at exec time.  I think
> qemu is the only software with such paranoid closing when just running
> an interface setup script..

We should try to use O_CLOEXEC everywhere, but at the same time QEMU
links to a large number of libraries, and we can't assume that they've
reliably used O_CLOEXEC. Non-QEMU owned code that is mapped in process
likely dwarfs QEMU owned code by a factor of x10.

With regards,
Daniel
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