Re: [Qemu-devel] [PATCH] linux-user: Use *at functions to implement interp_prefix

[Eric Blake]

On 02/13/2018 09:31 AM, Richard Henderson wrote:
> > I wonder if there are guest programs that make assumptions about
> > file descriptor numbers such that it would be worthwhile dup2'ing
> > the interp_dirfd away from the presumably low number fd it will
> > get by default into something larger...
>
> Hmm.  Using dup2(probe, probe) to test if the new (high) fd itself has not been
> allocated?

fcntl(F_DUPFD[_CLOEXEC]) is smarter than dup2/3, if you plan on 
atomically guaranteeing a dup to a not-in-use fd.

Will dup'ing to a high fd violate assumptions of programs that assume 
that open() and friends favor the next available fd by default, rather 
than having a gap?  (Probably not, but skipping fds is not usual, so 
it's worth asking.)


--
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org