Re: [Qemu-devel] [for-2.10 PATCH v2] 9pfs: local: fix fchmodat_nofollow() limitations

[Greg Kurz <groug@kaod.org>]

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On Wed, 9 Aug 2017 10:59:46 -0500
Eric Blake <eblake@redhat.com> wrote:

> On 08/09/2017 10:22 AM, Greg Kurz wrote:
> 
> >>>
> >>> The solution is to use O_PATH: openat() now succeeds in both cases, and we
> >>> can ensure the path isn't a symlink with fstat(). The associated entry in
> >>> "/proc/self/fd" can hence be safely passed to the regular chmod() syscall.    
> >>
> >> Hey - should we point this out as a viable solution to the glibc folks,
> >> since their current user-space emulation of AT_SYMLINK_NOFOLLOW is broken?
> >>  
> > 
> > Probably. What's the best way to do that ?  
> 
> I've added a comment to
> https://sourceware.org/bugzilla/show_bug.cgi?id=14578; you'll also want
> to point to the lkml discussion in that bug.  And reading that bug, it
> also looks like your hack with /proc/self/fd has been proposed by Rich
> Felker since 2013! (although fstat() didn't work until Linux 3.6, even
> though O_PATH predates that time) - so there is that one additional
> concern of whether we need to cater to the window of kernels where
> O_PATH exists but fstat() on that fd can't learn whether we opened a
> symlink.
> 

BTW, what happens with fstat() and O_PATH before Linux 3.6 ? Does it
fail or does it return something wrong in th stat buf ?

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