Re: [PATCH] fchmodat, lchmod: port to buggy Linux filesystems

[Pádraig Brady]

On 10/03/2020 11:52, Florian Weimer wrote:
> * Pádraig Brady:
>
> > On 09/03/2020 18:51, Paul Eggert wrote:
> > > On 3/9/20 10:30 AM, Pádraig Brady wrote:
> > >
> > > > A very similar "ENOTSUP" problem is being reported with coreutils-8.32
> > > > with `mknod -m 666 /dev/random c 1 8` when trying to build Fedora
> > > > rawhide in a chroot.
> > > > https://bugzilla.redhat.com/1811038
> > >
> > > I don't understand that bug report. The strace diff you mentioned in
> > > Comment 4 looks like the new mknod command is working. And yet the
> > > original bug report says new mknod command is complaining "Operation not
> > > supported".
> > >
> > > Is the problem that some filesystems don't work with the chmod
> > > /proc/self/fd/NNN trick, and that it worked for you (the strace diff)
> > > but not for Mohan (the original bug report)?
> >
> > Right, the strace is from my working mknod(1)
> > to show the differences between 8.31 and 8.32.
> >
> > I've requested an strace from the failing system.
>
> I guess it's possible that just isn't mounted at this point.
>
> The glibc implementation will definitely *not* add racy fallback in
> case /proc is not available, so this will not work until someone
> implements the required system call.
>
> It's not clear to my why the mknod command would need fchmodat at all.
> With the -m argument, it should simply set the umask to 0, and pass
> the mode bits to the mknod function.

umask is not used so as to cater for discrepancies between process and default 
ACL masks:
https://git.sv.gnu.org/gitweb/?p=coreutils.git;a=commitdiff;h=v8.21-51-ge7198a67b

An update re this issue.
The strace was supplied in https://bugzilla.redhat.com/1811038
which shows there is no fallback to chmod() in lchmod().
Now the gnulib code does fallback so this issue must be in the glibc 
implementation.
I also downloaded the problematic rpm, and used nm on the mknod binary
to confirm is was calling out to the glibc implementation.

cheers,
Pádraig