XFS reports lchmod failure, but changes file system contents

[Rich Felker]

On Wed, Feb 12, 2020 at 08:27:24PM +0000, Al Viro wrote:
> On Wed, Feb 12, 2020 at 03:19:51PM -0500, Rich Felker wrote:
> > On Wed, Feb 12, 2020 at 09:17:41PM +0100, Andreas Schwab wrote:
> > > On Feb 12 2020, Florian Weimer wrote:
> > > 
> > > > * Al Viro:
> > > >
> > > >> On Wed, Feb 12, 2020 at 08:15:08PM +0100, Florian Weimer wrote:
> > > >>
> > > >>> | Further, I've found some inconsistent behavior with ext4: chmod on the
> > > >>> | magic symlink fails with EOPNOTSUPP as in Florian's test, but fchmod
> > > >>> | on the O_PATH fd succeeds and changes the symlink mode. This is with
> > > >>> | 5.4. Cany anyone else confirm this? Is it a problem?
> > > >>> 
> > > >>> It looks broken to me because fchmod (as an inode-changing operation)
> > > >>> is not supposed to work on O_PATH descriptors.
> > > >>
> > > >> Why?  O_PATH does have an associated inode just fine; where does
> > > >> that "not supposed to" come from?
> > > >
> > > > It fails on most file systems right now.  I thought that was expected.
> > > > Other system calls (fsetxattr IIRC) do not work on O_PATH descriptors,
> > > > either.  I assumed that an O_PATH descriptor was not intending to
> > > > confer that capability.  Even openat fails.
> > > 
> > > According to open(2), this is expected:
> > > 
> > >        O_PATH (since Linux 2.6.39)
> > >               Obtain a file descriptor that can be used for two  purposes:  to
> > >               indicate a location in the filesystem tree and to perform opera-
> > >               tions that act purely at the file descriptor  level.   The  file
> > >               itself  is not opened, and other file operations (e.g., read(2),
> > >               write(2), fchmod(2), fchown(2), fgetxattr(2), ioctl(2), mmap(2))
> > >               fail with the error EBADF.
> > 
> > That text is outdated and should be corrected. Fixing fchmod fchown,
> > fstat, etc. to operate on O_PATH file descriptors was a very
> > intentional change in the kernel.
> 
> Wait.  First of all, in the testcase it's chmod(2) applied to /proc/*/fd/*; that's
> no different for O_PATH descriptors.  Location in the tree *is* associated with
> O_PATH fd; that's the only thing they exist for.
> 
> fchmod(2) will certainly fail for those, as it always had:
> int ksys_fchmod(unsigned int fd, umode_t mode)
> {
>         struct fd f = fdget(fd);
>         int err = -EBADF;
> 
>         if (f.file) {
>                 audit_file(f.file);
>                 err = chmod_common(&f.file->f_path, mode);
>                 fdput(f);  
>         }
>         return err;
> }
> 
> SYSCALL_DEFINE2(fchmod, unsigned int, fd, umode_t, mode)
> {
>         return ksys_fchmod(fd, mode);
> }
> 
> .... and that fdget() will give you -EBADF.  If you've managed to get
> fchmod(2) the syscall to give you anything other than that, I want
> to see details.

Sorry, it's my fault -- that's not the raw fchmod syscall but the
fchmod function, which falls back to using /proc on failure with EBADF
because this is necessary to support O_SEARCH/O_EXEC functionality
implemented through O_PATH file descriptors.

So the same thing is happening regardless of whether /proc is used
because /proc is the backend either way.

However, what I have found is that the same bug present on XFS is also
present on ext4. After:

chmod("/proc/self/fd/3", 0777)          = -1 EOPNOTSUPP (Not supported)

$ ls -l symlink
lrwxrwxrwx    1 dalias   users            3 Feb 12 13:48 symlink -> DNE

and after:

chmod("/proc/self/fd/3", 000)           = -1 EOPNOTSUPP (Not supported)

l---------    1 dalias   users            3 Feb 12 13:48 symlink -> DNE

So perhaps this is happening at a higher level in the kernel.

Apologies for the noise from confusing function call with syscall.

Rich