Re: XFS reports lchmod failure, but changes file system contents

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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That text is outdated and should be corrected.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.

Rich



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