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.
Andreas.
--
Andreas Schwab, schwab@xxxxxxxxxxxxxx
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