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 That text is outdated and should be corrected.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
