On Wed, Feb 12, 2020 at 09:26:11PM +0100, Florian Weimer wrote: > * Rich Felker: > > > 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 That text is outdated and should be corrected.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. > > I suppose we could do the S_ISLNK check, try fchmod, and if that > fails, go via /proc. Is this the direction you want to go in? It was, but Al Viro just pointed out to me that I was wrong. I think we could use fstat (which AIUI now works) to do the S_ISLNK check, so that it doesn't depend on /proc, but I don't see a way to do the chmod operation without /proc at this time. Rich
