* 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 That text is outdated and should be corrected.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?
