On Fri, Jul 28, 2023 at 09:58:26PM +1000, Aleksa Sarai wrote: > This allows userspace to avoid going through /proc/self/fd when dealing > with all types of file descriptors for chmod(), and makes fchmodat2() a > proper superset of all other chmod syscalls. > > The primary difference between fchmodat2(AT_EMPTY_PATH) and fchmod() is > that fchmod() doesn't operate on O_PATH file descriptors by design. To > quote open(2): > > > O_PATH (since Linux 2.6.39) > > [...] > > 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. > > However, procfs has allowed userspace to do this operation ever since > the introduction of O_PATH through magic-links, so adding this feature > is only an improvement for programs that have to mess around with > /proc/self/fd/$n today to get this behaviour. In addition, > fchownat(AT_EMPTY_PATH) has existed since the introduction of O_PATH and > allows chown() operations directly on O_PATH descriptors. > > Signed-off-by: Aleksa Sarai <cyphar@xxxxxxxxxx> It seems to me that this makes sense for fchmodat2. Acked-by: Alexey Gladkov <legion@xxxxxxxxxx> > --- > fs/open.c | 4 +++- > 1 file changed, 3 insertions(+), 1 deletion(-) > > diff --git a/fs/open.c b/fs/open.c > index e52d78e5a333..b8883ec286f5 100644 > --- a/fs/open.c > +++ b/fs/open.c > @@ -678,10 +678,12 @@ static int do_fchmodat(int dfd, const char __user *filename, umode_t mode, > int error; > unsigned int lookup_flags; > > - if (unlikely(flags & ~AT_SYMLINK_NOFOLLOW)) > + if (unlikely(flags & ~(AT_SYMLINK_NOFOLLOW | AT_EMPTY_PATH))) > return -EINVAL; > > lookup_flags = (flags & AT_SYMLINK_NOFOLLOW) ? 0 : LOOKUP_FOLLOW; > + if (flags & AT_EMPTY_PATH) > + lookup_flags |= LOOKUP_EMPTY; > > retry: > error = user_path_at(dfd, filename, lookup_flags, &path); > > --- > base-commit: 4859c257d295949c23f4074850a8c2ec31357abb > change-id: 20230728-fchmodat2-at_empty_path-310cf40c921f > > Best regards, > -- > Aleksa Sarai <cyphar@xxxxxxxxxx> > -- Rgrds, legion
