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fchmodat2: add support for AT_EMPTY_PATH

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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@cyphar.com>
Acked-by: Alexey Gladkov <legion@kernel.org>
Message-Id: <20230728-fchmodat2-at_empty_path-v1-1-f3add31d3516@cyphar.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>
This commit is contained in:
Aleksa Sarai 2023-07-28 21:58:26 +10:00 committed by Christian Brauner
parent 4859c257d2
commit 5daeb41a6f
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1 changed files with 3 additions and 1 deletions
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4
fs/open.c
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@ -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);