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Subject[PATCH 2/8] statx: reject unknown flags when using NULL path
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From: Eric Biggers <ebiggers@google.com>

The statx() system call currently accepts unknown flags when called with
a NULL path to operate on a file descriptor. Left unchanged, this could
make it hard to introduce new query flags in the future, since
applications may not be able to tell whether a given flag is supported.

Fix this by failing the system call with EINVAL if any flags other than
KSTAT_QUERY_FLAGS are specified in combination with a NULL path.

Arguably, we could still permit known lookup-related flags such as
AT_SYMLINK_NOFOLLOW. However, that would be inconsistent with how
sys_utimensat() behaves when passed a NULL path, which seems to be the
closest precedent. And given that the NULL path case is (I believe)
mainly intended to be used to implement a wrapper function like fstatx()
that doesn't have a path argument, I think rejecting lookup-related
flags too is probably the best choice.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: David Howells <dhowells@redhat.com>
---

fs/stat.c | 6 +++++-
1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/fs/stat.c b/fs/stat.c
index fa0be59340cc..df484a60846d 100644
--- a/fs/stat.c
+++ b/fs/stat.c
@@ -130,9 +130,13 @@ EXPORT_SYMBOL(vfs_getattr);
int vfs_statx_fd(unsigned int fd, struct kstat *stat,
u32 request_mask, unsigned int query_flags)
{
- struct fd f = fdget_raw(fd);
+ struct fd f;
int error = -EBADF;

+ if (query_flags & ~KSTAT_QUERY_FLAGS)
+ return -EINVAL;
+
+ f = fdget_raw(fd);
if (f.file) {
error = vfs_getattr(&f.file->f_path, stat,
request_mask, query_flags);
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