[PATCH v2 2/2] binder: Gracefully handle BINDER_TYPE_FDA objects with num_fds=0

[Alessandro Astone <ales.astone@gmail.com>]

Some android userspace is sending BINDER_TYPE_FDA objects with
num_fds=0. Like the previous patch, this is reproducible when
playing a video.

Before commit 09184ae9b575 BINDER_TYPE_FDA objects with num_fds=0
were 'correctly handled', as in no fixup was performed.

After commit 09184ae9b575 we aggregate fixup and skip regions in
binder_ptr_fixup structs and distinguish between the two by using
the skip_size field: if it's 0, then it's a fixup, otherwise skip.
When processing BINDER_TYPE_FDA objects with num_fds=0 we add a
skip region of skip_size=0, and this causes issues because now
binder_do_deferred_txn_copies will think this was a fixup region.

To address that, return early from binder_translate_fd_array to
avoid adding an empty skip region.

Fixes: 09184ae9b575 ("binder: defer copies of pre-patched txn data")
Signed-off-by: Alessandro Astone <ales.astone@gmail.com>
---
 drivers/android/binder.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/android/binder.c b/drivers/android/binder.c
index 31176edb1069..f3b639e89dd8 100644
--- a/drivers/android/binder.c
+++ b/drivers/android/binder.c
@@ -2491,6 +2491,9 @@ static int binder_translate_fd_array(struct list_head *pf_head,
 	struct binder_proc *proc = thread->proc;
 	int ret;
 
+	if (fda->num_fds == 0)
+		return 0;
+
 	fd_buf_size = sizeof(u32) * fda->num_fds;
 	if (fda->num_fds >= SIZE_MAX / sizeof(u32)) {
 		binder_user_error("%d:%d got transaction with invalid number of fds (%lld)\n",
-- 
2.35.1