pid: Replace struct pid 1-element array with flex-array

For pid namespaces, struct pid uses a dynamically sized array member,
"numbers".  This was implemented using the ancient 1-element fake
flexible array, which has been deprecated for decades.

Replace it with a C99 flexible array, refactor the array size
calculations to use struct_size(), and address elements via indexes.
Note that the static initializer (which defines a single element) works
as-is, and requires no special handling.

Without this, CONFIG_UBSAN_BOUNDS (and potentially
CONFIG_FORTIFY_SOURCE) will trigger bounds checks:

  https://lore.kernel.org/lkml/20230517-bushaltestelle-super-e223978c1ba6@brauner

Cc: Christian Brauner <[email protected]>
Cc: Jan Kara <[email protected]>
Cc: Jeff Xu <[email protected]>
Cc: Andreas Gruenbacher <[email protected]>
Cc: Daniel Verkamp <[email protected]>
Cc: "Paul E. McKenney" <[email protected]>
Cc: Jeff Xu <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Boqun Feng <[email protected]>
Cc: Luis Chamberlain <[email protected]>
Cc: Frederic Weisbecker <[email protected]>
Reported-by: [email protected]
[brauner: dropped unrelated changes and remove 0 with NULL cast]
Signed-off-by: Kees Cook <[email protected]>
Signed-off-by: Christian Brauner <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

1 files changed, 1 insertions, 1 deletions

diff --git a/include/linux/pid.h b/include/linux/pid.h
index b75de288a8c2..653a527574c4 100644
--- a/include/linux/pid.h
+++ b/include/linux/pid.h
@@ -67,7 +67,7 @@ struct pid
 	/* wait queue for pidfd notifications */
 	wait_queue_head_t wait_pidfd;
 	struct rcu_head rcu;
-	struct upid numbers[1];
+	struct upid numbers[];
 };
 
 extern struct pid init_struct_pid;