lib, stackdepot: check stackdepot handle before accessing slabs

Patch series "lib, stackdepot: check stackdepot handle before accessing slabs", v2.

PATCH-1: Checks validity of a stackdepot handle before proceeding to
access stackdepot slab/objects.

PATCH-2: Adds a helper in stackdepot, to allow users to print stack
entries just by specifying the stackdepot handle.  It also changes such
users to use this new interface.

PATCH-3: Adds a helper in stackdepot, to allow users to print stack
entries into buffers just by specifying the stackdepot handle and
destination buffer.  It also changes such users to use this new interface.

This patch (of 3):

stack_depot_save allocates slabs that will be used for storing objects in
future.If this slab allocation fails we may get to a situation where space
allocation for a new stack_record fails, causing stack_depot_save to
return 0 as handle.  If user of this handle ends up invoking
stack_depot_fetch with this handle value, current implementation of
stack_depot_fetch will end up using slab from wrong index.  To avoid this
check handle value at the beginning.

Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Imran Khan <[email protected]>
Suggested-by: Vlastimil Babka <[email protected]>
Acked-by: Vlastimil Babka <[email protected]>
Cc: Geert Uytterhoeven <[email protected]>
Cc: Andrey Ryabinin <[email protected]>
Cc: Alexander Potapenko <[email protected]>
Cc: Andrey Konovalov <[email protected]>
Cc: Dmitry Vyukov <[email protected]>
Cc: Maarten Lankhorst <[email protected]>
Cc: Maxime Ripard <[email protected]>
Cc: Thomas Zimmermann <[email protected]>
Cc: David Airlie <[email protected]>
Cc: Daniel Vetter <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

lib/stackdepot.c

@@ -231,6 +231,9 @@ unsigned int stack_depot_fetch(depot_stack_handle_t handle,
 	struct stack_record *stack;
 
 	*entries = NULL;
+	if (!handle)
+		return 0;
+
 	if (parts.slabindex > depot_index) {
 		WARN(1, "slab index %d out of bounds (%d) for stack id %08x\n",
 			parts.slabindex, depot_index, handle);