[PATCH] landlock: avoid memcpy static check warning

[Arnd Bergmann <arnd@kernel.org>]

From: Arnd Bergmann <arnd@arndb.de>

The fortified string helpers trigger a -Wrestrict warning when
gcc deducts that the size of the landlock_layer array can
overflow as a result of the flex_array_size() calculation:

In file included from arch/x86/include/asm/string.h:6,
                 from security/landlock/ruleset.c:16:
security/landlock/ruleset.c: In function 'create_rule':
arch/x86/include/asm/string_32.h:150:25: error: '__builtin_memcpy' accessing 4294967295 bytes at offsets 0 and 0 overlaps 6442450943 bytes at offset -2147483648 [-Werror=restrict]
  150 | #define memcpy(t, f, n) __builtin_memcpy(t, f, n)
      |                         ^~~~~~~~~~~~~~~~~~~~~~~~~
security/landlock/ruleset.c:139:9: note: in expansion of macro 'memcpy'
  139 |         memcpy(new_rule->layers, layers,
      |         ^~~~~~
  'create_rule': event 1
include/linux/compiler.h:69:46:
   68 |         (cond) ?                                        \
      |         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   69 |                 (__if_trace.miss_hit[1]++,1) :          \
      |                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~
      |                                              |
      |                                              (1) when the condition is evaluated to true
   70 |                 (__if_trace.miss_hit[0]++,0);           \
      |                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
include/linux/compiler.h:57:69: note: in expansion of macro '__trace_if_value'
   57 | #define __trace_if_var(cond) (__builtin_constant_p(cond) ? (cond) : __trace_if_value(cond))
      |                                                                     ^~~~~~~~~~~~~~~~
include/linux/compiler.h:55:28: note: in expansion of macro '__trace_if_var'
   55 | #define if(cond, ...) if ( __trace_if_var( !!(cond , ## __VA_ARGS__) ) )
      |                            ^~~~~~~~~~~~~~
include/linux/overflow.h:334:9: note: in expansion of macro 'if'
  334 |         if (check_mul_overflow(factor1, factor2, &bytes))
      |         ^~
  'create_rule': event 2

Out of these individually helpful checks (-Wrestrict, fortified
string helpers, flex_array_size), one of them has to go to avoid
the warning.

Seeing that the length of the array is already checked earlier
in this function, through both an explicit LANDLOCK_MAX_NUM_LAYERS
comparison and the implicit kzalloc_flex() having succeeded,
replace the flex_array_size() call with a direct multiplication.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
---
 security/landlock/ruleset.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/security/landlock/ruleset.c b/security/landlock/ruleset.c
index 181df7736bb9..26e0b7193a7b 100644
--- a/security/landlock/ruleset.c
+++ b/security/landlock/ruleset.c
@@ -137,7 +137,7 @@ create_rule(const struct landlock_id id,
 	new_rule->num_layers = new_num_layers;
 	/* Copies the original layer stack. */
 	memcpy(new_rule->layers, layers,
-	       flex_array_size(new_rule, layers, num_layers));
+	       sizeof(struct landlock_layer) * num_layers);
 	if (new_layer)
 		/* Adds a copy of @new_layer on the layer stack. */
 		new_rule->layers[new_rule->num_layers - 1] = *new_layer;
-- 
2.39.5