[PATCH v2] crypto/caam: Avoid GCC constprop bug warning

[Kees Cook <keescook@xxxxxxxxxxxx>]

GCC 12 appears to perform constant propagation incompletely(?) and can
no longer notice that "len" is always 0 when "data" is NULL. Expand the
check to avoid warnings about memcpy() having a NULL argument:

   ...
                    from drivers/crypto/caam/key_gen.c:8:
   drivers/crypto/caam/desc_constr.h: In function 'append_data.constprop':
   include/linux/fortify-string.h:48:33: warning: argument 2 null where non-null expected [-Wnonnull]
      48 | #define __underlying_memcpy     __builtin_memcpy
         |                                 ^
   include/linux/fortify-string.h:438:9: note: in expansion of macro '__underlying_memcpy'
     438 |         __underlying_##op(p, q, __fortify_size);                        \
         |         ^~~~~~~~~~~~~

The NULL was being propagated from:

        append_fifo_load_as_imm(desc, NULL, 0, LDST_CLASS_2_CCB |
                                FIFOLD_TYPE_MSG | FIFOLD_TYPE_LAST2);
...
static inline void append_##cmd##_as_imm(u32 * const desc, const void *data, \
                                         unsigned int len, u32 options) \
{ \
        PRINT_POS; \
        append_cmd_data(desc, data, len, CMD_##op | options); \
}
...
APPEND_CMD_PTR_TO_IMM(fifo_load, FIFO_LOAD);
...
static inline void append_cmd_data(u32 * const desc, const void *data, int len,
                                   u32 command)
{
        append_cmd(desc, command | IMMEDIATE | len);
        append_data(desc, data, len);
}

Cc: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
Cc: "Horia Geantă" <horia.geanta@xxxxxxx>
Cc: Pankaj Gupta <pankaj.gupta@xxxxxxx>
Cc: Gaurav Jain <gaurav.jain@xxxxxxx>
Cc: "David S. Miller" <davem@xxxxxxxxxxxxx>
Cc: linux-crypto@xxxxxxxxxxxxxxx
Reported-by: kernel test robot <lkp@xxxxxxxxx>
Link: https://lore.kernel.org/lkml/202210290446.qBayTfzl-lkp@xxxxxxxxx
Tested-by: Anders Roxell <anders.roxell@xxxxxxxxxx>
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
v1: https://lore.kernel.org/lkml/20221028210527.never.934-kees@xxxxxxxxxx/
v2: update comment (anders)
Note that this is about GCC, not sparse (any more).
---
 drivers/crypto/caam/desc_constr.h | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/crypto/caam/desc_constr.h b/drivers/crypto/caam/desc_constr.h
index 62ce6421bb3f..d9da4173af9d 100644
--- a/drivers/crypto/caam/desc_constr.h
+++ b/drivers/crypto/caam/desc_constr.h
@@ -163,7 +163,8 @@ static inline void append_data(u32 * const desc, const void *data, int len)
 {
 	u32 *offset = desc_end(desc);
 
-	if (len) /* avoid sparse warning: memcpy with byte count of 0 */
+	/* Avoid GCC warning: memcpy with NULL dest (but byte count of 0). */
+	if (data && len)
 		memcpy(offset, data, len);
 
 	(*desc) = cpu_to_caam32(caam32_to_cpu(*desc) +
-- 
2.34.1