wifi: ath12k: fix build vs old compiler

gcc 11.4.1-3 warns about memcpy() with overlapping pointers:

drivers/net/wireless/ath/ath12k/wow.c: In function ‘ath12k_wow_convert_8023_to_80211.constprop’:
./include/linux/fortify-string.h:114:33: error: ‘__builtin_memcpy’ accessing 18446744073709551611 or more bytes at offsets 0 and 0 overlaps 9223372036854775799 bytes at offset -9223372036854775804 [-Werror=restrict]
  114 | #define __underlying_memcpy     __builtin_memcpy
      |                                 ^
./include/linux/fortify-string.h:637:9: note: in expansion of macro ‘__underlying_memcpy’
  637 |         __underlying_##op(p, q, __fortify_size);                        \
      |         ^~~~~~~~~~~~~
./include/linux/fortify-string.h:682:26: note: in expansion of macro ‘__fortify_memcpy_chk’
  682 | #define memcpy(p, q, s)  __fortify_memcpy_chk(p, q, s,                  \
      |                          ^~~~~~~~~~~~~~~~~~~~
drivers/net/wireless/ath/ath12k/wow.c:190:25: note: in expansion of macro ‘memcpy’
  190 |                         memcpy(pat, eth_pat, eth_pat_len);
      |                         ^~~~~~
./include/linux/fortify-string.h:114:33: error: ‘__builtin_memcpy’ accessing 18446744073709551605 or more bytes at offsets 0 and 0 overlaps 9223372036854775787 bytes at offset -9223372036854775798 [-Werror=restrict]
  114 | #define __underlying_memcpy     __builtin_memcpy
      |                                 ^
./include/linux/fortify-string.h:637:9: note: in expansion of macro ‘__underlying_memcpy’
  637 |         __underlying_##op(p, q, __fortify_size);                        \
      |         ^~~~~~~~~~~~~
./include/linux/fortify-string.h:682:26: note: in expansion of macro ‘__fortify_memcpy_chk’
  682 | #define memcpy(p, q, s)  __fortify_memcpy_chk(p, q, s,                  \
      |                          ^~~~~~~~~~~~~~~~~~~~
drivers/net/wireless/ath/ath12k/wow.c:232:25: note: in expansion of macro ‘memcpy’
  232 |                         memcpy(pat, eth_pat, eth_pat_len);
      |                         ^~~~~~

The sum of size_t operands can overflow SIZE_MAX, triggering the
warning.
Address the issue using the suitable helper.

Fixes: 4a3c212 ("wifi: ath12k: add basic WoW functionalities")
Signed-off-by: Paolo Abeni <[email protected]>
Reviewed-by: Kees Cook <[email protected]>
Acked-by: Jeff Johnson <[email protected]>
Link: https://patch.msgid.link/3175f87d7227e395b330fd88fb840c1645084ea7.1721127979.git.pabeni@redhat.com
Signed-off-by: Jakub Kicinski <[email protected]>

Author: Paolo Abeni

drivers/net/wireless/ath/ath12k/wow.c

@@ -186,7 +186,7 @@ ath12k_wow_convert_8023_to_80211(struct ath12k *ar,
 	if (eth_pkt_ofs < ETH_ALEN) {
 		pkt_ofs = eth_pkt_ofs + a1_ofs;
 
-		if (eth_pkt_ofs + eth_pat_len < ETH_ALEN) {
+		if (size_add(eth_pkt_ofs, eth_pat_len) < ETH_ALEN) {
 			memcpy(pat, eth_pat, eth_pat_len);
 			memcpy(bytemask, eth_bytemask, eth_pat_len);
 
@@ -228,7 +228,7 @@ ath12k_wow_convert_8023_to_80211(struct ath12k *ar,
 	} else if (eth_pkt_ofs < prot_ofs) {
 		pkt_ofs = eth_pkt_ofs - ETH_ALEN + a3_ofs;
 
-		if (eth_pkt_ofs + eth_pat_len < prot_ofs) {
+		if (size_add(eth_pkt_ofs, eth_pat_len) < prot_ofs) {
 			memcpy(pat, eth_pat, eth_pat_len);
 			memcpy(bytemask, eth_bytemask, eth_pat_len);