wlcore: fix overlapping snprintf arguments in debugfs

gcc complains about undefined behavior in calling snprintf()
with the same buffer as input and output:

drivers/net/wireless/ti/wl18xx/debugfs.c: In function 'diversity_num_of_packets_per_ant_read':
drivers/net/wireless/ti/wl18xx/../wlcore/debugfs.h:86:3: error: 'snprintf' argument 4 overlaps destination object 'buf' [-Werror=restrict]
   86 |   snprintf(buf, sizeof(buf), "%s[%d] = %d\n",  \
      |   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   87 |     buf, i, stats->sub.name[i]);   \
      |     ~~~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/net/wireless/ti/wl18xx/debugfs.c:24:2: note: in expansion of macro 'DEBUGFS_FWSTATS_FILE_ARRAY'
   24 |  DEBUGFS_FWSTATS_FILE_ARRAY(a, b, c, wl18xx_acx_statistics)
      |  ^~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/net/wireless/ti/wl18xx/debugfs.c:159:1: note: in expansion of macro 'WL18XX_DEBUGFS_FWSTATS_FILE_ARRAY'
  159 | WL18XX_DEBUGFS_FWSTATS_FILE_ARRAY(diversity, num_of_packets_per_ant,

There are probably other ways of handling the debugfs file, without
using on-stack buffers, but a simple workaround here is to remember the
current position in the buffer and just keep printing in there.

Fixes: bcca1bb ("wlcore: add debugfs macro to help print fw statistics arrays")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20210323125723.1961432-1-arnd@kernel.org

drivers/net/wireless/ti/wlcore/boot.c

@@ -72,6 +72,7 @@ static int wlcore_validate_fw_ver(struct wl1271 *wl)
 	unsigned int *min_ver = (wl->fw_type == WL12XX_FW_TYPE_MULTI) ?
 		wl->min_mr_fw_ver : wl->min_sr_fw_ver;
 	char min_fw_str[32] = "";
+	int off = 0;
 	int i;
 
 	/* the chip must be exactly equal */
@@ -105,13 +106,15 @@ static int wlcore_validate_fw_ver(struct wl1271 *wl)
 	return 0;
 
 fail:
-	for (i = 0; i < NUM_FW_VER; i++)
+	for (i = 0; i < NUM_FW_VER && off < sizeof(min_fw_str); i++)
 		if (min_ver[i] == WLCORE_FW_VER_IGNORE)
-			snprintf(min_fw_str, sizeof(min_fw_str),
-				  "%s*.", min_fw_str);
+			off += snprintf(min_fw_str + off,
+					sizeof(min_fw_str) - off,
+					"*.");
 		else
-			snprintf(min_fw_str, sizeof(min_fw_str),
-				  "%s%u.", min_fw_str, min_ver[i]);
+			off += snprintf(min_fw_str + off,
+					sizeof(min_fw_str) - off,
+					"%u.", min_ver[i]);
 
 	wl1271_error("Your WiFi FW version (%u.%u.%u.%u.%u) is invalid.\n"
 		     "Please use at least FW %s\n"

drivers/net/wireless/ti/wlcore/debugfs.h

@@ -78,13 +78,14 @@ static ssize_t sub## _ ##name## _read(struct file *file,		\
 	struct wl1271 *wl = file->private_data;				\
 	struct struct_type *stats = wl->stats.fw_stats;			\
 	char buf[DEBUGFS_FORMAT_BUFFER_SIZE] = "";			\
+	int pos = 0;							\
 	int i;								\
 									\
 	wl1271_debugfs_update_stats(wl);				\
 									\
-	for (i = 0; i < len; i++)					\
-		snprintf(buf, sizeof(buf), "%s[%d] = %d\n",		\
-			 buf, i, stats->sub.name[i]);			\
+	for (i = 0; i < len && pos < sizeof(buf); i++)			\
+		pos += snprintf(buf + pos, sizeof(buf),			\
+			 "[%d] = %d\n", i, stats->sub.name[i]);		\
 									\
 	return wl1271_format_buffer(userbuf, count, ppos, "%s", buf);	\
 }									\