netdev - [PATCH net-next] pktgen: convert safe uses of strncpy() to strcpy() to avoid string truncation warning

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Date:   Tue, 17 Jul 2018 14:32:24 -0700
From:   Jakub Kicinski <jakub.kicinski@...ronome.com>
To:     davem@...emloft.net
Cc:     oss-drivers@...ronome.com, netdev@...r.kernel.org,
        Jakub Kicinski <jakub.kicinski@...ronome.com>
Subject: [PATCH net-next] pktgen: convert safe uses of strncpy() to strcpy() to avoid string truncation warning

GCC 8 complains:

net/core/pktgen.c: In function ‘pktgen_if_write’:
net/core/pktgen.c:1419:4: warning: ‘strncpy’ output may be truncated copying between 0 and 31 bytes from a string of length 127 [-Wstringop-truncation]
    strncpy(pkt_dev->src_max, buf, len);
    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
net/core/pktgen.c:1399:4: warning: ‘strncpy’ output may be truncated copying between 0 and 31 bytes from a string of length 127 [-Wstringop-truncation]
    strncpy(pkt_dev->src_min, buf, len);
    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
net/core/pktgen.c:1290:4: warning: ‘strncpy’ output may be truncated copying between 0 and 31 bytes from a string of length 127 [-Wstringop-truncation]
    strncpy(pkt_dev->dst_max, buf, len);
    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
net/core/pktgen.c:1268:4: warning: ‘strncpy’ output may be truncated copying between 0 and 31 bytes from a string of length 127 [-Wstringop-truncation]
    strncpy(pkt_dev->dst_min, buf, len);
    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

There is no bug here, but the code is not perfect either.  It copies
sizeof(pkt_dev->/member/) - 1 from user space into buf, and then does
a strcmp(pkt_dev->/member/, buf) hence assuming buf will be null-terminated
and shorter than pkt_dev->/member/ (pkt_dev->/member/ is never
explicitly null-terminated, and strncpy() doesn't have to null-terminate
so the assumption must be on buf).  The use of strncpy() without explicit
null-termination looks suspicious.  Convert to use straight strcpy().

strncpy() would also null-pad the output, but that's clearly unnecessary
since the author calls memset(pkt_dev->/member/, 0, sizeof(..)); prior
to strncpy(), anyway.

While at it format the code for "dst_min", "dst_max", "src_min" and
"src_max" in the same way by removing extra new lines in one case.

Signed-off-by: Jakub Kicinski <jakub.kicinski@...ronome.com>
Reviewed-by: Jiong Wang <jiong.wang@...ronome.com>
---
 net/core/pktgen.c | 10 ++++------
 1 file changed, 4 insertions(+), 6 deletions(-)

diff --git a/net/core/pktgen.c b/net/core/pktgen.c
index 49368e21d228..308ed04984de 100644
--- a/net/core/pktgen.c
+++ b/net/core/pktgen.c
@@ -1265,7 +1265,7 @@ static ssize_t pktgen_if_write(struct file *file,
 		buf[len] = 0;
 		if (strcmp(buf, pkt_dev->dst_min) != 0) {
 			memset(pkt_dev->dst_min, 0, sizeof(pkt_dev->dst_min));
-			strncpy(pkt_dev->dst_min, buf, len);
+			strcpy(pkt_dev->dst_min, buf);
 			pkt_dev->daddr_min = in_aton(pkt_dev->dst_min);
 			pkt_dev->cur_daddr = pkt_dev->daddr_min;
 		}
@@ -1280,14 +1280,12 @@ static ssize_t pktgen_if_write(struct file *file,
 		if (len < 0)
 			return len;
 
-
 		if (copy_from_user(buf, &user_buffer[i], len))
 			return -EFAULT;
-
 		buf[len] = 0;
 		if (strcmp(buf, pkt_dev->dst_max) != 0) {
 			memset(pkt_dev->dst_max, 0, sizeof(pkt_dev->dst_max));
-			strncpy(pkt_dev->dst_max, buf, len);
+			strcpy(pkt_dev->dst_max, buf);
 			pkt_dev->daddr_max = in_aton(pkt_dev->dst_max);
 			pkt_dev->cur_daddr = pkt_dev->daddr_max;
 		}
@@ -1396,7 +1394,7 @@ static ssize_t pktgen_if_write(struct file *file,
 		buf[len] = 0;
 		if (strcmp(buf, pkt_dev->src_min) != 0) {
 			memset(pkt_dev->src_min, 0, sizeof(pkt_dev->src_min));
-			strncpy(pkt_dev->src_min, buf, len);
+			strcpy(pkt_dev->src_min, buf);
 			pkt_dev->saddr_min = in_aton(pkt_dev->src_min);
 			pkt_dev->cur_saddr = pkt_dev->saddr_min;
 		}
@@ -1416,7 +1414,7 @@ static ssize_t pktgen_if_write(struct file *file,
 		buf[len] = 0;
 		if (strcmp(buf, pkt_dev->src_max) != 0) {
 			memset(pkt_dev->src_max, 0, sizeof(pkt_dev->src_max));
-			strncpy(pkt_dev->src_max, buf, len);
+			strcpy(pkt_dev->src_max, buf);
 			pkt_dev->saddr_max = in_aton(pkt_dev->src_max);
 			pkt_dev->cur_saddr = pkt_dev->saddr_max;
 		}
-- 
2.17.1