[PATCH v2] tile: avoid errors from truncating long strings in mpipe gxio

[Chris Metcalf]

Using strncpy() will just silently truncate long strings; we should
instead return an appropriate error.  Using strlcpy() would suffer from
the same problem.  Instead, use strnlen()+memcpy(), and add an
error-checking step to make sure the lengths are reasonable.

I called the convenience wrapper strscpy(), and a case could be made for
making it more generic (possibly with a better name), but that seems
outside the scope of this initial commit.

Signed-off-by: Chris Metcalf <cmetcalf@xxxxxxxxxx>
---
v2: use strnlen instead of strlen

 arch/tile/gxio/mpipe.c | 30 +++++++++++++++++++++++++-----
 1 file changed, 25 insertions(+), 5 deletions(-)

diff --git a/arch/tile/gxio/mpipe.c b/arch/tile/gxio/mpipe.c
index 5301a9ffbae1..27a56be8d583 100644
--- a/arch/tile/gxio/mpipe.c
+++ b/arch/tile/gxio/mpipe.c
@@ -29,6 +29,25 @@
 /* HACK: Avoid pointless "shadow" warnings. */
 #define link link_shadow
 
+/*
+ * Use this routine to avoid copying too-long strings.  Unlike strncpy
+ * or strlcpy, we don't enable programmers who don't check return codes;
+ * partially-copied strings can be problematic.  The routine returns
+ * the total number of bytes copied (including the trailing NUL) or
+ * zero if the buffer wasn't big enough.
+ */
+static size_t strscpy(char *dest, const char *src, size_t size)
+{
+	size_t ret = strnlen(src, size) + 1;
+	if (ret > size) {
+		if (size)
+			dest[0] = '\0';
+		return 0;
+	}
+	memcpy(dest, src, ret);
+	return ret;
+}
+
 int gxio_mpipe_init(gxio_mpipe_context_t *context, unsigned int mpipe_index)
 {
 	char file[32];
@@ -511,8 +530,8 @@ int gxio_mpipe_link_instance(const char *link_name)
 	if (!context)
 		return GXIO_ERR_NO_DEVICE;
 
-	strncpy(name.name, link_name, sizeof(name.name));
-	name.name[GXIO_MPIPE_LINK_NAME_LEN - 1] = '\0';
+	if (strscpy(name.name, link_name, sizeof(name.name)) == 0)
+		return GXIO_ERR_NO_DEVICE;
 
 	return gxio_mpipe_info_instance_aux(context, name);
 }
@@ -529,7 +548,8 @@ int gxio_mpipe_link_enumerate_mac(int idx, char *link_name, uint8_t *link_mac)
 
 	rv = gxio_mpipe_info_enumerate_aux(context, idx, &name, &mac);
 	if (rv >= 0) {
-		strncpy(link_name, name.name, sizeof(name.name));
+		if (strscpy(link_name, name.name, sizeof(name.name)) == 0)
+			return GXIO_ERR_INVAL_MEMORY_SIZE;
 		memcpy(link_mac, mac.mac, sizeof(mac.mac));
 	}
 
@@ -545,8 +565,8 @@ int gxio_mpipe_link_open(gxio_mpipe_link_t *link,
 	_gxio_mpipe_link_name_t name;
 	int rv;
 
-	strncpy(name.name, link_name, sizeof(name.name));
-	name.name[GXIO_MPIPE_LINK_NAME_LEN - 1] = '\0';
+	if (strscpy(name.name, link_name, sizeof(name.name)) == 0)
+		return GXIO_ERR_NO_DEVICE;
 
 	rv = gxio_mpipe_link_open_aux(context, name, flags);
 	if (rv < 0)
-- 
1.8.3.1

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