[PATCH rpcd] sys: mitigate possible strncpy string truncation

[Petr Štetiar]

gcc 10 with -O2 reports following:

 In function ‘strncpy’,
     inlined from ‘rpc_sys_packagelist’ at /opt/devel/openwrt/c-projects/rpcd/sys.c:244:4:
 /usr/include/x86_64-linux-gnu/bits/string_fortified.h:106:10: error: ‘__builtin_strncpy’ specified bound 128 equals destination size [-Werror=stringop-truncation]
   106 |   return __builtin___strncpy_chk (__dest, __src, __len, __bos (__dest));
       |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 In function ‘strncpy’,
     inlined from ‘rpc_sys_packagelist’ at /opt/devel/openwrt/c-projects/rpcd/sys.c:227:4:
 /usr/include/x86_64-linux-gnu/bits/string_fortified.h:106:10: error: ‘__builtin_strncpy’ specified bound 128 equals destination size [-Werror=stringop-truncation]
   106 |   return __builtin___strncpy_chk (__dest, __src, __len, __bos (__dest));
       |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Since it is not possible to avoid truncation by strncpy, it is necessary
to make sure the result of strncpy is properly NUL-terminated and the
NUL must be inserted explicitly, after strncpy has returned.

References: #10442
Reported-by: @alexeys85
Signed-off-by: Petr Štetiar <ynezz at true.cz>
---
 sys.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/sys.c b/sys.c
index 097e7057f7da..45b15746d93f 100644
--- a/sys.c
+++ b/sys.c
@@ -224,7 +224,8 @@ procstr:
 			continue;
 
 		if (!strcmp(var, "Package:")) {
-			strncpy(pkg, p1, sizeof(pkg));
+			strncpy(pkg, p1, sizeof(pkg) - 1);
+			pkg[sizeof(pkg) - 1] = '\0';
 			continue;
 		}
 
@@ -241,7 +242,8 @@ procstr:
 		}
 
 		if (!strcmp(var, "Version:")) {
-			strncpy(ver, p1, sizeof(ver));
+			strncpy(ver, p1, sizeof(ver) - 1);
+			ver[sizeof(ver) - 1] = '\0';
 			continue;
 		}