sunrpc: Suppress GCC -Os warning on user2netname

GCC with -Os warns that sprint might overflow:

  netname.c: In function ‘user2netname’:
  netname.c:51:28: error: ‘%s’ directive writing up to 255 bytes into a
  region of size between 239 and 249 [-Werror=format-overflow=]
     51 |   sprintf (netname, "%s.%d@%s", OPSYS, uid, dfltdom);
        |                            ^~               ~~~~~~~
  netname.c:51:3: note: ‘sprintf’ output between 8 and 273 bytes into a
  destination of size 256
     51 |   sprintf (netname, "%s.%d@%s", OPSYS, uid, dfltdom);
        |   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  cc1: all warnings being treated as errors

However the code does test prior the sprintf call that dfltdom plus
the required extra space for OPSYS, uid, and extra character will not
overflow and return 0 instead.

Checked on x86_64-linux-gnu and i686-linux-gnu.
Reviewed-by: Carlos O'Donell <carlos@redhat.com>
Tested-by: Carlos O'Donell <carlos@redhat.com>

1 files changed, 6 insertions, 0 deletions

diff --git a/sunrpc/netname.c b/sunrpc/netname.c
index bf7f0b8..c1d1c43 100644
--- a/sunrpc/netname.c
+++ b/sunrpc/netname.c
@@ -20,6 +20,7 @@
 #include <string.h>
 #include <rpc/rpc.h>
 #include <shlib-compat.h>
+#include <libc-diag.h>
 
 #include "nsswitch.h"
 
@@ -48,7 +49,12 @@ user2netname (char netname[MAXNETNAMELEN + 1], const uid_t uid,
   if ((strlen (dfltdom) + OPSYS_LEN + 3 + MAXIPRINT) > (size_t) MAXNETNAMELEN)
     return 0;
 
+  /* GCC with -Os warns that sprint might overflow while handling dfltdom,
+     however the above test does check if an overflow would happen.  */
+  DIAG_PUSH_NEEDS_COMMENT;
+  DIAG_IGNORE_Os_NEEDS_COMMENT (8, "-Wformat-overflow");
   sprintf (netname, "%s.%d@%s", OPSYS, uid, dfltdom);
+  DIAG_POP_NEEDS_COMMENT;
   i = strlen (netname);
   if (netname[i - 1] == '.')
     netname[i - 1] = '\0';