checkpatch: add check for sscanf without return use

Naked use sscanf can be troublesome because the pointed to variables may
not have been set.

Add a warning when the sscanf return value is not used.

For now, do not add __must_check to the sscanf prototype because that will
cause a couple of hundred new warnings when compiling a kernel.
Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

scripts/checkpatch.pl

@@ -4019,6 +4019,23 @@ sub process {
 			}
 		}
 
+# check for naked sscanf
+		if ($^V && $^V ge 5.10.0 &&
+		    defined $stat &&
+		    $stat =~ /\bsscanf\b/ &&
+		    ($stat !~ /$Ident\s*=\s*sscanf\s*$balanced_parens/ &&
+		     $stat !~ /\bsscanf\s*$balanced_parens\s*(?:$Compare)/ &&
+		     $stat !~ /(?:$Compare)\s*\bsscanf\s*$balanced_parens/)) {
+			my $lc = $stat =~ tr@\n@@;
+			$lc = $lc + $linenr;
+			my $stat_real = raw_line($linenr, 0);
+		        for (my $count = $linenr + 1; $count <= $lc; $count++) {
+				$stat_real = $stat_real . "\n" . raw_line($count, 0);
+			}
+			WARN("NAKED_SSCANF",
+			     "unchecked sscanf return value\n" . "$here\n$stat_real\n");
+		}
+
 # check for new externs in .h files.
 		if ($realfile =~ /\.h$/ &&
 		    $line =~ /^\+\s*(extern\s+)$Type\s*$Ident\s*\(/s) {