checkpatch: warn on self-assignments

The uninitialized_var() macro was removed recently via commit 63a0895d96
("compiler: Remove uninitialized_var() macro") as it's not a particularly
useful warning and its use can "paper over real bugs".

Add a checkpatch test to warn on self-assignments as a means to avoid
compiler warnings and as a back-door mechanism to reproduce the old
uninitialized_var macro behavior.

[akpm@linux-foundation.org: coding style fixes]

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Kees Cook <keescook@chromium.org>
Cc: Gustavo A. R. Silva <gustavoars@kernel.org>
Cc: Denis Efremov <efremov@linux.com>
Cc: Julia Lawall <julia.lawall@inria.fr>
Link: https://lkml.kernel.org/r/afc2cffdd315d3e4394af149278df9e8af7f49f4.camel@perches.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

scripts/checkpatch.pl

@@ -3899,6 +3899,17 @@ sub process {
 #ignore lines not being added
 		next if ($line =~ /^[^\+]/);
 
+# check for self assignments used to avoid compiler warnings
+# e.g.:	int foo = foo, *bar = NULL;
+#	struct foo bar = *(&(bar));
+		if ($line =~ /^\+\s*(?:$Declare)?([A-Za-z_][A-Za-z\d_]*)\s*=/) {
+			my $var = $1;
+			if ($line =~ /^\+\s*(?:$Declare)?$var\s*=\s*(?:$var|\*\s*\(?\s*&\s*\(?\s*$var\s*\)?\s*\)?)\s*[;,]/) {
+				WARN("SELF_ASSIGNMENT",
+				     "Do not use self-assignments to avoid compiler warnings\n" . $herecurr);
+			}
+		}
+
 # check for dereferences that span multiple lines
 		if ($prevline =~ /^\+.*$Lval\s*(?:\.|->)\s*$/ &&
 		    $line =~ /^\+\s*(?!\#\s*(?!define\s+|if))\s*$Lval/) {