checkpatch: try to avoid mask and shift errors

Shift has a higher precedence that mask so warn when a mask then shift
operation is done without parentheses around the mask.

This test works well for a right shift, but the left shift is pretty
commonly done correctly so only warn on the right shift.

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>

1 files changed, 8 insertions, 0 deletions

diff --git a/scripts/checkpatch.pl b/scripts/checkpatch.pl
index 853dc7f9f751..24d6702a95c2 100755
--- a/scripts/checkpatch.pl
+++ b/scripts/checkpatch.pl
@@ -4482,6 +4482,14 @@ sub process {
                         }
                 }
 
+# check for mask then right shift without a parentheses
+                if ($^V && $^V ge 5.10.0 &&
+                    $line =~ /$LvalOrFunc\s*\&\s*($LvalOrFunc)\s*>>/ &&
+                    $4 !~ /^\&/) { # $LvalOrFunc may be &foo, ignore if so
+                        WARN("MASK_THEN_SHIFT",
+                             "Possible precedence defect with mask then right shift - may need parentheses\n" . $herecurr);
+                }
+
 # check for bad placement of section $InitAttribute (e.g.: __initdata)
                 if ($line =~ /(\b$InitAttribute\b)/) {
                         my $attr = $1;