linux-kernel - [PATCH 1/3] checkpatch: ctx_statement_block: Fix preprocessor guard tracking

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Message-Id: <20210326035035.3057800-2-jwerner@chromium.org>
Date:   Thu, 25 Mar 2021 20:50:33 -0700
From:   Julius Werner <jwerner@...omium.org>
To:     Andy Whitcroft <apw@...onical.com>, Joe Perches <joe@...ches.com>
Cc:     Ivo Sieben <meltedpianoman@...il.com>,
        linux-kernel@...r.kernel.org, Julius Werner <jwerner@...omium.org>
Subject: [PATCH 1/3] checkpatch: ctx_statement_block: Fix preprocessor guard tracking

The preprocessor guard tracking in ctx_statement_block() is (and seems
to have always been) subtly broken whenever tracking over an #else: the
code is supposed to restore state from the current top of the stack
(like and #endif just without removing it). However, it indexes the
stack at [$#stack - 1]. In Perl, $# does not give you the length of an
array, it gives you the highest valid index. Therefore, the correct
index should just be [$#stack].

The preprocessor guard tracking also gets confused when
ctx_statement_block() was called on a line that is already inside a
preprocessor guard, and steps out of it within the same statement. This
happens commonly with constructs like this:

 #if CONFIG_XXX
   for (a = first_a(); !a_finished(); a = next_a()) {
 #else
   for (b = first_b(); !b_finished(); b = next_b()) {
 #endif
     ... loop body ...

The best course of action in this case is to not try to restore any
previous state (which we don't have) at all, so we should just keep our
current state if $#stack is already 0.

Signed-off-by: Julius Werner <jwerner@...omium.org>
---
 scripts/checkpatch.pl | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/scripts/checkpatch.pl b/scripts/checkpatch.pl
index df8b23dc1eb0af..ffccbd2033e579 100755
--- a/scripts/checkpatch.pl
+++ b/scripts/checkpatch.pl
@@ -1572,9 +1572,9 @@ sub ctx_statement_block {
 		# Handle nested #if/#else.
 		if ($remainder =~ /^#\s*(?:ifndef|ifdef|if)\s/) {
 			push(@stack, [ $type, $level ]);
-		} elsif ($remainder =~ /^#\s*(?:else|elif)\b/) {
-			($type, $level) = @{$stack[$#stack - 1]};
-		} elsif ($remainder =~ /^#\s*endif\b/) {
+		} elsif ($remainder =~ /^#\s*(?:else|elif)\b/ && $#stack > 0) {
+			($type, $level) = @{$stack[$#stack]};
+		} elsif ($remainder =~ /^#\s*endif\b/ && $#stack > 0) {
 			($type, $level) = @{pop(@stack)};
 		}

-- 
2.29.2