[PATCH v3] checkpatch: warn for non-standard fixes tag style

["Niklas Söderlund" <niklas.soderlund@corigine.com>]

Add a warning for fixes tags that does not fall in line with the
standards specified by the community.

Signed-off-by: Niklas Söderlund <niklas.soderlund@corigine.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Reviewed-by: Louis Peens <louis.peens@corigine.com>
---
* Changes since v2
- Change the pattern to match on 'fixes:?' to catch more malformed
  fixes tags.

* Changes since v1
- Update the documentation wording and add mention one cause of the
  message can be that email program splits the tag over multiple lines.
---
 Documentation/dev-tools/checkpatch.rst |  8 +++++
 scripts/checkpatch.pl                  | 41 ++++++++++++++++++++++++++
 2 files changed, 49 insertions(+)

diff --git a/Documentation/dev-tools/checkpatch.rst b/Documentation/dev-tools/checkpatch.rst
index b52452bc2963..8c8456a3bd18 100644
--- a/Documentation/dev-tools/checkpatch.rst
+++ b/Documentation/dev-tools/checkpatch.rst
@@ -612,6 +612,14 @@ Commit message
 
     See: https://www.kernel.org/doc/html/latest/process/submitting-patches.html#describe-your-changes
 
+  **BAD_FIXES_TAG**
+    The Fixes: tag is malformed or does not fall in line with the standards
+    specified by the community. This can occur if the tag have been split into
+    multiple lines (e.g., when pasted in email program with word wrapping
+    enabled).
+
+    See: https://www.kernel.org/doc/html/latest/process/submitting-patches.html#describe-your-changes
+
 
 Comparison style
 ----------------
diff --git a/scripts/checkpatch.pl b/scripts/checkpatch.pl
index 79e759aac543..a250bda52c66 100755
--- a/scripts/checkpatch.pl
+++ b/scripts/checkpatch.pl
@@ -3140,6 +3140,47 @@ sub process {
 			}
 		}
 
+# Check Fixes: styles is correct
+		if (!$in_header_lines && $line =~ /^fixes:?/i) {
+			my $orig_commit = "";
+			my $id = "0123456789ab";
+			my $title = "commit title";
+			my $tag_case = 1;
+			my $tag_space = 1;
+			my $id_length = 1;
+			my $id_case = 1;
+			my $title_has_quotes = 0;
+
+			if ($line =~ /(fixes:?)\s+([0-9a-f]{5,})\s+($balanced_parens)/i) {
+				my $tag = $1;
+				$orig_commit = $2;
+				$title = $3;
+
+				$tag_case = 0 if $tag eq "Fixes:";
+				$tag_space = 0 if ($line =~ /^fixes:? [0-9a-f]{5,} ($balanced_parens)/i);
+
+				$id_length = 0 if ($orig_commit =~ /^[0-9a-f]{12}$/i);
+				$id_case = 0 if ($orig_commit !~ /[A-F]/);
+
+				# Always strip leading/trailing parens then double quotes if existing
+				$title = substr($title, 1, -1);
+				if ($title =~ /^".*"$/) {
+					$title = substr($title, 1, -1);
+					$title_has_quotes = 1;
+				}
+			}
+
+			($id, $title) = git_commit_info($orig_commit, $id,
+							$title);
+
+			if ($tag_case || $tag_space || $id_length || $id_case ||
+			    !$title_has_quotes) {
+				WARN("BAD_FIXES_TAG",
+				     "Please use correct Fixes: style 'Fixes: <12 chars of sha1> (\"<title line>\")' - ie: 'Fixes: $id (\"$title\")'\n" . $herecurr);
+
+			}
+		}
+
 # Check email subject for common tools that don't need to be mentioned
 		if ($in_header_lines &&
 		    $line =~ /^Subject:.*\b(?:checkpatch|sparse|smatch)\b[^:]/i) {
-- 
2.37.3