checkpatch: warn for non-standard fixes tag style

Add a warning for fixes tags that does not follow community conventions.

Link: https://lkml.kernel.org/r/20220914100255.1048460-1-niklas.soderlund@corigine.com
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>
Reviewed-by: Philippe Schenker <philippe.schenker@toradex.com>
Acked-by: Dwaipayan Ray <dwaipayanray1@gmail.com>
Reviewed-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Acked-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Acked-by: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Documentation/dev-tools/checkpatch.rst

@@ -612,6 +612,13 @@ 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 follow the community conventions.
+    This can occur if the tag have been split into multiple lines (e.g., when
+    pasted in an email program with word wrapping enabled).
+
+    See: https://www.kernel.org/doc/html/latest/process/submitting-patches.html#describe-your-changes
+
 
 Comparison style
 ----------------

scripts/checkpatch.pl

@@ -3146,6 +3146,50 @@ sub process {
 			}
 		}
 
+# Check Fixes: styles is correct
+		if (!$in_header_lines &&
+		    $line =~ /^\s*fixes:?\s*(?:commit\s*)?[0-9a-f]{5,}\b/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 =~ /(\s*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);
+					$title_has_quotes = 1;
+				}
+			}
+
+			my ($cid, $ctitle) = git_commit_info($orig_commit, $id,
+							     $title);
+
+			if ($ctitle ne $title || $tag_case || $tag_space ||
+			    $id_length || $id_case || !$title_has_quotes) {
+				if (WARN("BAD_FIXES_TAG",
+				     "Please use correct Fixes: style 'Fixes: <12 chars of sha1> (\"<title line>\")' - ie: 'Fixes: $cid (\"$ctitle\")'\n" . $herecurr) &&
+				    $fix) {
+					$fixed[$fixlinenr] = "Fixes: $cid (\"$ctitle\")";
+				}
+			}
+		}
+
 # 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) {