checkpatch: extend author Signed-off-by check for split From: header

Checkpatch did not handle cases where the author From: header was split into multiple lines. The author identity could not be resolved and checkpatch generated a false NO_AUTHOR_SIGN_OFF warning.

A typical example is commit e33bcbab16d1 ("tee: add support for session's client UUID generation"). When checkpatch was run on this commit, it displayed:

"WARNING:NO_AUTHOR_SIGN_OFF: Missing Signed-off-by: line by nominal patch author ''"

This was due to split header lines not being handled properly and the author himself wrote in commit cd2614967d8b ("checkpatch: warn if missing author Signed-off-by"):

"Split From: headers are not fully handled: only the first part is compared."

Support split From: headers by correctly parsing the header extension lines. RFC 5322, Section-2.2.3 stated that each extended line must start with a WSP character (a space or htab). The solution was therefore to concatenate the lines which start with a WSP to get the correct long header.

---
 scripts/checkpatch.pl | 4 ++++
 1 file changed, 4 insertions(+)

index 504d2e431c60..9e65d21456f1 100755
@@ -2661,6 +2661,10 @@ sub process {
 # Check the patch for a From:
 		if (decode("MIME-Header", $line) =~ /^From:\s*(.*)/) {
 			$author = $1;
+			my $curline = $linenr;
+			while(defined($rawlines[$curline]) && ($rawlines[$curline++] =~ /^[ \t]\s*(.*)/)) {
+				$author .= $1;
+			}
 			$author = encode("utf8", $author) if ($line =~ /=\?utf-8\?/i);
 			$author =~ s/"//g;
 			$author = reformat_email($author);

2.27.0