commit: ignore additional signatures when parsing signed commits

When we create a commit with multiple signatures, neither of these
signatures includes the other.  Consequently, when we produce the
payload which has been signed so we can verify the commit, we must strip
off any other signatures, or the payload will differ from what was
signed.  Do so, and in preparation for verifying with multiple
algorithms, pass the algorithm we want to verify into
parse_signed_commit.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 34 insertions, 20 deletions

diff --git a/commit.c b/commit.c
index f128f18..d2908cb 100644
--- a/commit.c
+++ b/commit.c
@@ -1036,20 +1036,18 @@ static int do_sign_commit(struct strbuf *buf, const char *keyid)
 }
 
 int parse_signed_commit(const struct commit *commit,
-			struct strbuf *payload, struct strbuf *signature)
+			struct strbuf *payload, struct strbuf *signature,
+			const struct git_hash_algo *algop)
 {
 
 	unsigned long size;
 	const char *buffer = get_commit_buffer(commit, &size);
-	int in_signature, saw_signature = -1;
-	const char *line, *tail;
-	const char *gpg_sig_header = gpg_sig_headers[hash_algo_by_ptr(the_hash_algo)];
-	int gpg_sig_header_len = strlen(gpg_sig_header);
+	int in_signature = 0, saw_signature = 0, other_signature = 0;
+	const char *line, *tail, *p;
+	const char *gpg_sig_header = gpg_sig_headers[hash_algo_by_ptr(algop)];
 
 	line = buffer;
 	tail = buffer + size;
-	in_signature = 0;
-	saw_signature = 0;
 	while (line < tail) {
 		const char *sig = NULL;
 		const char *next = memchr(line, '\n', tail - line);
@@ -1057,9 +1055,15 @@ int parse_signed_commit(const struct commit *commit,
 		next = next ? next + 1 : tail;
 		if (in_signature && line[0] == ' ')
 			sig = line + 1;
-		else if (starts_with(line, gpg_sig_header) &&
-			 line[gpg_sig_header_len] == ' ')
-			sig = line + gpg_sig_header_len + 1;
+		else if (skip_prefix(line, gpg_sig_header, &p) &&
+			 *p == ' ') {
+			sig = line + strlen(gpg_sig_header) + 1;
+			other_signature = 0;
+		}
+		else if (starts_with(line, "gpgsig"))
+			other_signature = 1;
+		else if (other_signature && line[0] != ' ')
+			other_signature = 0;
 		if (sig) {
 			strbuf_add(signature, sig, next - sig);
 			saw_signature = 1;
@@ -1068,7 +1072,8 @@ int parse_signed_commit(const struct commit *commit,
 			if (*line == '\n')
 				/* dump the whole remainder of the buffer */
 				next = tail;
-			strbuf_add(payload, line, next - line);
+			if (!other_signature)
+				strbuf_add(payload, line, next - line);
 			in_signature = 0;
 		}
 		line = next;
@@ -1082,23 +1087,29 @@ int remove_signature(struct strbuf *buf)
 	const char *line = buf->buf;
 	const char *tail = buf->buf + buf->len;
 	int in_signature = 0;
-	const char *sig_start = NULL;
-	const char *sig_end = NULL;
+	struct sigbuf {
+		const char *start;
+		const char *end;
+	} sigs[2], *sigp = &sigs[0];
+	int i;
+	const char *orig_buf = buf->buf;
+
+	memset(sigs, 0, sizeof(sigs));
 
 	while (line < tail) {
 		const char *next = memchr(line, '\n', tail - line);
 		next = next ? next + 1 : tail;
 
 		if (in_signature && line[0] == ' ')
-			sig_end = next;
+			sigp->end = next;
 		else if (starts_with(line, "gpgsig")) {
 			int i;
 			for (i = 1; i < GIT_HASH_NALGOS; i++) {
 				const char *p;
 				if (skip_prefix(line, gpg_sig_headers[i], &p) &&
 				    *p == ' ') {
-					sig_start = line;
-					sig_end = next;
+					sigp->start = line;
+					sigp->end = next;
 					in_signature = 1;
 				}
 			}
@@ -1106,15 +1117,18 @@ int remove_signature(struct strbuf *buf)
 			if (*line == '\n')
 				/* dump the whole remainder of the buffer */
 				next = tail;
+			if (in_signature && sigp - sigs != ARRAY_SIZE(sigs))
+				sigp++;
 			in_signature = 0;
 		}
 		line = next;
 	}
 
-	if (sig_start)
-		strbuf_remove(buf, sig_start - buf->buf, sig_end - sig_start);
+	for (i = ARRAY_SIZE(sigs) - 1; i >= 0; i--)
+		if (sigs[i].start)
+			strbuf_remove(buf, sigs[i].start - orig_buf, sigs[i].end - sigs[i].start);
 
-	return sig_start != NULL;
+	return sigs[0].start != NULL;
 }
 
 static void handle_signed_tag(struct commit *parent, struct commit_extra_header ***tail)
@@ -1165,7 +1179,7 @@ int check_commit_signature(const struct commit *commit, struct signature_check *
 
 	sigc->result = 'N';
 
-	if (parse_signed_commit(commit, &payload, &signature) <= 0)
+	if (parse_signed_commit(commit, &payload, &signature, the_hash_algo) <= 0)
 		goto out;
 	ret = check_signature(payload.buf, payload.len, signature.buf,
 		signature.len, sigc);