sequencer: fix "rebase -i --root" corrupting author header

When "git rebase -i --root" creates a new root commit (say, by swapping
in a different commit for the root), it corrupts the commit's "author"
header with trailing garbage:

    author A U Thor <author@example.com> @1112912773 -07000or@example.com

This is a result of read_author_ident() neglecting to NUL-terminate the
buffer into which it composes the "author" header.

(Note that the "@" preceding the timestamp and the extra "0" in the
timezone are separate bugs which will be fixed subsequently.)

Security considerations: Construction of the "author" header by
read_author_ident() happens in-place and in parallel with parsing the
content of "rebase-merge/author-script" which occupies the same buffer.
This is possible because the constructed "author" header is always
smaller than the content of "rebase-merge/author-script". Despite
neglecting to NUL-terminate the constructed "author" header, memory is
never accessed (either by read_author_ident() or its caller) beyond the
allocated buffer since a NUL-terminator is present at the end of the
loaded "rebase-merge/author-script" content, and additional NUL's are
inserted as part of the parsing process.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Acked-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

sequencer.c

@@ -741,7 +741,7 @@ static const char *read_author_ident(struct strbuf *buf)
 		return NULL;
 	}
 
-	buf->len = out - buf->buf;
+	strbuf_setlen(buf, out - buf->buf);
 	return buf->buf;
 }
 

t/t3404-rebase-interactive.sh

@@ -1202,7 +1202,7 @@ rebase_setup_and_clean () {
 		test_might_fail git branch -D $1 &&
 		test_might_fail git rebase --abort
 	" &&
-	git checkout -b $1 master
+	git checkout -b $1 ${2:-master}
 }
 
 test_expect_success 'drop' '
@@ -1379,4 +1379,12 @@ test_expect_success 'rebase -i --gpg-sign=<key-id> overrides commit.gpgSign' '
 	test_i18ngrep "$SQ-S\"S I Gner\"$SQ" err
 '
 
+test_expect_success 'valid author header after --root swap' '
+	rebase_setup_and_clean author-header no-conflict-branch &&
+	set_fake_editor &&
+	FAKE_LINES="2 1" git rebase -i --root &&
+	git cat-file commit HEAD^ >out &&
+	grep "^author ..*> @[0-9][0-9]* [-+][0-9][0-9]*$" out
+'
+
 test_done