builtin/blame: fix out-of-bounds read with excessive --abbrev

In 6411a0a (builtin/blame: fix type of `length` variable when
emitting object ID, 2024-12-06) we have fixed the type of the `length`
variable. In order to avoid a cast from `size_t` to `int` in the call to
printf(3p) with the "%.*s" formatter we have converted the code to
instead use fwrite(3p), which accepts the length as a `size_t`.

It was reported though that this makes us read over the end of the OID
array when the provided `--abbrev=` length exceeds the length of the
object ID. This is because fwrite(3p) of course doesn't stop when it
sees a NUL byte, whereas printf(3p) does.

Fix the bug by reverting back to printf(3p) and culling the provided
length to `GIT_MAX_HEXSZ` to keep it from overflowing when cast to an
`int`.

Reported-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: pks-t

builtin/blame.c

@@ -505,7 +505,8 @@ static void emit_other(struct blame_scoreboard *sb, struct blame_entry *ent, int
 			length--;
 			putchar('?');
 		}
-		fwrite(hex, 1, length, stdout);
+
+		printf("%.*s", (int)(length < GIT_MAX_HEXSZ ? length : GIT_MAX_HEXSZ), hex);
 		if (opt & OUTPUT_ANNOTATE_COMPAT) {
 			const char *name;
 			if (opt & OUTPUT_SHOW_EMAIL)

t/t8002-blame.sh

@@ -126,6 +126,14 @@ test_expect_success '--no-abbrev works like --abbrev with full length' '
 	check_abbrev $hexsz --no-abbrev
 '
 
+test_expect_success 'blame --abbrev gets truncated' '
+	check_abbrev $hexsz --abbrev=9000 HEAD
+'
+
+test_expect_success 'blame --abbrev gets truncated with boundary commit' '
+	check_abbrev $hexsz --abbrev=9000 ^HEAD
+'
+
 test_expect_success '--exclude-promisor-objects does not BUG-crash' '
 	test_must_fail git blame --exclude-promisor-objects one
 '