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From: "Martin Ågren" <martin.agren@gmail.com>
To: git@vger.kernel.org
Cc: Jonathan Tan <jonathantanmy@google.com>
Subject: [PATCH] t1450: fix quoting of NUL byte when corrupting pack
Date: Sun,  2 Aug 2020 00:06:11 +0200
Message-ID: <20200801220611.10453-1-martin.agren@gmail.com> (raw)

We use

  printf '\0'

to generate a NUL byte which we then `dd` into the packfile to ensure
that we modify the first byte of the first object, thereby
(probabilistically) invalidating the checksum. Except the single quotes
we're using are interpreted to match with the ones we enclose the whole
test in. So we actually execute

  printf \0

and end up injecting the ASCII code for "0", 0x30, instead.

The comment right above this `printf` invocation says that "at least one
of [the type bits] is not zero, so setting the first byte to 0 is
sufficient". Substituting "0x30" for "0" in that comment won't do: we'd
need to reason about which bits go where and just what the packfile
looks like that we're modifying in this test.

Let's avoid all of that by actually executing

  printf "\0"

to generate a NUL byte, as intended.

Signed-off-by: Martin Ågren <martin.agren@gmail.com>
---
 If my reading is correct, when we substitute 0x30, the type will be 3
 (blob) and the size will be zero. So there might actually exist
 formally valid packfiles where this byte that we're modifying is
 already zero. What matters in the end is whether we might be using such
 a packfile in this exact test and from what I can tell, no, we won't be
 doing that.

 t/t1450-fsck.sh | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/t/t1450-fsck.sh b/t/t1450-fsck.sh
index 344a2aad82..af2a2c4682 100755
--- a/t/t1450-fsck.sh
+++ b/t/t1450-fsck.sh
@@ -714,7 +714,7 @@ test_expect_success 'fsck fails on corrupt packfile' '
 	# at least one of which is not zero, so setting the first byte to 0 is
 	# sufficient.)
 	chmod a+w .git/objects/pack/pack-$pack.pack &&
-	printf '\0' | dd of=.git/objects/pack/pack-$pack.pack bs=1 conv=notrunc seek=12 &&
+	printf "\0" | dd of=.git/objects/pack/pack-$pack.pack bs=1 conv=notrunc seek=12 &&
 
 	test_when_finished "rm -f .git/objects/pack/pack-$pack.*" &&
 	remove_object $hsh &&
-- 
2.28.0.81.ge8ab941b67


             reply index

Thread overview: 16+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-08-01 22:06 Martin Ågren [this message]
2020-08-02  0:45 ` Junio C Hamano
2020-08-02 14:30   ` Martin Ågren
2020-08-02 17:22     ` Eric Sunshine
2020-08-06 20:08     ` [PATCH v2 0/2] t: don't spuriously close and reopen quotes Martin Ågren
2020-08-06 20:08       ` [PATCH v2 1/2] " Martin Ågren
2020-08-06 20:26         ` Eric Sunshine
2020-08-07  8:45           ` Martin Ågren
2020-08-07 16:17             ` Eric Sunshine
2020-08-07 17:16               ` Junio C Hamano
2020-08-06 20:08       ` [PATCH v2 2/2] t4104: modernize and simplify quoting Martin Ågren
2020-08-02  1:00 ` [PATCH] t1450: fix quoting of NUL byte when corrupting pack Chris Torek
2020-08-02  1:02   ` Chris Torek
2020-08-02 14:35     ` Martin Ågren
2020-08-02 16:20       ` Chris Torek
2020-08-02 17:57         ` Junio C Hamano

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