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From: Junio C Hamano <gitster@pobox.com>
To: "Martin Ågren" <martin.agren@gmail.com>
Cc: git@vger.kernel.org, Jonathan Tan <jonathantanmy@google.com>
Subject: Re: [PATCH] t1450: fix quoting of NUL byte when corrupting pack
Date: Sat, 01 Aug 2020 17:45:11 -0700
Message-ID: <xmqqmu3d27vs.fsf@gitster.c.googlers.com> (raw)
In-Reply-To: <20200801220611.10453-1-martin.agren@gmail.com> ("Martin =?utf-8?Q?=C3=85gren=22's?= message of "Sun, 2 Aug 2020 00:06:11 +0200")

Martin Ågren <martin.agren@gmail.com> writes:

> 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.

Thanks.  Very well explained.

I wonder if it is an easy way to find similar problems without too
much hand-parsing of the test scripts.  Inside a modern test_expect_*
that begins and ends the test body with a single quote, any line
that has a single quote that is not quoted could be suspect, but
that would probably give us too many false positive.


> 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 &&

  reply	other threads:[~2020-08-02  0:46 UTC|newest]

Thread overview: 16+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-08-01 22:06 Martin Ågren
2020-08-02  0:45 ` Junio C Hamano [this message]
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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