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From: Jonathan Tan <>
To: Junio C Hamano <>
Cc: Git mailing list <>,
	Mark Thomas <>,
	Jeff Hostetler <>,
	Kevin David <>
Subject: Re: Proposal for missing blob support in Git repos
Date: Tue, 2 May 2017 10:21:59 -0700	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

On Mon, May 1, 2017 at 6:41 PM, Junio C Hamano <> wrote:
> Jonathan Tan <> writes:
>> On 05/01/2017 04:29 PM, Junio C Hamano wrote:
>>> Jonathan Tan <> writes:
>>>> Thanks for your comments. If you're referring to the codepath
>>>> involving write_sha1_file() (for example, builtin/hash-object ->
>>>> index_fd or builtin/unpack-objects), that is fine because
>>>> write_sha1_file() invokes freshen_packed_object() and
>>>> freshen_loose_object() directly to check if the object already exists
>>>> (and thus does not invoke the new mechanism in this patch).
>>> Is that a good thing, though?  It means that you an attacker can
>>> feed one version to the remote object store your "grab blob" hook
>>> gets the blobs from, and have you add a colliding object locally,
>>> and the usual "are we recording the same object as existing one?"
>>> check is bypassed.
>> If I understand this correctly, what you mean is the situation where
>> the hook adds an object to the local repo, overriding another object
>> of the same name?
> No.
> write_sha1_file() pays attention to objects already in the local
> object store to avoid hash collisions that can be used to replace a
> known-to-be-good object and that is done as a security measure.
> What I am reading in your response was that this new mechanism
> bypasses that, and I was wondering if that is a good thing.

Oh, what I meant was that write_sha1_file() bypasses the new
mechanism, not that the new mechanism bypasses the checks in

To be clear, here's what happens when write_sha1_file() is invoked
(before and after this patch - this patch does not affect
write_sha1_file at all):
1. (some details omitted)
2. call freshen_packed_object
3, call freshen_loose_object if necessary
4. write object (if freshen_packed_object and freshen_loose_object do
not both return 0)

Nothing changes in this patch (whether a hook is defined or not).

And here's what happens when has_sha1_file (or another function listed
in the commit message) is invoked:
1. check for existence of packed object of the requested name
2. check for existence of loose object of the requested name
3. check again for existence of packed object of the requested name
4. if a hook is defined, invoke the hook and repeat 1-3

Here, in step 4, the hook could do whatever it wants to the repository.

  reply	other threads:[~2017-05-02 17:22 UTC|newest]

Thread overview: 12+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-04-26 22:13 Proposal for missing blob support in Git repos Jonathan Tan
2017-05-01  3:57 ` Junio C Hamano
2017-05-01 19:12   ` Jonathan Tan
2017-05-01 23:29     ` Junio C Hamano
2017-05-02  0:33       ` Jonathan Tan
2017-05-02  0:38         ` Brandon Williams
2017-05-02  1:41         ` Junio C Hamano
2017-05-02 17:21           ` Jonathan Tan [this message]
2017-05-02 18:32             ` Ævar Arnfjörð Bjarmason
2017-05-02 21:45               ` Jonathan Tan
2017-05-04  4:29                 ` Junio C Hamano
2017-05-04 17:09                   ` Jonathan Tan

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