threads / bug / 38978

Kernel bug caused by 'git apply' misapplying a patch with ambiguous chunk

Subject: Kernel bug caused by 'git apply' misapplying a patch with ambiguous chunk

## tl;dr

3 messages between Apr 2, 2015 and Apr 2, 2015.

replies: 2people: 2as markdown or json

Mikko Perttunen· Apr 2, 2015, 15:25 UTC · lore

Hello everyone,

we recently ran into a kernel bug caused by git misapplying this patch: https://lkml.org/lkml/2014/7/3/896 .

The chunk '@@ -653,6 +655,7 @@' in tegra124.dtsi (the second file in the patch) has ambiguous context (there are several almost identical PHY nodes in the file). Git applied the chunk to the second PHY node when it should have been applied to the first node.

You can reproduce this by checking out, for example, version 3.16 of the Linux kernel and applying the patch from the above link and then looking at

   arch/arm/boot/dts/tegra124.dtsi

and verifying that git has added the 'nvidia,has-utmi-pad-registers' property to the second, instead of the first 'phy' node.

Of course this is probably rather hard to fix on the applying end; but perhaps format-patch could check for ambiguous chunks and either warn the user or increase the context size automatically, or apply could warn about the chunk being ambiguous?

Thanks, Mikko Perttunen

Junio C Hamano· Apr 2, 2015, 15:59 UTC · re: Mikko Perttunen · lore

Re: Kernel bug caused by 'git apply' misapplying a patch with ambiguous chunk

Mikko Perttunen <mikko.perttunen@kapsi.fi> writes:

> Of course this is probably rather hard to fix on the applying end; but
> perhaps format-patch could check for ambiguous chunks and either warn
> the user or increase the context size automatically, or apply could
> warn about the chunk being ambiguous?

Interesting thought. Let me rephrase to make sure I got your thought process correctly.

Imagine you started from an original that had two cut-and-pasted codeblocks A and B in the same file and updated one of them, say A, and then sent out the patch that turns A into A1.

Meanwhile, somebody started from the same original and updated the same codeblock A in the upstream already to A2. Your patch applies cleanly to codeblock B and turns it to A1, which is a mispatch. And you cannot even detect the problem while applying.

But if you are starting from the original with idential A and B, format-patch can see that the resulting patch to turn A to A1 can be misunderstood to be a patch to change B to A1 instead. So in that case, you _could_ detect.

But imagine if you started from an original that had A and C, that are clearly different. Your change turns A into A1. In the meantime, the upstream started from the same original, and changed C into B that looks identical to A.

The same thing would happen to your patch when you try to apply it. "git apply" could try to diagnose this situation and warn. But you cannot check when your format-patch produces a patch that turns <A,C> into <A1,C>, as there is no ambiguity in the original.

So,

 - format-patch could try to help, but it won't be a complete
   solution.

 - apply could try to help, but it won't be a complete solution.

I am not sure if having "both" would make it complete, but I doubt it.

Mikko Perttunen· Apr 2, 2015, 16:13 UTC · re: Junio C Hamano · lore

Re: Kernel bug caused by 'git apply' misapplying a patch with ambiguous chunk

On 04/02/2015 06:59 PM, Junio C Hamano wrote:

Show 45 quoted lines

> Mikko Perttunen <mikko.perttunen@kapsi.fi> writes:
>
>> Of course this is probably rather hard to fix on the applying end; but
>> perhaps format-patch could check for ambiguous chunks and either warn
>> the user or increase the context size automatically, or apply could
>> warn about the chunk being ambiguous?
>
> Interesting thought.  Let me rephrase to make sure I got your
> thought process correctly.
>
> Imagine you started from an original that had two cut-and-pasted
> codeblocks A and B in the same file and updated one of them, say A,
> and then sent out the patch that turns A into A1.
>
> Meanwhile, somebody started from the same original and updated the
> same codeblock A in the upstream already to A2. Your patch applies
> cleanly to codeblock B and turns it to A1, which is a mispatch.  And
> you cannot even detect the problem while applying.
>
> But if you are starting from the original with idential A and B,
> format-patch can see that the resulting patch to turn A to A1 can be
> misunderstood to be a patch to change B to A1 instead. So in that
> case, you _could_ detect.
>
> But imagine if you started from an original that had A and C, that
> are clearly different.  Your change turns A into A1.  In the
> meantime, the upstream started from the same original, and changed C
> into B that looks identical to A.
>
> The same thing would happen to your patch when you try to apply it.
> "git apply" could try to diagnose this situation and warn.  But you
> cannot check when your format-patch produces a patch that turns <A,C>
> into <A1,C>, as there is no ambiguity in the original.
>
> So,
>
>   - format-patch could try to help, but it won't be a complete
>     solution.
>
>   - apply could try to help, but it won't be a complete solution.
>
> I am not sure if having "both" would make it complete, but I doubt
> it.
>
>

I agree. I think you can also create a situation where neither would detect the problem, if the upstream changes C such that A->A1 can be applied on it and at the same time changes A such that A->A1 no longer applies. However, such a situation should be rare, so I think these checks would be useful.

In this case, the ambiguity existed the whole time both on the submitter's end and in upstream. Upstream just added more stuff into the file causing the line numbers to shift. So in this case a check in either subcommand would have caught it.

Mikko