Re: [FFmpeg-devel] rebasing security

[Kacper Michajlow <kasper93@gmail.com>]

On Mon, 4 Aug 2025 at 23:38, Marton Balint <cus@passwd.hu> wrote:
>
>
>
> On Mon, 4 Aug 2025, Alexander Strasser via ffmpeg-devel wrote:
>
> > Hi Michael,
> > hi all!
> >
> > I think it's a good time to bring stuff like this up for discussion.
> >
> > On 2025-08-03 21:02 +0200, Michael Niedermayer wrote:
> >>
> >> On Sun, Aug 03, 2025 at 05:31:39PM +0200, Michael Niedermayer wrote:
> >> [...]
> >>> The solutions are obvious:
> >>> 1. ignore security and supply chain attacks
> >>> 2. use merges not rebases on the server
> >>> 3. rebase locally, use fast forward only
> >>> 4. verify on server rebases
> >>
> >> Maybe not everyone understood the problem. So let me try a different
> >> explanation. Without any signatures.
> >>
> >> In the ML workflow: (for simplicity we assume reviewer and commiter is the same person)
> >> 1. someone posts a patch
> >> 2. patch is locally applied or rebased
> >> 3. commit is reviewed
> >> 4. commit is tested
> >> 5. commit is pushed
> >>
> >> Here the only way to get bad code in, is through the reviewer
> >> If the reviewer doesnt miss anything and his setup is not compromised
> >> then what he pushes is teh reviewed code
> >>
> >> if its manipulated after its pushed git should light up like a christmess tree
> >> on the next "git pull --rebase"
> >>
> >>
> >> With the rebase on webapp (gitlab or forgejo) workflow
> >> 1. someone posts a pull request
> >> 2. pr is reviewed
> >> 3. pr is approved
> >> 4. pr is rebased
> >> 5. pr is tested
> >> 6, pr is pushed
> >>
> >> now here of course the same reviewer trust or compromised scenarios exist
> >> but we also have an extra one and that is the server
> >> because the server strips the signatures during rebase it can modify the
> >> commit. And this happens after review and because a rebase was litterally
> >> requested by the reviewer its not likely to be noticed as something out of
> >> place
> >
> > If I understand the original point you wanted to discuss correctly,
> > than this is not a question of rebase or merge but one of letting
> > **commits happen on the forge**. If it happens it bears the
> > possibility of modification on the server the forge is running on.
> >
> > TL;DR: I think it's fine the way it's setup now.
> >
> > I'm not against letting rebase/merges happen on the server because
> > otherwise we would lose a lot of advantages and comfort we get by
> > using a forge for PRs.
> >
> > Only alternative I see is to do PRs on the forge and doing merging
> > manually by the same person that ensures reviewed PR is not changed
> > and pushes (after rebase or with a clean merge commit) from their
> > machine.
>
> Two things came to my mind about the current forgejo workflow.
>
> - Previously it was pretty clear from git history who actually committed
>    a change from the comitter field. With using forgejo the comitter
>    field no longer shows the person who actually *committed* the change to
>    the main repo, but it is inherited from the original pull request commit
>    instead, so it simply shows the original author of the patch.

I don't think this is accurate. Committer field is set to the person
who clicks the "merge" button. Same as they would manually git push
the patches.

Slightly related, I don't like how simple the web ui commit log of
forgejo is, it doesn't show commiter at all. For me this information
is as important as the author. I'm keeping notes on forgejo usage and
will share it when the time comes, it has some annoying limitations
compared to other forges.

> - A pull request is writable both by the reviewer and the author up to
>    the point when it is actually committed to the main repo. So force
>    pushes from an author can happen anytime during this timeline:
>      - reviewer reads changes
>      - approves the changes
>      - rebases the branch
>      - sets it up to auto merge
>      - CI actually runs
>      - forgejo auto-merge
>    A reviewer may not realize the new force push from the author. Maybe
>    forgejo handle some force pushes in this timeline gracefully and aborts,
>    or ignores them, I am not sure. It still looks a bit fragile, my
>    expectation as a reviewer would be that what I saw when I finished the
>    review and clicked on the Approve button will get comitted, when I later
>    click on the merge button.

There is a timeline of events. If PR is approved, you can see if there
were new events (comments/pushes) after that. This is close to the
"merge" button and you should see "approve" as the last event in the
timeline to be sure nothing changes.

Also if there were rebases after approval, the approved tick mark (in
Reviewers list) becomes yellow instead of green.

It is also possible to configure to completely discard previous
approval if any push happened. But this hinders the workflow if there
is trivial rebase done to run CI on the latest merge base. Reviewer
would have to approve again. This works if the reviewer is the same
person who will merge, but if both author and reviewer are
maintainers, we should trust each other and respect approver to not
include unwanted changes after this point.

Saying that, I think (if possible) it should be configured to clear
the approval status if the **user** (not contributor) pushes to this
branch. This way the reviewer has to recheck it, before merge.

- Kacper
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