Subject: Re: [PATCH 02/10] builtin/fast-import: fix segfault with unsafe SHA1

From: Patrick Steinhardt <[email-masked]>
Date: Fri, 3 Jan 2025 14:08:01 +0100

On Mon, Dec 30, 2024 at 12:22:34PM -0500, Taylor Blau wrote:
> On Mon, Dec 30, 2024 at 03:24:02PM +0100, Patrick Steinhardt wrote:
> > diff --git a/builtin/fast-import.c b/builtin/fast-import.c
> > index 1fa2929a01b7dfee52b653248bba802884f6be6a..0f86392761abbe6acb217fef7f4fe7c3ff5ac1fa 100644
> > --- a/builtin/fast-import.c
> > +++ b/builtin/fast-import.c
> > @@ -1106,7 +1106,7 @@ static void stream_blob(uintmax_t len, struct object_id *oidout, uintmax_t mark)
> >  		|| (pack_size + PACK_SIZE_THRESHOLD + len) < pack_size)
> >  		cycle_packfile();
> >
> > -	the_hash_algo->init_fn(&checkpoint.ctx);
> > +	the_hash_algo->unsafe_init_fn(&checkpoint.ctx);
> 
> This will obviously fix the issue at hand, but I don't think this is any
> less brittle than before. The hash function implementation here needs to
> agree with that used in the hashfile API. This change makes that
> happen, but only using side information that the hashfile API uses the
> unsafe variants.

Yup, I only cared about fixing the segfault because we're close to the
v2.48 release. I agree that the overall state is still extremely brittle
right now.

[snip]
> I think we should perhaps combine forces here. My ideal end-state is to
> have the unsafe_hash_algo() stuff land from my earlier series, then have
> these two fixes (adjusted to the new world order as above), and finally
> the Meson fixes after that.
> 
> Does that seem like a plan to you? If so, I can put everything together
> and send it out (if you're OK with me forging your s-o-b).

I think the ideal state would be if the hashing function used was stored
as part of `struct git_hash_ctx`. So the flow basically becomes for
example:

    ```
    struct git_hash_ctx ctx;
    struct object_id oid;

    git_hash_sha1_init(&ctx);
    git_hash_update(&ctx, data);
    git_hash_final_oid(&oid, &ctx);
    ```

Note how the intermediate calls don't need to know which hash function
you used to initialize the `struct git_hash_ctx` -- the structure itself
should remember what it has been initilized with and do the right thing.

Patrick