From: Jeff King <peff@peff.net>
Date: Sat, 18 Jan 2025 09:23:51 GMT
Subject: Re: [PATCH 1/3] packfile: factor out --pack_header argument parsing
Message-ID: <20250118092351.GB3474411@coredump.intra.peff.net>
In-Reply-To: <xmqqo705hxfz.fsf@gitster.g>

On Fri, Jan 17, 2025 at 02:45:04PM -0800, Junio C Hamano wrote:

> Jeff King <peff@peff.net> writes:
> 
> >  			} else if (starts_with(arg, "--pack_header=")) {
> > -				struct pack_header *hdr;
> > -				char *c;
> > -
> > -				hdr = (struct pack_header *)input_buffer;
> > -				hdr->hdr_signature = htonl(PACK_SIGNATURE);
> > -				hdr->hdr_version = htonl(strtoul(arg + 14, &c, 10));
> 
> Interesting.  So the file-scope static input_buffer[] sits in the
> BSS and happens to be well aligned not to cause the problem, but ...

I suspect it _is_ a problem, but either:

  - The OP's test case was small enough to trigger unpack-objects, not
    index-pack. Possibly:

      git index-pack --stdin --pack_header=2,2 <no-header.pack

    would fail for them.

  - We simply got lucky with alignment based on the other things in BSS,
    the whim of the compiler, etc. But it is an accident waiting to
    happen.

-Peff