Re: [IGNORETHIS/PATCH] Choosing the sha1 prefix of your commits

From: Jeff King <peff@peff.net>
Date: Oct 20, 2011, 07:13 UTC
Message-ID: <20111020071356.GA14945@sigill.intra.peff.net>
In-Reply-To: <7vr5289mlu.fsf@alter.siamese.dyndns.org>

On Wed, Oct 19, 2011 at 11:57:17PM -0700, Junio C Hamano wrote:

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> Jeff King <peff@peff.net> writes:
> 
> > Agreed. Having hidden cruft makes birthday collision attacks easier (or
> > it will, if sha1 ever gets broken to that point).  Unfortunately, there
> > is a _ton_ of code which assumes that commit messages are
> > NUL-terminated, as they always have been since e871b64 (2005-05-25).
> 
> I think that commit is irrelevant, as long as read_sha1_file() returns the
> contents as <ptr,len> pair, which has been the case forever. It's just the
> matter of propagating the length back up the callchain.

It's not that the commit is bad or the source of problems. My point is that the assumption that commit messages are NUL-terminated has been there for a really long time, so there are lots of spots in the code that sloppily run string functions on them. Every one of those needs to be found and fixed (e.g., I remember seeing this in for-each-ref.c:find_subpos recently).

It's not impossible, of course, or even really that hard. It's just a giant pain, and I wonder if the effort is worth it.

Show 9 quoted lines

> A naïve implementation to add "len" member to struct commit would increase
> the size of the in-core commit object by sizeof(unsigned long), which we
> may want to avoid. Traversals that care nothing but the topology of the
> history would have to waste that memory and these things tend to add up
> (8-byte ulong * 250k commits = 2MB).
> 
> Perhaps change the type of "buf" member in struct commit to a pointer to a
> <ptr,len> pair, or something? Or perhaps a few megabytes wasted between
> friends we do not care much about?

I think you'd have to convert the struct (even if not every piece of code is converted to use it) and profile.

-Peff