Re: [PATCH] Put sha1dc on a diet

From: Marc Stevens <marc.stevens@cwi.nl>
Date: Mar 13, 2017, 21:15 UTC
Message-ID: <1392458356.3351662.1489439723458.JavaMail.zimbra@cwi.nl>
In-Reply-To: <20170313210023.bumtp6wyw6blmymp@sigill.intra.peff.net>

I think I now understand. The Makefile indeed seems to fail to correctly rebuild when a header has changed.

As the performance branch has removed the 'int bigendian' from SHA1_CTX in lib/sha1.h, the perf-branch and master-branch are binary incompatible. So the command-line utility does not get fully recompiled and instead of the value of found_collision will read a different value of SHA1_CTX.

So be careful to always do a 'make clean' for now.

-- Marc

----- Original Message -----
From: "Jeff King" <peff@peff.net>
To: "Marc Stevens" <Marc.Stevens@cwi.nl>
Cc: "Linus Torvalds" <torvalds@linux-foundation.org>, "Dan Shumow" <danshu@microsoft.com>, "Junio C Hamano" <gitster@pobox.com>, "Git Mailing List" <git@vger.kernel.org>
Sent: Monday, March 13, 2017 10:00:23 PM
Subject: Re: [PATCH] Put sha1dc on a diet

On Mon, Mar 13, 2017 at 09:47:54PM +0100, Marc Stevens wrote:

> Linus:
> I would be surprised, the dependencies should be automatically determined.
> 
> BTW Did you make local changes to this perf branch?

I can reproduce it with:

  cd sha1collisiondetection
  git clean -dqfx ;# make sure we are starting from scratch

  git checkout 9c8e73cadb35776d3310e3f8ceda7183fa75a39f
  make
  bin/sha1dcsum $file

  git checkout 55d1db0980501e582f6cd103a04f493995b1df78
  make
  bin/sha1dcsum $file

The final call to sha1dcsum will report a collision, even though the first one did not.

It also reproduces with the original snippet I posted. I didn't notice because I was just collecting the timings then (and I originally noticed the problem on the versions I had pulled into Git, where it works as expected; but then I am just pulling in the two source files, without all of the libtool magic).

-Peff