Re: pack corruption post-mortem

From: Nicolas Pitre <nico@fluxnic.net>
Date: Oct 19, 2013, 14:41 UTC
Message-ID: <alpine.LFD.2.03.1310191024000.1951@syhkavp.arg>
In-Reply-To: <CACsJy8ABdE8mZMVFZkqYMC4ZeN_baWw=XcYOZYBCZytWP97rRw@mail.gmail.com>

On Sat, 19 Oct 2013, Duy Nguyen wrote:

Show 20 quoted lines

> On Wed, Oct 16, 2013 at 3:34 PM, Jeff King <peff@peff.net> wrote:
> > I was recently presented with a repository with a corrupted packfile,
> > and was asked if the data was recoverable. This post-mortem describes
> > the steps I took to investigate and fix the problem. I thought others
> > might find the process interesting, and it might help somebody in the
> > same situation.
> >
> > I started with an fsck, which found a problem with exactly one object
> > (I've used $pack and $obj below to keep the output readable, and also
> > because I'll refer to them later):
> >
> >     $ git fsck
> >     error: $pack SHA1 checksum mismatch
> >     error: index CRC mismatch for object $obj from $pack at offset 51653873
> >     error: inflate: data stream error (incorrect data check)
> >     error: cannot unpack $obj from $pack at offset 51653873
> 
> I wonder if we should protect the sha-1 and pathname tables in packv4
> with CRC too. A bit flipped in there could cause stream of corrupt
> objects and make it hard to pinpoint the corrupt location..

It turns out that we already have this covered.

The SHA1 used in the name of the pack file is actually the SHA1 checksum of the SHA1 table.

The path and ident tables are already protected by the CRC32 in the zlib deflated stream.

Normal objects are also zlib deflated (except for their header) but you need to inflate them in order to have this CRC verified, which the pack data copy tries to avoid. Hence the separate CRC32 in the index file in that case.

However the pack v4 tables are very unlikely to be reused as is from one pack to another.

Nicolas