Re: pack corruption post-mortem

On Sat, 19 Oct 2013, Duy Nguyen wrote:

On Wed, Oct 16, 2013 at 3:34 PM, Jeff King [off-list ref] wrote:

quoted

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