git@vger.kernel.org list mirror (unofficial, one of many)
 help / color / mirror / code / Atom feed
From: Nicolas Pitre <nico@fluxnic.net>
To: Duy Nguyen <pclouds@gmail.com>
Cc: Git Mailing List <git@vger.kernel.org>, Jeff King <peff@peff.net>
Subject: Re: pack corruption post-mortem
Date: Sat, 19 Oct 2013 10:41:08 -0400 (EDT)	[thread overview]
Message-ID: <alpine.LFD.2.03.1310191024000.1951@syhkavp.arg> (raw)
In-Reply-To: <CACsJy8ABdE8mZMVFZkqYMC4ZeN_baWw=XcYOZYBCZytWP97rRw@mail.gmail.com>

On Sat, 19 Oct 2013, Duy Nguyen wrote:

> 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

  reply	other threads:[~2013-10-19 14:41 UTC|newest]

Thread overview: 16+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2013-10-16  8:34 Jeff King
2013-10-16  8:59 ` Duy Nguyen
2013-10-16 15:41 ` Martin Fick
2013-10-17  0:35   ` Jeff King
2013-10-17 15:47     ` Junio C Hamano
2013-10-25  7:55       ` Jeff King
2013-10-17  1:06   ` Duy Nguyen
2013-10-19 10:32 ` Duy Nguyen
2013-10-19 14:41   ` Nicolas Pitre [this message]
2013-10-19 19:17     ` Shawn Pearce
2013-10-20 20:56       ` Nicolas Pitre
2013-10-20  4:44     ` Duy Nguyen
2013-10-20 21:08       ` Nicolas Pitre
2015-04-01 21:08 ` [PATCH] howto: document more tools for recovery corruption Jeff King
2015-04-01 22:21   ` Junio C Hamano
2015-04-02  0:49     ` Jeff King

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

  List information: http://vger.kernel.org/majordomo-info.html

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=alpine.LFD.2.03.1310191024000.1951@syhkavp.arg \
    --to=nico@fluxnic.net \
    --cc=git@vger.kernel.org \
    --cc=pclouds@gmail.com \
    --cc=peff@peff.net \
    --subject='Re: pack corruption post-mortem' \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

Code repositories for project(s) associated with this inbox:

	https://80x24.org/mirrors/git.git

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).