rsync - May 2013 - [Bug 9894] New: Rsync can silently zero out chunks in a file

https://bugzilla.samba.org/show_bug.cgi?id=9894

           Summary: Rsync can silently zero out chunks in a file
           Product: rsync
           Version: 3.0.9
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: critical
          Priority: P5
         Component: core
        AssignedTo: wayned at samba.org
        ReportedBy: an.m at outlook.com
         QAContact: rsync-qa at samba.org


map_ptr() handles read failures by zeroing out remaining chunk and returning
the partially filled/all zero buffers to callers. It also fills error details
in map_struct->status.

It is caller's responsibility to check the status and handle this condition
appropriately.

It is probably OK to ignore this when we are generating checksums (as it might
lead to a false mismatch, resulting in transfer of a non matching block)

But in context of transfer (send_deflated_token()) when this error is ignored
it leads to silent data corruption for receiver.

Possible solutions are to log errors and disconnect such connection, or retry
till timeout happens. 

No point in giving user a false impression that xfr is successful. Request dev
to look at it with priority.

Cheers,
Ankur

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https://bugzilla.samba.org/show_bug.cgi?id=9894

Wayne Davison <wayned at samba.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|                            |WORKSFORME

--- Comment #1 from Wayne Davison <wayned at samba.org> 2013-05-19
18:50:19 UTC ---
Rsync marks a file with a read error with a checksum that doesn't match the
file content so that the receiver knows that the file it generated is not
valid.

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https://bugzilla.samba.org/show_bug.cgi?id=9894

--- Comment #2 from Ankur Mishra <an.m at outlook.com> 2013-05-20 12:41:31
UTC ---
(In reply to comment #1)
> Rsync marks a file with a read error with a checksum that doesn't match
the
> file content so that the receiver knows that the file it generated is not
> valid.
Thanks for your response Wayne. Apologies for long comment below, but it would
be really helpful if you could go through it.

My setup is rsyncd on one machine (source of a file), and rsync client on
another machine (destination). 

When this happens (checksum mismatch due to read failure) does receiver
(destination) throw an error? What I see is that receiver is not retuning an
error code. From receiver point of view this looks like a success.

I have tested this with both with and without --checksum with fresh sync (i.e.,
receiver didn't have the file)

This is the experiment I did:

1. Ran rsyncd on one machine
2. Modified the rsyncd map_ptr() to mimic read failures
3. Ran rsync from a different machine
4. I could see rsync returning without any error, and a partially zeroed out
file (of same length)
5. I observed same symptoms as #4 on a flash based embedded system during i/o
stress on server side.

Is there any specific option I should be using while running rsync to catch
checksum errors?

Thanks,
Ankur

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