Re: [PATCH 3/8] Add support for a generic wal reading facility dubbed XLogReader

On Tuesday, October 30, 2012 04:24:21 PM Alvaro Herrera wrote:
> Tom Lane escribió:
> > Andres Freund <andres@2ndquadrant.com> writes:
> > > On Tuesday, October 30, 2012 03:20:03 PM Alvaro Herrera wrote:
> > >> Am I the only one who finds this rather bizarre?  Maybe this was okay
> > >> when xlog data would only come from WAL files stored in the data
> > >> directory at recovery, but if we're now receiving these from a remote
> > >> sender over the network I wonder if we should be protecting against
> > >> malicious senders.  (This is not related to this patch anyway.)
> > > 
> > > You can't use a CRC against malicous users anyway, its not
> > > cryptographically secure in any meaning of the word,
> > 
> > More to the point, if a bad guy has got control of your WAL stream,
> > crashing the startup process with a ridiculous length word is several
> > orders of magnitude less than the worst thing he can find to do to you.
> > So the above argument seems completely nonsensical to me.
> 
> Well, I wasn't talking just about the record length, but about the
> record in general.  The length just came up because it's what I noticed.
> 
> And yeah, I was thinking in one sum for the header and another one for
> the data.  If we're using CRC to detect end of WAL, what sense does it
> make to have to read the whole record if we can detect the end by just
> looking at the header?  (I obviously see that we need to checksum the
> data as well; and having a second CRC field would bloat the record.)

Well, the header is written first. In the header we can detect somewhat 
accurately that were beyond the current end-of-wal by looking at ->xl_prev and 
doing some validity checks, but thats not applicable for the data. A valid 
looking header doesn't mean that the whole, potentially multi-megabyte, record 
data is valid, we could have crashed while writing the data.

Greetings,

Andres
-- 
Andres Freund		http://www.2ndQuadrant.com/
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