Horton

Re: pg_verify_checksums and -fno-strict-aliasing

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Michael Banck <michael.banck@credativ.de>, Magnus Hagander <magnus@hagander.net>, Fabien COELHO <coelho@cri.ensmp.fr>, pgsql-hackers <pgsql-hackers@postgresql.org>
Date: 2018-08-30T21:46:06Z
Lists: pgsql-hackers 
Hi,

On 2018-08-30 17:19:28 -0400, Tom Lane wrote:
> So, I've been fooling around trying to get it to work without
> -fno-strict-aliasing, but with little luck so far.

The problem presumably is that pg_checksum_block() accesses the relevant
fields as an uint32, whereas pg_checksum_page() accesses it as a
PageHeader. That's an aliasing violation.  *One* cast from char* to
either type is fine, it's accessing under both those types that's
problematic.

One way to fix it would be to memcpy in/out the modified PageHeader, or
just do offset math and memcpy to that offset.

Greetings,

Andres Freund

Commits

  1. Avoid using potentially-under-aligned page buffers.

  2. Code review for pg_verify_checksums.c.

  3. Make checksum_impl.h safe to compile with -fstrict-aliasing.