Horton

Re: pg_verify_checksums and -fno-strict-aliasing

Fabien COELHO <coelho@cri.ensmp.fr>

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From: Fabien COELHO <coelho@cri.ensmp.fr>
To: Michael Banck <michael.banck@credativ.de>
Cc: pgsql-hackers <pgsql-hackers@postgresql.org>
Date: 2018-08-30T11:25:14Z
Lists: pgsql-hackers 
> I noticed that pg_verify_checksums computes bogus checksums if I compile
> it with '-O2 -Wall' but without -fno-strict-aliasing. Also I am getting
> a compile warning then:
>
> [...]
>
> If I add -fno-strict-aliasing to $CFLAGS, the problem goes away.
>
> Is this something to worry about, or just pilot error cause I am not
> using the same $CFLAGS as for the rest of the build? I originally
> noticed this problem with my external fork of pg_verify_checksums.

It looks like the code is using aliasing where the standard says it should 
not, which breaks compiler optimization, and the added options tells the 
compiler to not assume that the code conforms to the standard...

As PostgreSQL source is expected to conform to some C standard (unsure 
which one right now, possibly c89 but maybe it is beginning to switch to 
c99, a young 19 years old standard), I'd suggest that the right fix is 
rather to actually remove the aliasing issue.

-- 
Fabien.

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.