| From: | Andres Freund <andres(at)anarazel(dot)de> |
|---|---|
| To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
| Cc: | Michael Banck <michael(dot)banck(at)credativ(dot)de>, Magnus Hagander <magnus(at)hagander(dot)net>, Fabien COELHO <coelho(at)cri(dot)ensmp(dot)fr>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: pg_verify_checksums and -fno-strict-aliasing |
| Date: | 2018-08-30 21:46:06 |
| Message-ID: | 20180830214606.xpm4bho5nmhfuzlx@alap3.anarazel.de |
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| 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
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