Re: pg_verify_checksums and -fno-strict-aliasing

On 2018-08-30 14:46:06 -0700, Andres Freund wrote:
> 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.

It took me a bit to reproduce the issue (due to sheer stupidity on my
part: no, changing the flags passed to gcc to link pg_verify_checksums
doesn't do the trick), but the above indeed fixes the issue for me.

The attached is just for demonstration that the approach works.

Greetings,

Andres Freund