From: Tom Lane <tgl@sss.pgh.pa.us>
To: Mark Dilger <mark.dilger@enterprisedb.com>
cc: Robert Haas <robertmhaas@gmail.com>, Peter Geoghegan <pg@bowt.ie>,
        "Andrey M. Borodin" <x4mmm@yandex-team.ru>,
        Stephen Frost <sfrost@snowman.net>,
        Michael Paquier <michael@paquier.xyz>, Amul Sul <sulamul@gmail.com>,
        Dilip Kumar <dilipbalaut@gmail.com>,
        Andres Freund <andres@anarazel.de>,
        PostgreSQL-development <pgsql-hackers@postgresql.org>
Subject: Re: new heapcheck contrib module
In-reply-to: <42432.1603399756@sss.pgh.pa.us>
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Comments: In-reply-to Tom Lane <tgl@sss.pgh.pa.us>
	message dated "Thu, 22 Oct 2020 16:49:16 -0400"
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Content-ID: <43208.1603400779.1@sss.pgh.pa.us>
Date: Thu, 22 Oct 2020 17:06:19 -0400
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I wrote:
> Mark Dilger <mark.dilger@enterprisedb.com> writes:
>> It is seeking to position 32 and writing '\x77\x77\x77\x77'.  x86_64 is
>> little-endian, and ppc32 and sparc64 are both big-endian, right?

> They are, but that should not meaningfully affect the results of
> that corruption step.  You zapped only one line pointer not
> several, but it would look the same regardless of endiannness.

Oh, wait a second.  ItemIdData has the flag bits in the middle:

typedef struct ItemIdData
{
    unsigned    lp_off:15,        /* offset to tuple (from start of page) */
                lp_flags:2,       /* state of line pointer, see below */
                lp_len:15;        /* byte length of tuple */
} ItemIdData;

meaning that for that particular bit pattern, one endianness
is going to see the flags as 01 (LP_NORMAL) and the other as 10
(LP_REDIRECT).  The offset/len are corrupt either way, but
I'd certainly expect that amcheck would produce different
complaints about those two cases.  So it's unsurprising if
this test case's output is endian-dependent.

			regards, tom lane