From: Tom Lane <tgl@sss.pgh.pa.us>
To: Tomas Vondra <tomas.vondra@2ndquadrant.com>
cc: marxin.liska@gmail.com
cc: pgsql-bugs@lists.postgresql.org
Subject: Re: BUG #15121: Multiple UBSAN errors
In-reply-to: <7df78014-c931-6c75-6f92-ec48f390b2b9@2ndquadrant.com>
References: <152140318583.11955.7731957058248462341@wrigleys.postgresql.org>
 <7df78014-c931-6c75-6f92-ec48f390b2b9@2ndquadrant.com>
Comments: In-reply-to Tomas Vondra <tomas.vondra@2ndquadrant.com>
	message dated "Mon, 19 Mar 2018 01:34:05 +0100"
MIME-Version: 1.0
Content-Type: text/plain; charset="us-ascii"
Content-ID: <20077.1521421379.1@sss.pgh.pa.us>
Content-Transfer-Encoding: quoted-printable
Date: Sun, 18 Mar 2018 21:02:59 -0400
Message-ID: <20078.1521421379@sss.pgh.pa.us>
Precedence: bulk

Tomas Vondra <tomas.vondra@2ndquadrant.com> writes:
> On 03/18/2018 08:59 PM, PG Bug reporting form wrote:
>> Building current trunk with -fsanitize=3Dundefined I see following erro=
rs with
>> make check:

>> pg_crc32c_sse42.c:37:18: runtime error: load of misaligned address
>> 0x7fffffffd484 for type 'const uint64', which requires 8 byte alignment
>> 0x7fffffffd484: note: pointer points here

> This comes from this call in pg_comp_crc32c_sse42
>     crc =3D (uint32) _mm_crc32_u64(crc, *((const uint64 *) p));
> ...
> So, not a bug.

Agreed.  That's Intel-only code so it doesn't need to be strict
about alignment.

>> arrayfuncs.c:3740:17: runtime error: member access within misaligned ad=
dress
>> 0x0000028b937c for type 'struct ExpandedObjectHeader', which requires 8=
 byte
>> alignment

> Again, the line numbers don't really match the code I have, but I guess
> it's the same issue as for pg_comp_crc32c_sse42. This is apparently
> related to array serialization, and I guess we have a compact structure
> (intentionally, to make it smaller), and we accept the unaligned access.

No, there's no intentional misalignment in the array stuff; if there
were, it'd fail on alignment-picky hardware.

What I think might be happening is that the compiler is taking the
fact that the pointer is declared as AnyArrayType *, where

typedef union AnyArrayType
{
	ArrayType	flt;
	ExpandedArrayHeader xpn;
} AnyArrayType;

to assume that the pointer must be aligned on an 8-byte boundary because
ExpandedArrayHeader would require that, even if we're only accessing the
"flt" member.  Maybe that's a live problem, though we've seen no related
trouble reports.  It'd require the compiler to generate 8-byte-aligned
instructions for accessing the ArrayType header, which doesn't seem all
that probable.

			regards, tom lane