From: Tom Lane <tgl@sss.pgh.pa.us>
To: Tomas Vondra <tomas.vondra@2ndquadrant.com>
Cc: marxin.liska@gmail.com
Subject: Re: BUG #15121: Multiple UBSAN errors
Date: Sun, 18 Mar 2018 21:02:59 -0400
Message-ID: <20078.1521421379@sss.pgh.pa.us> (raw)
In-Reply-To: <7df78014-c931-6c75-6f92-ec48f390b2b9@2ndquadrant.com>
References: <152140318583.11955.7731957058248462341@wrigleys.postgresql.org>
<7df78014-c931-6c75-6f92-ec48f390b2b9@2ndquadrant.com>
Tomas Vondra <tomas.vondra@2ndquadrant.com> writes:
> On 03/18/2018 08:59 PM, PG Bug reporting form wrote:
>> Building current trunk with -fsanitize=undefined I see following errors 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 = (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 address
>> 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
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To: pgsql-bugs@postgresql.org
Cc: tgl@sss.pgh.pa.us, tomas.vondra@2ndquadrant.com, marxin.liska@gmail.com
Subject: Re: BUG #15121: Multiple UBSAN errors
In-Reply-To: <20078.1521421379@sss.pgh.pa.us>
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