From: Tom Lane <tgl@sss.pgh.pa.us>
To: pgsql-hackers@lists.postgresql.org
Cc: Fabien COELHO <coelho@cri.ensmp.fr>
Subject: Overflow hazard in pgbench
Date: Sun, 27 Jun 2021 13:39:03 -0400
Message-ID: <73927.1624815543@sss.pgh.pa.us> (raw)
moonjelly just reported an interesting failure [1]. It seems that
with the latest bleeding-edge gcc, this code is misoptimized:
/* check random range */
if (imin > imax)
{
pg_log_error("empty range given to random");
return false;
}
else if (imax - imin < 0 || (imax - imin) + 1 < 0)
{
/* prevent int overflows in random functions */
pg_log_error("random range is too large");
return false;
}
such that the second if-test doesn't fire. Now, according to the C99
spec this code is broken, because the compiler is allowed to assume
that signed integer overflow doesn't happen, whereupon the second
if-block is provably unreachable. The failure still represents a gcc
bug, because we're using -fwrapv which should disable that assumption.
However, not all compilers have that switch, so it'd be better to code
this in a spec-compliant way. I suggest applying the attached in
branches that have the required functions.
[1] https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=moonjelly&dt=2021-06-26%2007%3A03%3A17
regards, tom lane
Attachments:avoid-pgbench-overflow-hazard-1.patch (text/x-diff, 744B) downloaddiff --git a/src/bin/pgbench/pgbench.c b/src/bin/pgbench/pgbench.cindex e61055b6b7..c4023bfa27 100644--- a/src/bin/pgbench/pgbench.c+++ b/src/bin/pgbench/pgbench.c@@ -2450,7 +2450,8 @@ evalStandardFunc(CState *st,
case PGBENCH_RANDOM_ZIPFIAN:
{
int64 imin,
- imax;+ imax,+ delta;
Assert(nargs >= 2);
@@ -2464,7 +2465,8 @@ evalStandardFunc(CState *st,
pg_log_error("empty range given to random");
return false;
}
- else if (imax - imin < 0 || (imax - imin) + 1 < 0)+ else if (pg_sub_s64_overflow(imax, imin, &delta) ||+ pg_add_s64_overflow(delta, 1, &delta))
{
/* prevent int overflows in random functions */
pg_log_error("random range is too large");
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To: pgsql-hackers@postgresql.org
Cc: tgl@sss.pgh.pa.us, pgsql-hackers@lists.postgresql.org, coelho@cri.ensmp.fr
Subject: Re: Overflow hazard in pgbench
In-Reply-To: <73927.1624815543@sss.pgh.pa.us>
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