Re: [HEADS-UP] Problem with clang in 9-stable [was: r268244 (stable/9) seems to break "sysctl hw.ncpu"]

On Sat, Jul 05, 2014 at 01:49:44PM +0100, David Chisnall wrote:
> On 4 Jul 2014, at 19:18, David Wolfskill <david_at_catwhisker.org> wrote:
> 
> > clang -O2 -pipe  -std=gnu99 -Qunused-arguments  -fstack-protector -Wsystem-headers -Werror -Wall -Wno-format-y2k -W -Wno-unused-parameter -Wstrict-prototypes -Wmissing-prototypes -Wpointer-arith -Wno-uninitialized -Wno-pointer-sign -Wno-empty-body -Wno-string-plus-int -Wno-unused-const-variable -Wno-tautological-compare -Wno-unused-value -Wno-parentheses-equality -Wno-unused-function -Wno-enum-conversion  -o sysctl sysctl.o 
> 
> This compile line is turning off a lot of warnings.  In particular, -Wno-uninitialized and -Wno-parentheses-equality are likely to hide warnings that refer to real errors.  It sounds like this case was one of them - if these warnings were on then we'd have got a build failure rather than an executable that depended on undefined behaviour.
> ....

So I checked src/sbin/sysctl/Makefile first; it's fairly "vanilla":

#       _at_(#)Makefile    8.1 (Berkeley) 6/6/93
# $FreeBSD: stable/9/sbin/sysctl/Makefile 203917 2010-02-15 14:08:06Z
uqs $

PROG=   sysctl
WARNS?= 3
MAN=    sysctl.8

.include <bsd.prog.mk>

And the -Wno-uninitialized (at least) comes from bsd.sys.mk:

.if ${WARNS} >= 2 && ${WARNS} <= 4
# XXX Delete -Wuninitialized by default for now -- the compiler doesn't
# XXX always get it right.
CWARNFLAGS+=    -Wno-uninitialized
.endif # WARNS >=2 && WARNS <= 4

A bit later, we see the origin of -Wno-parentheses-equality:

# Clang has more warnings enabled by default, and when using -Wall, so if WARNS
# is set to low values, these have to be disabled explicitly.
.if ${COMPILER_TYPE} == "clang" && !defined(EARLY_BUILD)
.if ${WARNS} <= 6
CWARNFLAGS+=    -Wno-empty-body -Wno-string-plus-int -Wno-unused-const-variable
.endif # WARNS <= 6
.if ${WARNS} <= 3
CWARNFLAGS+=    -Wno-tautological-compare -Wno-unused-value\
                -Wno-parentheses-equality -Wno-unused-function -Wno-enum-conversion
.endif # WARNS <= 3
.if ${WARNS} <= 2
CWARNFLAGS+=    -Wno-switch -Wno-switch-enum -Wno-knr-promoted-parameter
.endif # WARNS <= 2
.if ${WARNS} <= 1
CWARNFLAGS+=    -Wno-parentheses
.endif # WARNS <= 1
.if defined(NO_WARRAY_BOUNDS)
CWARNFLAGS+=    -Wno-array-bounds
.endif # NO_WARRAY_BOUNDS
.endif # CLANG


I don't know that there's likely to be a huge amount of interest
in addressing the issue for stable/9, but stable/10 looks similar,
and while I see some differences in head, the code in head's
bsd.sys.mk may well be functionally equivalent.

I'm happy to help test if someone wants to put together patches to
(at least) reduce the extent to which we have executables depending
on undefined behavior.

Peace,
david
-- 
David H. Wolfskill				david_at_catwhisker.org
Taliban: Evil cowards with guns afraid of truth from a 14-year old girl.

See http://www.catwhisker.org/~david/publickey.gpg for my public key.