From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <gcc-bugzilla@gcc.gnu.org>
Received: by sourceware.org (Postfix, from userid 48)
	id 987713858C56; Mon,  4 Mar 2024 08:45:58 +0000 (GMT)
DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 987713858C56
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gcc.gnu.org;
	s=default; t=1709541958;
	bh=4oTJzjBQds8C5cU7nf82sRGc/A/QiTB0As9lfOJN8U8=;
	h=From:To:Subject:Date:In-Reply-To:References:From;
	b=ogYorixEeuNt+OQ8JTD0twR3Ug2zOr/GWfimbDrcBq3oAJU28XUheCMCsNIqt5bdV
	 Apnl+KvTPX3sIoQO6/0mnYjhj8rD20rZIkwdpJwYDk21Zdyjnlm+xr+Q0ygm2f+LtU
	 YtcrNYy5tt67mC6y4elV1Dmmterci/KES8j2Udv4=
From: "akihiko.odaki at daynix dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug sanitizer/114217] -fsanitize=alignment false positive with
 intended unaligned struct member access
Date: Mon, 04 Mar 2024 08:45:57 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: changed
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: sanitizer
X-Bugzilla-Version: 13.2.0
X-Bugzilla-Keywords: 
X-Bugzilla-Severity: normal
X-Bugzilla-Who: akihiko.odaki at daynix dot com
X-Bugzilla-Status: RESOLVED
X-Bugzilla-Resolution: INVALID
X-Bugzilla-Priority: P3
X-Bugzilla-Assigned-To: unassigned at gcc dot gnu.org
X-Bugzilla-Target-Milestone: ---
X-Bugzilla-Flags: 
X-Bugzilla-Changed-Fields: 
Message-ID: <bug-114217-4-oFTFQ38ojP@http.gcc.gnu.org/bugzilla/>
In-Reply-To: <bug-114217-4@http.gcc.gnu.org/bugzilla/>
References: <bug-114217-4@http.gcc.gnu.org/bugzilla/>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
X-Bugzilla-URL: http://gcc.gnu.org/bugzilla/
Auto-Submitted: auto-generated
MIME-Version: 1.0
List-Id: <gcc-bugs.sourceware.org>

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D114217

--- Comment #13 from Akihiko Odaki <akihiko.odaki at daynix dot com> ---
(In reply to Jakub Jelinek from comment #12)
> (In reply to Akihiko Odaki from comment #11)
> > So there are two constructs invoking UBs but ignored by UBSan: 1)
>=20
> That is an understatement. UBSan is a best effort which diagnoses some fo=
rms
> of undefined behavior.  There are tons of undefined behavior UBSan doesn't
> catch,
> most importantly e.g. aliasing violations, but far from limited to just t=
hat.
> If a program is diagnostic free with -fsanitize=3Dundefined,address , it
> doesn't mean it is UB free, but the goal is that if there is diagnostic,
> there is a real UB in the program.

Right. I just listed the two relevant constructs and don't intend to say th=
ey
are the only case that UBSan doesn't catch.

>=20
> You are basically asking for the PR80797 fix to be reverted just because =
you
> aren't willing to fix UB in your code.  That is not going to happen, we've
> been diagnosing this for almost 7 years now, I think clang even for 11
> years, it is a real UB and other projects have been able to cope with it.=
=20
> By reverting the change new UB in other programs couldn't be discovered.

I'm not asking for reverting PR80797. See f() and f2() I wrote earlier:
u64 f(struct dir_entry *entry)
{
    return get_unaligned(&entry->offset);
}

u64 f2(u64 *offset)
{
    return get_unaligned(offset);
}

Both f(NULL) and f2(NULL) should be caught and there should be no discrepan=
cy
in behavior for these two functions. However, there is a discrepancy when it
comes to -fsanitize=3Dalignment.

I'm not saying ignoring UB in f() is the only sensible option either. Catch=
ing
UBs in both f() and f2() is a logical option.=