From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 60BAC292B44; Tue, 16 Sep 2025 21:50:17 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1758059417; cv=none; b=Lyq0Agmm48GES1XC+UFidB4G31XkjTC1+TWF4dVAVLl5RUWW0s19YUMxZfdLA6VdgUpQtMRWJnibf7HzwQFH61cjU2/4SpQSh6s6nK8v5vOqSxRjGYBF2Yues/PVtS4pfitb6fF0PQy24OrPyMn7isAVXB/hnAlXZQUhqZX+KYA= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1758059417; c=relaxed/simple; bh=ZQ5vXZCra2+RwJM46CnghsruiNCvFvmuzVBMwnQAxV8=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=WJ8/3SoLTggNUB5tcboIfyE1Vo/WYrLqGPvvF9aoxlcRIA2tOEJdz/mM215LR2TsmoZ3X/dDF45qKOeOHqbSJYDSv19zWV0zmQSZwmHauESZiqx+eNfJItMa7jhIsS16bmAArCzJ/qRbNdS9Qlq4jx0b7ThsiTfd3LEdVaiKaK4= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=V+TrpXzU; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="V+TrpXzU" Received: by smtp.kernel.org (Postfix) with ESMTPSA id B7972C4CEEB; Tue, 16 Sep 2025 21:50:13 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1758059416; bh=ZQ5vXZCra2+RwJM46CnghsruiNCvFvmuzVBMwnQAxV8=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=V+TrpXzUl2eTgi4QAIadZ/pNqv4SSebpKU3um6A7/anUcuUGYZyj9EL6D0IGTRP7l LGw9Vctspzd2wyYumY6+hlmE5QoNEyrVnAial2rZOdpr6lJ2kAfh0l4YBsjHEZPx75 uqyKE07qlPxAeISvNqNsGLvGISga6JTeIVmT4nBWYhNyE5eUDLXu9mmj2DrMmAyFg8 dkXAjw1NJS5PNR1FKKPpQBvzDmq2fVa4WxBysqzBwlYh8XiWX/MzgZVcNluuzMEW99 xRLj940q2hWbWyuSkN/iWBNliBKT3Sw5fBuGq+8+9IZt49pC1glVE13Dd7fnXFtJ+z wx+sp4PwAsr6w== Date: Tue, 16 Sep 2025 14:50:11 -0700 From: Nathan Chancellor To: Thomas Gleixner Cc: LKML , Linus Torvalds , Peter Zijlstra , kernel test robot , Russell King , linux-arm-kernel@lists.infradead.org, Christophe Leroy , Darren Hart , Davidlohr Bueso , =?iso-8859-1?Q?Andr=E9?= Almeida , x86@kernel.org, Alexander Viro , Christian Brauner , Jan Kara , linux-fsdevel@vger.kernel.org Subject: Re: [patch V2a 2/6] kbuild: Disable CC_HAS_ASM_GOTO_OUTPUT on clang < version 17 Message-ID: <20250916215011.GA596283@ax162> References: <20250916163004.674341701@linutronix.de> <20250916163252.100835216@linutronix.de> <20250916184440.GA1245207@ax162> <87ikhi9lhg.ffs@tglx> <87frcm9kvv.ffs@tglx> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87frcm9kvv.ffs@tglx> On Tue, Sep 16, 2025 at 10:56:36PM +0200, Thomas Gleixner wrote: > clang < 17 fails to use scope local labels with CONFIG_CC_HAS_ASM_GOTO_OUTPUT=y: > > { > __label__ local_lbl; > ... > unsafe_get_user(uval, uaddr, local_lbl); > ... > return 0; > local_lbl: > return -EFAULT; > } > > when two such scopes exist in the same function: > > error: cannot jump from this asm goto statement to one of its possible targets > > There are other failure scenarios. Shuffling code around slightly makes it > worse and fail even with one instance. > > That issue prevents using local labels for a cleanup based user access > mechanism. > > After failed attempts to provide a simple enough test case for the 'depends > on' test in Kconfig, the initial cure was to mark ASM goto broken on clang > versions < 17 to get this road block out of the way. > > But Nathan pointed out that this is a known clang issue and indeed affects > clang < version 17 in combination with cleanup(). It's not even required to > use local labels for that. > > The clang issue tracker has a small enough test case, which can be used as > a test in the 'depends on' section of CC_HAS_ASM_GOTO_OUTPUT: > > void bar(void **); > void* baz(); I would recommend void* baz(void); here and in the actual test to preemptively harden against the possibility of a future where -Wstrict-prototypes is turned on as an error by default (as unlikely as this may be, it has been brought up before [1]), as I would not want this to get silently disabled. > int foo (void) { > { > asm goto("jmp %l0"::::l0); > return 0; > l0: > return 1; > } > void *x __attribute__((cleanup(bar))) = baz(); > { > asm goto("jmp %l0"::::l1); > return 42; > l1: > return 0xff; > } > } > > Add another dependency to config CC_HAS_ASM_GOTO_OUTPUT for it and use the > clang issue tracker test case for detection by condensing it to obfuscated > C-code contest format. This reliably catches the problem on clang < 17 and > did not show any issues on the non known to be broken GCC versions. > > That test might be sufficient to catch all issues and therefore could > replace the existing test, but keeping that around does no harm either. > > Thanks to Nathan for pointing to the relevant clang issue! > > Suggested-by: Nathan Chancellor > Signed-off-by: Thomas Gleixner > Cc: Nathan Chancellor > Link: https://github.com/ClangBuiltLinux/linux/issues/1886 > Link: https://github.com/llvm/llvm-project/commit/f023f5cdb2e6c19026f04a15b5a935c041835d14 Reviewed-by: Nathan Chancellor > --- > V2a: Use the reproducer from llvm > V2: New patch > --- > init/Kconfig | 3 +++ > 1 file changed, 3 insertions(+) > > --- a/init/Kconfig > +++ b/init/Kconfig > @@ -99,7 +99,10 @@ config GCC_ASM_GOTO_OUTPUT_BROKEN > config CC_HAS_ASM_GOTO_OUTPUT > def_bool y > depends on !GCC_ASM_GOTO_OUTPUT_BROKEN > + # Find basic issues Maybe "Detect basic support" or something like that? This is not really an "issues" test, more of a "does the compiler support it at all?" test if I understand correctly. > depends on $(success,echo 'int foo(int x) { asm goto ("": "=r"(x) ::: bar); return x; bar: return 0; }' | $(CC) -x c - -c -o /dev/null) > + # Detect buggy clang, fixed in clang-17 > + depends on $(success,echo 'void b(void **);void* c();int f(void){{asm goto("jmp %l0"::::l0);return 0;l0:return 1;}void *x __attribute__((cleanup(b))) = c();{asm goto("jmp %l0"::::l1);return 2;l1:return 1;}}' | $(CC) -x c - -c -o /dev/null) > > config CC_HAS_ASM_GOTO_TIED_OUTPUT > depends on CC_HAS_ASM_GOTO_OUTPUT