linux-kernel - Re: llist code relies on undefined behaviour, upsets llvm/clang

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Message-ID: <20170116090540.GE3159@twins.programming.kicks-ass.net>
Date:   Mon, 16 Jan 2017 10:05:40 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Anton Blanchard <anton@...ba.org>
Cc:     behanw@...verseincode.com, ying.huang@...el.com,
        akpm@...ux-foundation.org, oleg@...hat.com,
        Segher Boessenkool <segher@...nel.crashing.org>, mingo@...e.hu,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: llist code relies on undefined behaviour, upsets llvm/clang

On Mon, Jan 16, 2017 at 08:36:00AM +1100, Anton Blanchard wrote:
> Hi,
> 
> I was debugging a hang on a ppc64le kernel built with clang, and it
> looks to be undefined behaviour with pointer wrapping in the llist code.
> 
> A test case is below. llist_for_each_entry() does container_of() on a
> NULL pointer, which wraps our pointer negative, then adds the same
> offset back in and expects to get back to NULL. Unfortunately clang
> decides that this can never be NULL and optimises it into an infinite
> loop.
> 
> Build with -DFIX, such that the llist_node has a zero offset from the
> start of the struct, and things work.
> 
> Is anyone other than ppc64le building kernels with llvm/clang these
> days? This should reproduce on ARM64 and x86-64.

Last I checked I couldn't build a x86_64 kernel with llvm. So no, not
something I've ever ran into.


Also, I would argue that this is broken in llvm, the kernel very much
relies on things like this all over the place. Sure, we're way outside
of what the C language spec says, but who bloody cares ;-)

If llvm wants to compile the kernel, it needs to learn the C dialect the
kernel uses.