Re: [PATCH] ppc: Fix BPF JIT for ABIv2

From: Michael Ellerman
Date: Tue Jun 21 2016 - 07:39:41 EST


On Tue, 2016-06-21 at 14:28 +0530, Naveen N. Rao wrote:
> On 2016/06/20 03:56PM, Thadeu Lima de Souza Cascardo wrote:
> > On Sun, Jun 19, 2016 at 11:19:14PM +0530, Naveen N. Rao wrote:
> > > On 2016/06/17 10:00AM, Thadeu Lima de Souza Cascardo wrote:
> > > >
> > > > Hi, Michael and Naveen.
> > > >
> > > > I noticed independently that there is a problem with BPF JIT and ABIv2, and
> > > > worked out the patch below before I noticed Naveen's patchset and the latest
> > > > changes in ppc tree for a better way to check for ABI versions.
> > > >
> > > > However, since the issue described below affect mainline and stable kernels,
> > > > would you consider applying it before merging your two patchsets, so that we can
> > > > more easily backport the fix?
> > >
> > > Hi Cascardo,
> > > Given that this has been broken on ABIv2 since forever, I didn't bother
> > > fixing it. But, I can see why this would be a good thing to have for
> > > -stable and existing distros. However, while your patch below may fix
> > > the crash you're seeing on ppc64le, it is not sufficient -- you'll need
> > > changes in bpf_jit_asm.S as well.
> >
> > Hi, Naveen.
> >
> > Any tips on how to exercise possible issues there? Or what changes you think
> > would be sufficient?
>
> The calling convention is different with ABIv2 and so we'll need changes
> in bpf_slow_path_common() and sk_negative_common().

How big would those changes be? Do we know?

How come no one reported this was broken previously? This is the first I've
heard of it being broken.

> However, rather than enabling classic JIT for ppc64le, are we better off
> just disabling it?
>
> --- a/arch/powerpc/Kconfig
> +++ b/arch/powerpc/Kconfig
> @@ -128,7 +128,7 @@ config PPC
> select IRQ_FORCED_THREADING
> select HAVE_RCU_TABLE_FREE if SMP
> select HAVE_SYSCALL_TRACEPOINTS
> - select HAVE_CBPF_JIT
> + select HAVE_CBPF_JIT if CPU_BIG_ENDIAN
> select HAVE_ARCH_JUMP_LABEL
> select ARCH_HAVE_NMI_SAFE_CMPXCHG
> select ARCH_HAS_GCOV_PROFILE_ALL
>
>
> Michael,
> Let me know your thoughts on whether you intend to take this patch or
> Cascardo's patch for -stable before the eBPF patches. I can redo my
> patches accordingly.

This patch sounds like the best option at the moment for something we can
backport. Unless the changes to fix it are minimal.

cheers