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

["Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com>]

On 2016/06/21 11:47AM, Thadeu Lima de Souza Cascardo wrote:
> On Tue, Jun 21, 2016 at 09:15:48PM +1000, Michael Ellerman wrote:
> > 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?

I don't think it'd be that much -- I will take a stab at this today.

> > 
> > How come no one reported this was broken previously? This is the first I've
> > heard of it being broken.
> > 
> 
> I just heard of it less than two weeks ago, and only could investigate it last
> week, when I realized mainline was also affected.
> 
> It looks like the little-endian support for classic JIT were done before the
> conversion to ABIv2. And as JIT is disabled by default, no one seems to have
> exercised it.

Yes, my thoughts too. I didn't previously think much about this as JIT 
wouldn't be enabled by default. It's interesting though that no one else 
reported this as an issue before.

> 
> > > 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.

Right -- I will take a look today to see what changes would be needed.

- Naveen