linux-kernel - Re: [PATCH bpf-next 1/1] arm64: bpf: Add BPF exception tables

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Message-ID: <20200730211453.GA79372@lophozonia>
Date:   Thu, 30 Jul 2020 23:14:53 +0200
From:   Jean-Philippe Brucker <jean-philippe@...aro.org>
To:     Daniel Borkmann <daniel@...earbox.net>
Cc:     Qian Cai <cai@....pw>, linux-arm-kernel@...ts.infradead.org,
        bpf@...r.kernel.org, songliubraving@...com, andriin@...com,
        catalin.marinas@....com, john.fastabend@...il.com, ast@...nel.org,
        zlim.lnx@...il.com, kpsingh@...omium.org, yhs@...com,
        will@...nel.org, kafai@...com, sfr@...b.auug.org.au,
        linux-next@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH bpf-next 1/1] arm64: bpf: Add BPF exception tables

On Thu, Jul 30, 2020 at 09:47:39PM +0200, Daniel Borkmann wrote:
> On 7/30/20 4:22 PM, Jean-Philippe Brucker wrote:
> > On Thu, Jul 30, 2020 at 08:28:56AM -0400, Qian Cai wrote:
> > > On Tue, Jul 28, 2020 at 05:21:26PM +0200, Jean-Philippe Brucker wrote:
> > > > When a tracing BPF program attempts to read memory without using the
> > > > bpf_probe_read() helper, the verifier marks the load instruction with
> > > > the BPF_PROBE_MEM flag. Since the arm64 JIT does not currently recognize
> > > > this flag it falls back to the interpreter.
> > > > 
> > > > Add support for BPF_PROBE_MEM, by appending an exception table to the
> > > > BPF program. If the load instruction causes a data abort, the fixup
> > > > infrastructure finds the exception table and fixes up the fault, by
> > > > clearing the destination register and jumping over the faulting
> > > > instruction.
> > > > 
> > > > To keep the compact exception table entry format, inspect the pc in
> > > > fixup_exception(). A more generic solution would add a "handler" field
> > > > to the table entry, like on x86 and s390.
> > > > 
> > > > Signed-off-by: Jean-Philippe Brucker <jean-philippe@...aro.org>
> > > 
> > > This will fail to compile on arm64,
> > > 
> > > https://gitlab.com/cailca/linux-mm/-/blob/master/arm64.config
> > > 
> > > arch/arm64/mm/extable.o: In function `fixup_exception':
> > > arch/arm64/mm/extable.c:19: undefined reference to `arm64_bpf_fixup_exception'
> > 
> > Thanks for the report, I attached a fix. Daniel, can I squash it and
> > resend as v2 or is it too late?
> 
> If you want I can squash your attached snippet into the original patch of
> yours. If you want to send a v2 that is fine as well of course. Let me know.

Yes please squash it into the original patch, sorry for the mess

Thanks,
Jean