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

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Message-ID: <20200730142213.GB1529030@myrica>
Date:   Thu, 30 Jul 2020 16:22:13 +0200
From:   Jean-Philippe Brucker <jean-philippe@...aro.org>
To:     Qian Cai <cai@....pw>
Cc:     linux-arm-kernel@...ts.infradead.org, bpf@...r.kernel.org,
        songliubraving@...com, andriin@...com, daniel@...earbox.net,
        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 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?

I'd be more confident if my patches sat a little longer on the list so
arm64 folks have a chance to review them. This isn't my first silly
mistake...

Thanks,
Jean

View attachment "0001-arm64-bpf-Fix-build-for-CONFIG_BPF_JIT.patch" of type "text/plain" (1690 bytes)