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

[Daniel Borkmann]

On 7/29/20 7:28 PM, Song Liu wrote:
> On Tue, Jul 28, 2020 at 8:37 AM Jean-Philippe Brucker
> <jean-philippe at linaro.org> 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 at linaro.org>
> 
> This patch looks good to me.
> 
> Acked-by: Song Liu <songliubraving at fb.com>

+1, applied, thanks a lot!

> It is possible to add a selftest for this? I thought about this a
> little bit, but
> didn't get a good idea.

Why not adding a test_verifier.c test case which calls into bpf_get_current_task()
to fetch pointer to current and then read out some field via BPF_PROBE_MEM which
should then succeed on x86/s390x/arm64 but be skipped on the other archs? Jean-Philippe,
could you look into following up with such test case(s)?

Thanks,
Daniel