Re: [PATCH bpf-next,v3] riscv, bpf: Add BPF exception tables

[tongtiangen]

On 2021/10/28 7:11, Daniel Borkmann wrote:
> On 10/27/21 6:55 PM, Björn Töpel wrote:
> > On Wed, 27 Oct 2021 at 13:03, Tong Tiangen <tongtiangen@xxxxxxxxxx> 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 riscv 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.
> > >
> > > A more generic solution would add a "handler" field to the table entry,
> > > like on x86 and s390.
> > >
> > > The same issue in ARM64 is fixed in:
> > > commit 800834285361 ("bpf, arm64: Add BPF exception tables")
> > >
> > > Signed-off-by: Tong Tiangen <tongtiangen@xxxxxxxxxx>
> > > Tested-by: Pu Lehui <pulehui@xxxxxxxxxx>
> > > ---
> > > v3:
> > > Modify according to Björn's comments, mainly code optimization.
> >
> > Thank you!
> >
> > I ran this patch against the test_bpf.ko, and selftests/bpf -- no
> > regressions, and after the patch is applied more tests passes. Yay!
> >
> > On a related note. The RISC-V selftests/bpf is in a pretty lousy
> > state. I'll send a cleanup patch for them soonish. E.g.:
>
> Thanks for testing!
>
> > * RISC-V is missing in bpf_tracing.h (libbpf)
> > * Some programs don't converge in 16 steps, I had to increase it to ~32
> > * The selftest/bpf Makefile needed some RV specific changes
> > * ...a lot of tests still don't pass, and needs to be looked in to
>
> Sounds good, please ship them. ;)
>
> > Feel free to add:
> >
> > Acked-by: Björn Töpel <bjorn@xxxxxxxxxx>
>
> Applied, thanks! Tong, if you have a chance, please follow up with Mark's
> suggestion to align the extable infra to arm64/x86.

Thanks, Mark's suggestion is good. I will improve this part if I have the opportunity.
> Thanks,
> Daniel
> .