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

[tongtiangen]

On 2021/10/27 19:50, Mark Rutland wrote:
> On Wed, Oct 27, 2021 at 11:18:22AM +0000, Tong Tiangen 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")
>
> > +#ifdef CONFIG_BPF_JIT
> > +int rv_bpf_fixup_exception(const struct exception_table_entry *ex, struct pt_regs *regs);
> > +#endif
> > +
> >  int fixup_exception(struct pt_regs *regs)
> >  {
> >  	const struct exception_table_entry *fixup;
> >
> >  	fixup = search_exception_tables(regs->epc);
> > -	if (fixup) {
> > -		regs->epc = fixup->fixup;
> > -		return 1;
> > -	}
> > -	return 0;
> > +	if (!fixup)
> > +		return 0;
> > +
> > +#ifdef CONFIG_BPF_JIT
> > +	if (regs->epc >= BPF_JIT_REGION_START && regs->epc < BPF_JIT_REGION_END)
> > +		return rv_bpf_fixup_exception(fixup, regs);
> > +#endif
> > +
> > +	regs->epc = fixup->fixup;
> > +	return 1;
> >  }
>
> As a heads-up, on the extable front, both arm64 and x86 are moving to
> having an enumerated "type" field to select the handler:
>
> x86:
>
>   https://lore.kernel.org/lkml/20210908132525.211958725@xxxxxxxxxxxxx/
>
> arm64:
>
>   https://lore.kernel.org/linux-arm-kernel/20211019160219.5202-11-mark.rutland@xxxxxxx/
>
> ... and going forwards, riscv might want to do likewise.
>
> Thanks,
> Mark.
> .

Thanks mark, I'm very interested in this change.