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

[Mark Rutland]

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.