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

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Date:   Wed, 27 Oct 2021 21:26:38 +0800
From:   tongtiangen <tongtiangen@...wei.com>
To:     Mark Rutland <mark.rutland@....com>
CC:     Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Palmer Dabbelt <palmerdabbelt@...gle.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Björn Töpel <bjorn@...nel.org>,
        Luke Nelson <luke.r.nels@...il.com>,
        Xi Wang <xi.wang@...il.com>,
        "Alexei Starovoitov" <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        "Andrii Nakryiko" <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        <linux-riscv@...ts.infradead.org>, <linux-kernel@...r.kernel.org>,
        <netdev@...r.kernel.org>, <bpf@...r.kernel.org>
Subject: Re: [PATCH bpf-next,v3] riscv, bpf: Add BPF exception tables



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@linutronix.de/
>
> arm64:
>
>   https://lore.kernel.org/linux-arm-kernel/20211019160219.5202-11-mark.rutland@arm.com/
>
> ... and going forwards, riscv might want to do likewise.
>
> Thanks,
> Mark.
> .
>

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