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Message-ID: <0ade59ea-6863-4d68-607c-22e4b9405a0d@huaweicloud.com>
Date: Tue, 6 Dec 2022 11:37:49 +0800
From: Pu Lehui <pulehui@...weicloud.com>
To: Björn Töpel <bjorn@...nel.org>,
bpf@...r.kernel.org, linux-riscv@...ts.infradead.org,
linux-kernel@...r.kernel.org
Cc: Alexei Starovoitov <ast@...nel.org>,
Daniel Borkmann <daniel@...earbox.net>,
Andrii Nakryiko <andrii@...nel.org>,
Martin KaFai Lau <martin.lau@...ux.dev>,
Song Liu <song@...nel.org>, Yonghong Song <yhs@...com>,
John Fastabend <john.fastabend@...il.com>,
KP Singh <kpsingh@...nel.org>,
Stanislav Fomichev <sdf@...gle.com>,
Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
Paul Walmsley <paul.walmsley@...ive.com>,
Palmer Dabbelt <palmer@...belt.com>,
Albert Ou <aou@...s.berkeley.edu>,
Pu Lehui <pulehui@...wei.com>
Subject: Re: [PATCH bpf v2] riscv, bpf: Emit fixed-length instructions for
BPF_PSEUDO_FUNC
On 2022/12/2 18:54, Björn Töpel wrote:
> Pu Lehui <pulehui@...weicloud.com> writes:
>
>> From: Pu Lehui <pulehui@...wei.com>
>>
>> For BPF_PSEUDO_FUNC instruction, verifier will refill imm with
>> correct addresses of bpf_calls and then run last pass of JIT.
>> Since the emit_imm of RV64 is variable-length, which will emit
>> appropriate length instructions accorroding to the imm, it may
>> broke ctx->offset, and lead to unpredictable problem, such as
>> inaccurate jump. So let's fix it with fixed-length instructions.
>>
>> Fixes: 69c087ba6225 ("bpf: Add bpf_for_each_map_elem() helper")
>> Signed-off-by: Pu Lehui <pulehui@...wei.com>
>> Suggested-by: Björn Töpel <bjorn@...osinc.com>
>> ---
>> arch/riscv/net/bpf_jit_comp64.c | 20 +++++++++++++++++++-
>> 1 file changed, 19 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/riscv/net/bpf_jit_comp64.c b/arch/riscv/net/bpf_jit_comp64.c
>> index eb99df41fa33..9723f34f7a06 100644
>> --- a/arch/riscv/net/bpf_jit_comp64.c
>> +++ b/arch/riscv/net/bpf_jit_comp64.c
>> @@ -139,6 +139,19 @@ static bool in_auipc_jalr_range(s64 val)
>> val < ((1L << 31) - (1L << 11));
>> }
>>
>> +/* Emit fixed-length instructions for address */
>> +static void emit_addr(u8 rd, u64 addr, struct rv_jit_context *ctx)
>> +{
>> + u64 ip = (u64)(ctx->insns + ctx->ninsns);
>> + s64 off = addr - ip;
>> + s64 upper = (off + (1 << 11)) >> 12;
>> + s64 lower = ((off & 0xfff) << 52) >> 52;
>> +
>> + emit(rv_auipc(rd, upper), ctx);
>> + emit(rv_addi(rd, rd, lower), ctx);
>> +}
>
> Nice! Two instructions are better than 6! :-)
>
> One final thing. Please add a sanity check, that the range is correct,
> e.g.:
>
> if (!(addr && in_auipc_addi_range(off)))
> return -1;
>
Hi Björn,
Sorry for replying so late. For BPF_PSEUDO_FUNC instruction, verifier
will set insn[0].imm and insn[1].imm to 1 that make addr to 0x100000001
before extra pass, and also ctx->insns is NULL in iteration stage, all
of these make off out of range of AUIPC-ADDI range, and return failed.
We could add some special handling at different stages, but that seems a
little weird. By the way, I do not really like emit_addr function with
return value.
While a proper address is at least 2B alignment, and the valid address
is from 0xffffffff00000000 to 0xffffffffffffffff, we can make address
shifed 1 place to right, and addr >> 1 will always in the range of
AUIPC-ADDI range. We can get rid of the range detection. The
implementation is as follows:
static void emit_addr(u8 rd, u64 addr, struct rv_jit_context *ctx)
{
s64 imm = addr >> 1;
s64 upper = (imm + (1 << 11)) >> 12;
s64 lower = imm & 0xfff;
emit(rv_lui(rd, upper), ctx);
emit(rv_addi(rd, rd, lower), ctx);
emit(rv_slli(rd, rd, 1), ctx);
}
What do you think?
Regards,
Lehui
> Have a look at emit_jump_and_link().
>
>
> Thanks!
> Björn
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