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Subject: Re: [PATCH 4/4] bpf powerpc: Add addr > TASK_SIZE_MAX explicit check
To:     Christophe Leroy <christophe.leroy@csgroup.eu>
Cc:     naveen.n.rao@linux.ibm.com, mpe@ellerman.id.au, ast@kernel.org,
        daniel@iogearbox.net, songliubraving@fb.com,
        netdev@vger.kernel.org, john.fastabend@gmail.com,
        andrii@kernel.org, kpsingh@kernel.org, paulus@samba.org,
        sandipan@linux.ibm.com, yhs@fb.com, bpf@vger.kernel.org,
        linuxppc-dev@lists.ozlabs.org, kafai@fb.com,
        linux-kernel@vger.kernel.org,
        Ravi Bangoria <ravi.bangoria@linux.ibm.com>
References: <20210706073211.349889-1-ravi.bangoria@linux.ibm.com>
 <20210706073211.349889-5-ravi.bangoria@linux.ibm.com>
 <74f55f12-c7da-a06d-c3a5-6869b907e3f6@csgroup.eu>
From:   Ravi Bangoria <ravi.bangoria@linux.ibm.com>
Message-ID: <8c4fb89e-626e-fd0d-5703-e3916924785a@linux.ibm.com>
Date:   Wed, 7 Jul 2021 09:36:33 +0530
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>> @@ -763,6 +771,14 @@ int bpf_jit_build_body(struct bpf_prog *fp, u32 *image, struct codegen_context *
>>           /* dst = *(u16 *)(ul) (src + off) */
>>           case BPF_LDX | BPF_MEM | BPF_H:
>>           case BPF_LDX | BPF_PROBE_MEM | BPF_H:
>> +            if (BPF_MODE(code) == BPF_PROBE_MEM) {
>> +                EMIT(PPC_RAW_ADDI(b2p[TMP_REG_1], src_reg, off));
>> +                PPC_LI64(b2p[TMP_REG_2], TASK_SIZE_MAX);
>> +                EMIT(PPC_RAW_CMPLD(b2p[TMP_REG_1], b2p[TMP_REG_2]));
>> +                PPC_BCC(COND_GT, (ctx->idx + 4) * 4);
>> +                EMIT(PPC_RAW_XOR(dst_reg, dst_reg, dst_reg));
>> +                PPC_JMP((ctx->idx + 2) * 4);
>> +            }
> 
> That code seems strictly identical to the previous one and the next one.
> Can you refactor in a function ?

I'll check this.

> 
>>               EMIT(PPC_RAW_LHZ(dst_reg, src_reg, off));
>>               if (insn_is_zext(&insn[i + 1]))
>>                   addrs[++i] = ctx->idx * 4;
>> @@ -773,6 +789,14 @@ int bpf_jit_build_body(struct bpf_prog *fp, u32 *image, struct codegen_context *
>>           /* dst = *(u32 *)(ul) (src + off) */
>>           case BPF_LDX | BPF_MEM | BPF_W:
>>           case BPF_LDX | BPF_PROBE_MEM | BPF_W:
>> +            if (BPF_MODE(code) == BPF_PROBE_MEM) {
>> +                EMIT(PPC_RAW_ADDI(b2p[TMP_REG_1], src_reg, off));
>> +                PPC_LI64(b2p[TMP_REG_2], TASK_SIZE_MAX);
>> +                EMIT(PPC_RAW_CMPLD(b2p[TMP_REG_1], b2p[TMP_REG_2]));
>> +                PPC_BCC(COND_GT, (ctx->idx + 4) * 4);
>> +                EMIT(PPC_RAW_XOR(dst_reg, dst_reg, dst_reg));
>> +                PPC_JMP((ctx->idx + 2) * 4);
>> +            }
>>               EMIT(PPC_RAW_LWZ(dst_reg, src_reg, off));
>>               if (insn_is_zext(&insn[i + 1]))
>>                   addrs[++i] = ctx->idx * 4;
>> @@ -783,6 +807,20 @@ int bpf_jit_build_body(struct bpf_prog *fp, u32 *image, struct codegen_context *
>>           /* dst = *(u64 *)(ul) (src + off) */
>>           case BPF_LDX | BPF_MEM | BPF_DW:
>>           case BPF_LDX | BPF_PROBE_MEM | BPF_DW:
>> +            if (BPF_MODE(code) == BPF_PROBE_MEM) {
>> +                EMIT(PPC_RAW_ADDI(b2p[TMP_REG_1], src_reg, off));
>> +                PPC_LI64(b2p[TMP_REG_2], TASK_SIZE_MAX);
>> +                EMIT(PPC_RAW_CMPLD(b2p[TMP_REG_1], b2p[TMP_REG_2]));
>> +                if (off % 4)
> 
> That test is worth a comment.

(off % 4) test is based on how PPC_BPF_LL() emits instruction.

> 
> And I'd prefer
> 
>      if (off & 3) {
>          PPC_BCC(COND_GT, (ctx->idx + 5) * 4);
>          EMIT(PPC_RAW_XOR(dst_reg, dst_reg, dst_reg));
>          PPC_JMP((ctx->idx + 3) * 4);
>      } else {
>          PPC_BCC(COND_GT, (ctx->idx + 4) * 4);
>          EMIT(PPC_RAW_XOR(dst_reg, dst_reg, dst_reg));
>          PPC_JMP((ctx->idx + 2) * 4);
>      }

Yes this is neat.

Thanks for the review,
Ravi