Re: [PATCH v2 04/10] powerpc/bpf: Fix BPF_SUB when imm == 0x80000000

["Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com>]

Christophe Leroy wrote:
> 
> 
> Le 05/10/2021 à 22:25, Naveen N. Rao a écrit :
>> We aren't handling subtraction involving an immediate value of
>> 0x80000000 properly. Fix the same.
>> 
>> Fixes: 156d0e290e969c ("powerpc/ebpf/jit: Implement JIT compiler for extended BPF")
>> Signed-off-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
>> ---
>> Changelog:
>> - Split up BPF_ADD and BPF_SUB cases per Christophe's comments
>> 
>>   arch/powerpc/net/bpf_jit_comp64.c | 27 +++++++++++++++++----------
>>   1 file changed, 17 insertions(+), 10 deletions(-)
>> 
>> diff --git a/arch/powerpc/net/bpf_jit_comp64.c b/arch/powerpc/net/bpf_jit_comp64.c
>> index d67f6d62e2e1ff..6626e6c17d4ed2 100644
>> --- a/arch/powerpc/net/bpf_jit_comp64.c
>> +++ b/arch/powerpc/net/bpf_jit_comp64.c
>> @@ -330,18 +330,25 @@ int bpf_jit_build_body(struct bpf_prog *fp, u32 *image, struct codegen_context *
>>   			EMIT(PPC_RAW_SUB(dst_reg, dst_reg, src_reg));
>>   			goto bpf_alu32_trunc;
>>   		case BPF_ALU | BPF_ADD | BPF_K: /* (u32) dst += (u32) imm */
>> -		case BPF_ALU | BPF_SUB | BPF_K: /* (u32) dst -= (u32) imm */
>>   		case BPF_ALU64 | BPF_ADD | BPF_K: /* dst += imm */
>> +			if (!imm) {
>> +				goto bpf_alu32_trunc;
>> +			} else if (imm >= -32768 && imm < 32768) {
>> +				EMIT(PPC_RAW_ADDI(dst_reg, dst_reg, IMM_L(imm)));
>> +			} else {
>> +				PPC_LI32(b2p[TMP_REG_1], imm);
>> +				EMIT(PPC_RAW_ADD(dst_reg, dst_reg, b2p[TMP_REG_1]));
>> +			}
>> +			goto bpf_alu32_trunc;
>> +		case BPF_ALU | BPF_SUB | BPF_K: /* (u32) dst -= (u32) imm */
>>   		case BPF_ALU64 | BPF_SUB | BPF_K: /* dst -= imm */
>> -			if (BPF_OP(code) == BPF_SUB)
>> -				imm = -imm;
>> -			if (imm) {
>> -				if (imm >= -32768 && imm < 32768)
>> -					EMIT(PPC_RAW_ADDI(dst_reg, dst_reg, IMM_L(imm)));
>> -				else {
>> -					PPC_LI32(b2p[TMP_REG_1], imm);
>> -					EMIT(PPC_RAW_ADD(dst_reg, dst_reg, b2p[TMP_REG_1]));
>> -				}
>> +			if (!imm) {
>> +				goto bpf_alu32_trunc;
>> +			} else if (imm > -32768 && imm < 32768) {
> 
> Why do you exclude imm == 32768 ?
> 
> Reviewed-by: Christophe Leroy <christophe.leroy@csgroup.eu>

Good catch -- that was from an earlier version where this was shared 
across BPF_ADD and BPF_SUB. I missed updating this section before 
posting.

Michael, please consider squashing in the below diff into this patch.

Thanks!
- Naveen


---
diff --git a/arch/powerpc/net/bpf_jit_comp64.c b/arch/powerpc/net/bpf_jit_comp64.c
index f5a804d8c95bc1..0fdc1ff86e4f1c 100644
--- a/arch/powerpc/net/bpf_jit_comp64.c
+++ b/arch/powerpc/net/bpf_jit_comp64.c
@@ -368,7 +368,7 @@ int bpf_jit_build_body(struct bpf_prog *fp, u32 *image, struct codegen_context *
 		case BPF_ALU64 | BPF_SUB | BPF_K: /* dst -= imm */
 			if (!imm) {
 				goto bpf_alu32_trunc;
-			} else if (imm > -32768 && imm < 32768) {
+			} else if (imm > -32768 && imm <= 32768) {
 				EMIT(PPC_RAW_ADDI(dst_reg, dst_reg, IMM_L(-imm)));
 			} else {
 				PPC_LI32(b2p[TMP_REG_1], imm);