[PATCH 6/9] powerpc/bpf: Fix BPF_SUB when imm == 0x80000000

[Naveen N. Rao]

Christophe Leroy wrote:
> 
> 
> Le 01/10/2021 à 23:14, 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 at linux.vnet.ibm.com>
>> ---
>>   arch/powerpc/net/bpf_jit_comp64.c | 16 ++++++++--------
>>   1 file changed, 8 insertions(+), 8 deletions(-)
>> 
>> diff --git a/arch/powerpc/net/bpf_jit_comp64.c b/arch/powerpc/net/bpf_jit_comp64.c
>> index ffb7a2877a8469..4641a50e82d50d 100644
>> --- a/arch/powerpc/net/bpf_jit_comp64.c
>> +++ b/arch/powerpc/net/bpf_jit_comp64.c
>> @@ -333,15 +333,15 @@ int bpf_jit_build_body(struct bpf_prog *fp, u32 *image, struct codegen_context *
>>   		case BPF_ALU | BPF_SUB | BPF_K: /* (u32) dst -= (u32) imm */
>>   		case BPF_ALU64 | BPF_ADD | BPF_K: /* dst += 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);
>> +			if (imm > -32768 && imm < 32768) {
>> +				EMIT(PPC_RAW_ADDI(dst_reg, dst_reg,
>> +					BPF_OP(code) == BPF_SUB ? IMM_L(-imm) : IMM_L(imm)));
>> +			} else {
>> +				PPC_LI32(b2p[TMP_REG_1], imm);
>> +				if (BPF_OP(code) == BPF_SUB)
>> +					EMIT(PPC_RAW_SUB(dst_reg, dst_reg, b2p[TMP_REG_1]));
>> +				else
>>   					EMIT(PPC_RAW_ADD(dst_reg, dst_reg, b2p[TMP_REG_1]));
>> -				}
>>   			}
>>   			goto bpf_alu32_trunc;
> 
> There is now so few code common to both BPF_ADD and BPF_SUB that you 
> should make them different cases.
> 
> While at it, why not also use ADDIS if imm is 32 bits ? That would be an 
> ADDIS/ADDI instead of LIS/ORI/ADD

Sure. I wanted to limit the change for this fix. We can do a separate 
patch to optimize code generation for BPF_ADD.


- Naveen