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

["Naveen N. Rao" <naveen.n.rao@xxxxxxxxxxxxxxxxxx>]

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@xxxxxxxxxxxxxxxxxx>
> > ---
> > 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@xxxxxxxxxx>

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);