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

[Naveen N. Rao]

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....@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.le...@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);