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

[Christophe Leroy]

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....@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

                case BPF_ALU | BPF_MUL | BPF_X: /* (u32) dst *= (u32) src */