[PATCH bpf-next v3 2/9] arm32, bpf: add support for sign-extension load instruction

Puranjay Mohan
2023-09-07T23:06:00Z

The cpuv4 added the support of an instruction that is similar to load
but also sign-extends the result after the load.

BPF_MEMSX | <size> | BPF_LDX means dst = *(signed size *) (src + offset)
here <size> can be one of BPF_B, BPF_H, BPF_W.

ARM32 has instructions to load a byte or a half word with sign
extension into a 32bit register. As the JIT uses two 32 bit registers
to simulate a 64-bit BPF register, an extra instruction is emitted to
sign-extent the result up to the second register.

Signed-off-by: Puranjay Mohan <puranjay12@gmail.com>
Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
---
arch/arm/net/bpf_jit_32.c | 74 ++++++++++++++++++++++++++++++++++++++-
arch/arm/net/bpf_jit_32.h | 2 ++
2 files changed, 75 insertions(+), 1 deletion(-)

diff --git a/arch/arm/net/bpf_jit_32.c b/arch/arm/net/bpf_jit_32.c
index c03600fe86f6..1b3bd05878ab 100644
--- a/arch/arm/net/bpf_jit_32.c
+++ b/arch/arm/net/bpf_jit_32.c
@@ -333,6 +333,9 @@ static u32 arm_bpf_ldst_imm8(u32 op, u8 rt, u8 rn, s16 imm8)
#define ARM_LDRD_I(rt, rn, off) arm_bpf_ldst_imm8(ARM_INST_LDRD_I, rt, rn, off)
#define ARM_LDRH_I(rt, rn, off) arm_bpf_ldst_imm8(ARM_INST_LDRH_I, rt, rn, off)

+#define ARM_LDRSH_I(rt, rn, off) arm_bpf_ldst_imm8(ARM_INST_LDRSH_I, rt, rn, off)
+#define ARM_LDRSB_I(rt, rn, off) arm_bpf_ldst_imm8(ARM_INST_LDRSB_I, rt, rn, off)
+
#define ARM_STR_I(rt, rn, off) arm_bpf_ldst_imm12(ARM_INST_STR_I, rt, rn, off)
#define ARM_STRB_I(rt, rn, off) arm_bpf_ldst_imm12(ARM_INST_STRB_I, rt, rn, off)
#define ARM_STRD_I(rt, rn, off) arm_bpf_ldst_imm8(ARM_INST_STRD_I, rt, rn, off)
@@ -1026,6 +1029,24 @@ static bool is_ldst_imm(s16 off, const u8 size)
return -off_max <= off && off <= off_max;
}

+static bool is_ldst_imm8(s16 off, const u8 size)
+{
+ s16 off_max = 0;
+
+ switch (size) {
+ case BPF_B:
+ off_max = 0xff;
+ break;
+ case BPF_W:
+ off_max = 0xfff;
+ break;
+ case BPF_H:
+ off_max = 0xff;
+ break;
+ }
+ return -off_max <= off && off <= off_max;
+}
+
/* *(size *)(dst + off) = src */
static inline void emit_str_r(const s8 dst, const s8 src[],
s16 off, struct jit_ctx *ctx, const u8 sz){
@@ -1105,6 +1126,50 @@ static inline void emit_ldx_r(const s8 dst[], const s8 src,
arm_bpf_put_reg64(dst, rd, ctx);
}

+/* dst = *(signed size*)(src + off) */
+static inline void emit_ldsx_r(const s8 dst[], const s8 src,
+ s16 off, struct jit_ctx *ctx, const u8 sz){
+ const s8 *tmp = bpf2a32[TMP_REG_1];
+ const s8 *rd = is_stacked(dst_lo) ? tmp : dst;
+ s8 rm = src;
+ int add_off;
+
+ if (!is_ldst_imm8(off, sz)) {
+ /*
+ * offset does not fit in the load/store immediate,
+ * construct an ADD instruction to apply the offset.
+ */
+ add_off = imm8m(off);
+ if (add_off > 0) {
+ emit(ARM_ADD_I(tmp[0], src, add_off), ctx);
+ rm = tmp[0];
+ } else {
+ emit_a32_mov_i(tmp[0], off, ctx);
+ emit(ARM_ADD_R(tmp[0], tmp[0], src), ctx);
+ rm = tmp[0];
+ }
+ off = 0;
+ }
+
+ switch (sz) {
+ case BPF_B:
+ /* Load a Byte with sign extension*/
+ emit(ARM_LDRSB_I(rd[1], rm, off), ctx);
+ break;
+ case BPF_H:
+ /* Load a HalfWord with sign extension*/
+ emit(ARM_LDRSH_I(rd[1], rm, off), ctx);
+ break;
+ case BPF_W:
+ /* Load a Word*/
+ emit(ARM_LDR_I(rd[1], rm, off), ctx);
+ break;
+ }
+ /* Carry the sign extension to upper 32 bits */
+ emit(ARM_ASR_I(rd[0], rd[1], 31), ctx);
+ arm_bpf_put_reg64(dst, rd, ctx);
+}
+
/* Arithmatic Operation */
static inline void emit_ar_r(const u8 rd, const u8 rt, const u8 rm,
const u8 rn, struct jit_ctx *ctx, u8 op,
@@ -1603,8 +1668,15 @@ static int build_insn(const struct bpf_insn *insn, struct jit_ctx *ctx)
case BPF_LDX | BPF_MEM | BPF_H:
case BPF_LDX | BPF_MEM | BPF_B:
case BPF_LDX | BPF_MEM | BPF_DW:
+ /* LDSX: dst = *(signed size *)(src + off) */
+ case BPF_LDX | BPF_MEMSX | BPF_B:
+ case BPF_LDX | BPF_MEMSX | BPF_H:
+ case BPF_LDX | BPF_MEMSX | BPF_W:
rn = arm_bpf_get_reg32(src_lo, tmp2[1], ctx);
- emit_ldx_r(dst, rn, off, ctx, BPF_SIZE(code));
+ if (BPF_MODE(insn->code) == BPF_MEMSX)
+ emit_ldsx_r(dst, rn, off, ctx, BPF_SIZE(code));
+ else
+ emit_ldx_r(dst, rn, off, ctx, BPF_SIZE(code));
break;
/* speculation barrier */
case BPF_ST | BPF_NOSPEC:
diff --git a/arch/arm/net/bpf_jit_32.h b/arch/arm/net/bpf_jit_32.h
index e0b593a1498d..79c7373fadce 100644
--- a/arch/arm/net/bpf_jit_32.h
+++ b/arch/arm/net/bpf_jit_32.h
@@ -79,9 +79,11 @@
#define ARM_INST_LDST__IMM12 0x00000fff
#define ARM_INST_LDRB_I 0x05500000
#define ARM_INST_LDRB_R 0x07d00000
+#define ARM_INST_LDRSB_I 0x015000d0
#define ARM_INST_LDRD_I 0x014000d0
#define ARM_INST_LDRH_I 0x015000b0
#define ARM_INST_LDRH_R 0x019000b0
+#define ARM_INST_LDRSH_I 0x015000f0
#define ARM_INST_LDR_I 0x05100000
#define ARM_INST_LDR_R 0x07900000

--
2.39.2