[PATCH bpf-next v3 3/9] arm32, bpf: add support for sign-extension mov instruction

Puranjay Mohan
2023-09-07T23:06:01Z

The cpuv4 added a new BPF_MOVSX instruction that sign extends the src
before moving it to the destination.

BPF_ALU | BPF_MOVSX sign extends 8-bit and 16-bit operands into 32-bit
operands, and zeroes the remaining upper 32 bits.

BPF_ALU64 | BPF_MOVSX sign extends 8-bit, 16-bit, and 32-bit operands
into 64-bit operands.

The offset field of the instruction is used to tell the number of bit to
use for sign-extension. BPF_MOV and BPF_MOVSX have the same code but the
former sets offset to 0 and the later one sets the offset to 8, 16 or 32

The behaviour of this instruction is dst = (s8,s16,s32)src

On ARM32 the implementation uses LSH and ARSH to extend the 8/16 bits to
a 32-bit register and then it is sign extended to the upper 32-bit
register using ARSH. For 32-bit we just move it to the destination
register and use ARSH to extend it to the upper 32-bit 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 | 35 ++++++++++++++++++++++++++++++-----
1 file changed, 30 insertions(+), 5 deletions(-)

diff --git a/arch/arm/net/bpf_jit_32.c b/arch/arm/net/bpf_jit_32.c
index 1b3bd05878ab..29a1ccf761fd 100644
--- a/arch/arm/net/bpf_jit_32.c
+++ b/arch/arm/net/bpf_jit_32.c
@@ -747,12 +747,16 @@ static inline void emit_a32_alu_r64(const bool is64, const s8 dst[],
}

/* dst = src (4 bytes)*/
-static inline void emit_a32_mov_r(const s8 dst, const s8 src,
+static inline void emit_a32_mov_r(const s8 dst, const s8 src, const u8 off,
struct jit_ctx *ctx) {
const s8 *tmp = bpf2a32[TMP_REG_1];
s8 rt;

rt = arm_bpf_get_reg32(src, tmp[0], ctx);
+ if (off && off != 32) {
+ emit(ARM_LSL_I(rt, rt, 32 - off), ctx);
+ emit(ARM_ASR_I(rt, rt, 32 - off), ctx);
+ }
arm_bpf_put_reg32(dst, rt, ctx);
}

@@ -761,15 +765,15 @@ static inline void emit_a32_mov_r64(const bool is64, const s8 dst[],
const s8 src[],
struct jit_ctx *ctx) {
if (!is64) {
- emit_a32_mov_r(dst_lo, src_lo, ctx);
+ emit_a32_mov_r(dst_lo, src_lo, 0, ctx);
if (!ctx->prog->aux->verifier_zext)
/* Zero out high 4 bytes */
emit_a32_mov_i(dst_hi, 0, ctx);
} else if (__LINUX_ARM_ARCH__ < 6 &&
ctx->cpu_architecture < CPU_ARCH_ARMv5TE) {
/* complete 8 byte move */
- emit_a32_mov_r(dst_lo, src_lo, ctx);
- emit_a32_mov_r(dst_hi, src_hi, ctx);
+ emit_a32_mov_r(dst_lo, src_lo, 0, ctx);
+ emit_a32_mov_r(dst_hi, src_hi, 0, ctx);
} else if (is_stacked(src_lo) && is_stacked(dst_lo)) {
const u8 *tmp = bpf2a32[TMP_REG_1];

@@ -785,6 +789,24 @@ static inline void emit_a32_mov_r64(const bool is64, const s8 dst[],
}
}

+/* dst = (signed)src */
+static inline void emit_a32_movsx_r64(const bool is64, const u8 off, const s8 dst[], const s8 src[],
+ struct jit_ctx *ctx) {
+ const s8 *tmp = bpf2a32[TMP_REG_1];
+ const s8 *rt;
+
+ rt = arm_bpf_get_reg64(dst, tmp, ctx);
+
+ emit_a32_mov_r(dst_lo, src_lo, off, ctx);
+ if (!is64) {
+ if (!ctx->prog->aux->verifier_zext)
+ /* Zero out high 4 bytes */
+ emit_a32_mov_i(dst_hi, 0, ctx);
+ } else {
+ emit(ARM_ASR_I(rt[0], rt[1], 31), ctx);
+ }
+}
+
/* Shift operations */
static inline void emit_a32_alu_i(const s8 dst, const u32 val,
struct jit_ctx *ctx, const u8 op) {
@@ -1450,7 +1472,10 @@ static int build_insn(const struct bpf_insn *insn, struct jit_ctx *ctx)
emit_a32_mov_i(dst_hi, 0, ctx);
break;
}
- emit_a32_mov_r64(is64, dst, src, ctx);
+ if (insn->off)
+ emit_a32_movsx_r64(is64, insn->off, dst, src, ctx);
+ else
+ emit_a32_mov_r64(is64, dst, src, ctx);
break;
case BPF_K:
/* Sign-extend immediate value to destination reg */
--
2.39.2