[PATCH bpf-next v3 5/9] arm32, bpf: add support for 32-bit signed division

Puranjay Mohan
2023-09-07T23:06:04Z

The cpuv4 added a new BPF_SDIV instruction that does signed division.
The encoding is similar to BPF_DIV but BPF_SDIV sets offset=1.

ARM32 already supports 32-bit BPF_DIV which can be easily extended to
support BPF_SDIV as ARM32 has the SDIV instruction. When the CPU is not
ARM-v7, we implement that SDIV/SMOD with the function call similar to
the implementation of DIV/MOD.

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 | 38 ++++++++++++++++++++++++++++++--------
arch/arm/net/bpf_jit_32.h | 2 ++
2 files changed, 32 insertions(+), 8 deletions(-)

diff --git a/arch/arm/net/bpf_jit_32.c b/arch/arm/net/bpf_jit_32.c
index a08eba850ddf..6939546f4ddf 100644
--- a/arch/arm/net/bpf_jit_32.c
+++ b/arch/arm/net/bpf_jit_32.c
@@ -228,6 +228,16 @@ static u32 jit_mod32(u32 dividend, u32 divisor)
return dividend % divisor;
}

+static s32 jit_sdiv32(s32 dividend, s32 divisor)
+{
+ return dividend / divisor;
+}
+
+static s32 jit_smod32(s32 dividend, s32 divisor)
+{
+ return dividend % divisor;
+}
+
static inline void _emit(int cond, u32 inst, struct jit_ctx *ctx)
{
inst |= (cond << 28);
@@ -477,17 +487,18 @@ static inline int epilogue_offset(const struct jit_ctx *ctx)
return to - from - 2;
}

-static inline void emit_udivmod(u8 rd, u8 rm, u8 rn, struct jit_ctx *ctx, u8 op)
+static inline void emit_udivmod(u8 rd, u8 rm, u8 rn, struct jit_ctx *ctx, u8 op, u8 sign)
{
const int exclude_mask = BIT(ARM_R0) | BIT(ARM_R1);
const s8 *tmp = bpf2a32[TMP_REG_1];
+ u32 dst;

#if __LINUX_ARM_ARCH__ == 7
if (elf_hwcap & HWCAP_IDIVA) {
- if (op == BPF_DIV)
- emit(ARM_UDIV(rd, rm, rn), ctx);
- else {
- emit(ARM_UDIV(ARM_IP, rm, rn), ctx);
+ if (op == BPF_DIV) {
+ emit(sign ? ARM_SDIV(rd, rm, rn) : ARM_UDIV(rd, rm, rn), ctx);
+ } else {
+ emit(sign ? ARM_SDIV(ARM_IP, rm, rn) : ARM_UDIV(ARM_IP, rm, rn), ctx);
emit(ARM_MLS(rd, rn, ARM_IP, rm), ctx);
}
return;
@@ -515,8 +526,19 @@ static inline void emit_udivmod(u8 rd, u8 rm, u8 rn, struct jit_ctx *ctx, u8 op)
emit(ARM_PUSH(CALLER_MASK & ~exclude_mask), ctx);

/* Call appropriate function */
- emit_mov_i(ARM_IP, op == BPF_DIV ?
- (u32)jit_udiv32 : (u32)jit_mod32, ctx);
+ if (sign) {
+ if (op == BPF_DIV)
+ dst = (u32)jit_sdiv32;
+ else
+ dst = (u32)jit_smod32;
+ } else {
+ if (op == BPF_DIV)
+ dst = (u32)jit_udiv32;
+ else
+ dst = (u32)jit_mod32;
+ }
+
+ emit_mov_i(ARM_IP, dst, ctx);
emit_blx_r(ARM_IP, ctx);

/* Restore caller-saved registers from stack */
@@ -1551,7 +1573,7 @@ static int build_insn(const struct bpf_insn *insn, struct jit_ctx *ctx)
rt = src_lo;
break;
}
- emit_udivmod(rd_lo, rd_lo, rt, ctx, BPF_OP(code));
+ emit_udivmod(rd_lo, rd_lo, rt, ctx, BPF_OP(code), off);
arm_bpf_put_reg32(dst_lo, rd_lo, ctx);
if (!ctx->prog->aux->verifier_zext)
emit_a32_mov_i(dst_hi, 0, ctx);
diff --git a/arch/arm/net/bpf_jit_32.h b/arch/arm/net/bpf_jit_32.h
index 79c7373fadce..438f0e1f91a0 100644
--- a/arch/arm/net/bpf_jit_32.h
+++ b/arch/arm/net/bpf_jit_32.h
@@ -139,6 +139,7 @@
#define ARM_INST_TST_I 0x03100000

#define ARM_INST_UDIV 0x0730f010
+#define ARM_INST_SDIV 0x0710f010

#define ARM_INST_UMULL 0x00800090

@@ -267,6 +268,7 @@
#define ARM_TST_I(rn, imm) _AL3_I(ARM_INST_TST, 0, rn, imm)

#define ARM_UDIV(rd, rn, rm) (ARM_INST_UDIV | (rd) << 16 | (rn) | (rm) << 8)
+#define ARM_SDIV(rd, rn, rm) (ARM_INST_SDIV | (rd) << 16 | (rn) | (rm) << 8)

#define ARM_UMULL(rd_lo, rd_hi, rn, rm) (ARM_INST_UMULL | (rd_hi) << 16 \
| (rd_lo) << 12 | (rm) << 8 | rn)
--
2.39.2