From 669cf41f912cec76c9f8a47033aeb6177befef06 Mon Sep 17 00:00:00 2001 From: Xu Kuohai Date: Wed, 16 Mar 2022 12:26:21 -0400 Subject: [PATCH] bpf, arm64: adjust the offset of str/ldr(immediate) to positive number The BPF STX/LDX instruction uses offset relative to the FP to address stack space. Since the BPF_FP locates at the top of the frame, the offset is usually a negative number. However, arm64 str/ldr immediate instruction requires that offset be a positive number. Therefore, this patch tries to convert the offsets. The method is to find the negative offset furthest from the FP firstly. Then add it to the FP, calculate a bottom position, called FPB, and then adjust the offsets in other STR/LDX instructions relative to FPB. FPB is saved using the callee-saved register x27 of arm64 which is not used yet. Before adjusting the offset, the patch checks every instruction to ensure that the FP does not change in run-time. If the FP may change, no offset is adjusted. For example, for the following bpftrace command: bpftrace -e 'kprobe:do_sys_open { printf("opening: %s\n", str(arg1)); }' Without this patch, jited code(fragment): 0: bti c 4: stp x29, x30, [sp, #-16]! 8: mov x29, sp c: stp x19, x20, [sp, #-16]! 10: stp x21, x22, [sp, #-16]! 14: stp x25, x26, [sp, #-16]! 18: mov x25, sp 1c: mov x26, #0x0 // #0 20: bti j 24: sub sp, sp, #0x90 28: add x19, x0, #0x0 2c: mov x0, #0x0 // #0 30: mov x10, #0xffffffffffffff78 // #-136 34: str x0, [x25, x10] 38: mov x10, #0xffffffffffffff80 // #-128 3c: str x0, [x25, x10] 40: mov x10, #0xffffffffffffff88 // #-120 44: str x0, [x25, x10] 48: mov x10, #0xffffffffffffff90 // #-112 4c: str x0, [x25, x10] 50: mov x10, #0xffffffffffffff98 // #-104 54: str x0, [x25, x10] 58: mov x10, #0xffffffffffffffa0 // #-96 5c: str x0, [x25, x10] 60: mov x10, #0xffffffffffffffa8 // #-88 64: str x0, [x25, x10] 68: mov x10, #0xffffffffffffffb0 // #-80 6c: str x0, [x25, x10] 70: mov x10, #0xffffffffffffffb8 // #-72 74: str x0, [x25, x10] 78: mov x10, #0xffffffffffffffc0 // #-64 7c: str x0, [x25, x10] 80: mov x10, #0xffffffffffffffc8 // #-56 84: str x0, [x25, x10] 88: mov x10, #0xffffffffffffffd0 // #-48 8c: str x0, [x25, x10] 90: mov x10, #0xffffffffffffffd8 // #-40 94: str x0, [x25, x10] 98: mov x10, #0xffffffffffffffe0 // #-32 9c: str x0, [x25, x10] a0: mov x10, #0xffffffffffffffe8 // #-24 a4: str x0, [x25, x10] a8: mov x10, #0xfffffffffffffff0 // #-16 ac: str x0, [x25, x10] b0: mov x10, #0xfffffffffffffff8 // #-8 b4: str x0, [x25, x10] b8: mov x10, #0x8 // #8 bc: ldr x2, [x19, x10] [...] With this patch, jited code(fragment): 0: bti c 4: stp x29, x30, [sp, #-16]! 8: mov x29, sp c: stp x19, x20, [sp, #-16]! 10: stp x21, x22, [sp, #-16]! 14: stp x25, x26, [sp, #-16]! 18: stp x27, x28, [sp, #-16]! 1c: mov x25, sp 20: sub x27, x25, #0x88 24: mov x26, #0x0 // #0 28: bti j 2c: sub sp, sp, #0x90 30: add x19, x0, #0x0 34: mov x0, #0x0 // #0 38: str x0, [x27] 3c: str x0, [x27, #8] 40: str x0, [x27, #16] 44: str x0, [x27, #24] 48: str x0, [x27, #32] 4c: str x0, [x27, #40] 50: str x0, [x27, #48] 54: str x0, [x27, #56] 58: str x0, [x27, #64] 5c: str x0, [x27, #72] 60: str x0, [x27, #80] 64: str x0, [x27, #88] 68: str x0, [x27, #96] 6c: str x0, [x27, #104] 70: str x0, [x27, #112] 74: str x0, [x27, #120] 78: str x0, [x27, #128] 7c: ldr x2, [x19, #8] [...] Signed-off-by: Xu Kuohai --- arch/arm64/net/bpf_jit_comp.c | 84 ++++++++++++++++++++++++++++++++--- 1 file changed, 79 insertions(+), 5 deletions(-) diff --git a/arch/arm64/net/bpf_jit_comp.c b/arch/arm64/net/bpf_jit_comp.c index e56274ce001a..9e1fd7a1e08f 100644 --- a/arch/arm64/net/bpf_jit_comp.c +++ b/arch/arm64/net/bpf_jit_comp.c @@ -26,6 +26,7 @@ #define TMP_REG_2 (MAX_BPF_JIT_REG + 1) #define TCALL_CNT (MAX_BPF_JIT_REG + 2) #define TMP_REG_3 (MAX_BPF_JIT_REG + 3) +#define FP_BOTTOM (MAX_BPF_JIT_REG + 4) #define check_imm(bits, imm) do { \ if ((((imm) > 0) && ((imm) >> (bits))) || \ @@ -63,6 +64,7 @@ static const int bpf2a64[] = { [TCALL_CNT] = A64_R(26), /* temporary register for blinding constants */ [BPF_REG_AX] = A64_R(9), + [FP_BOTTOM] = A64_R(27), }; struct jit_ctx { @@ -73,6 +75,7 @@ struct jit_ctx { int exentry_idx; __le32 *image; u32 stack_size; + int fpb_offset; }; static inline void emit(const u32 insn, struct jit_ctx *ctx) @@ -234,9 +237,9 @@ static noinline bool is_lsi_offset(s16 offset, int scale) /* Tail call offset to jump into */ #if IS_ENABLED(CONFIG_ARM64_BTI_KERNEL) -#define PROLOGUE_OFFSET 8 +#define PROLOGUE_OFFSET 10 #else -#define PROLOGUE_OFFSET 7 +#define PROLOGUE_OFFSET 9 #endif static int build_prologue(struct jit_ctx *ctx, bool ebpf_from_cbpf) @@ -248,6 +251,7 @@ static int build_prologue(struct jit_ctx *ctx, bool ebpf_from_cbpf) const u8 r9 = bpf2a64[BPF_REG_9]; const u8 fp = bpf2a64[BPF_REG_FP]; const u8 tcc = bpf2a64[TCALL_CNT]; + const u8 fpb = bpf2a64[FP_BOTTOM]; const int idx0 = ctx->idx; int cur_offset; @@ -286,9 +290,11 @@ static int build_prologue(struct jit_ctx *ctx, bool ebpf_from_cbpf) emit(A64_PUSH(r6, r7, A64_SP), ctx); emit(A64_PUSH(r8, r9, A64_SP), ctx); emit(A64_PUSH(fp, tcc, A64_SP), ctx); + emit(A64_PUSH(fpb, A64_R(28), A64_SP), ctx); /* Set up BPF prog stack base register */ emit(A64_MOV(1, fp, A64_SP), ctx); + emit(A64_SUB_I(1, fpb, fp, ctx->fpb_offset), ctx); if (!ebpf_from_cbpf) { /* Initialize tail_call_cnt */ @@ -553,11 +559,13 @@ static void build_epilogue(struct jit_ctx *ctx) const u8 r8 = bpf2a64[BPF_REG_8]; const u8 r9 = bpf2a64[BPF_REG_9]; const u8 fp = bpf2a64[BPF_REG_FP]; + const u8 fpb = bpf2a64[FP_BOTTOM]; /* We're done with BPF stack */ emit(A64_ADD_I(1, A64_SP, A64_SP, ctx->stack_size), ctx); /* Restore fs (x25) and x26 */ + emit(A64_POP(fpb, A64_R(28), A64_SP), ctx); emit(A64_POP(fp, A64_R(26), A64_SP), ctx); /* Restore callee-saved register */ @@ -645,12 +653,14 @@ static int add_exception_handler(const struct bpf_insn *insn, static int build_insn(const struct bpf_insn *insn, struct jit_ctx *ctx, bool extra_pass) { + u8 dst = bpf2a64[insn->dst_reg]; + u8 src = bpf2a64[insn->src_reg]; + s16 off = insn->off; + const u8 fp = bpf2a64[BPF_REG_FP]; const u8 code = insn->code; - const u8 dst = bpf2a64[insn->dst_reg]; - const u8 src = bpf2a64[insn->src_reg]; const u8 tmp = bpf2a64[TMP_REG_1]; const u8 tmp2 = bpf2a64[TMP_REG_2]; - const s16 off = insn->off; + const u8 fpb = bpf2a64[FP_BOTTOM]; const s32 imm = insn->imm; const int i = insn - ctx->prog->insnsi; const bool is64 = BPF_CLASS(code) == BPF_ALU64 || @@ -1012,6 +1022,10 @@ static int build_insn(const struct bpf_insn *insn, struct jit_ctx *ctx, case BPF_LDX | BPF_PROBE_MEM | BPF_W: case BPF_LDX | BPF_PROBE_MEM | BPF_H: case BPF_LDX | BPF_PROBE_MEM | BPF_B: + if (ctx->fpb_offset > 0 && src == fp) { + src = fpb; + off += ctx->fpb_offset; + } switch (BPF_SIZE(code)) { case BPF_W: if (is_lsi_offset(off, 2)) { @@ -1070,6 +1084,10 @@ static int build_insn(const struct bpf_insn *insn, struct jit_ctx *ctx, case BPF_ST | BPF_MEM | BPF_H: case BPF_ST | BPF_MEM | BPF_B: case BPF_ST | BPF_MEM | BPF_DW: + if (ctx->fpb_offset > 0 && dst == fp) { + dst = fpb; + off += ctx->fpb_offset; + } /* Load imm to a register then store it */ emit_a64_mov_i(1, tmp, imm, ctx); switch (BPF_SIZE(code)) { @@ -1113,6 +1131,10 @@ static int build_insn(const struct bpf_insn *insn, struct jit_ctx *ctx, case BPF_STX | BPF_MEM | BPF_H: case BPF_STX | BPF_MEM | BPF_B: case BPF_STX | BPF_MEM | BPF_DW: + if (ctx->fpb_offset > 0 && dst == fp) { + dst = fpb; + off += ctx->fpb_offset; + } switch (BPF_SIZE(code)) { case BPF_W: if (is_lsi_offset(off, 2)) { @@ -1167,6 +1189,56 @@ static int build_insn(const struct bpf_insn *insn, struct jit_ctx *ctx, return 0; } +/* + * Return 0 if FP may change at runtime, otherwise find the minimum negative + * offset to FP and converts it to positive number. + */ +static int find_fpb_offset(struct bpf_prog *prog) +{ + int i; + int offset = 0; + + for (i = 0; i < prog->len; i++) { + const struct bpf_insn *insn = &prog->insnsi[i]; + + switch (BPF_CLASS(insn->code)) { + case BPF_STX: + case BPF_ST: + if (BPF_MODE(insn->code) == BPF_ATOMIC) { + if ((insn->imm == BPF_XCHG || + insn->imm == (BPF_ADD | BPF_FETCH) || + insn->imm == (BPF_AND | BPF_FETCH) || + insn->imm == (BPF_OR | BPF_FETCH) || + insn->imm == (BPF_XOR | BPF_FETCH)) && + insn->src_reg == BPF_REG_FP) { + return 0; + } + } + if (BPF_MODE(insn->code) == BPF_MEM && + insn->dst_reg == BPF_REG_FP) { + if (insn->off < offset) + offset = insn->off; + } + break; + + case BPF_JMP32: + case BPF_JMP: + break; + + case BPF_ALU: + case BPF_ALU64: + case BPF_LDX: + case BPF_LD: + default: + if (insn->dst_reg == BPF_REG_FP) + return 0; + } + } + + /* safely be converted to a positive 'int', since insn->off is 's16' */ + return -offset; +} + static int build_body(struct jit_ctx *ctx, bool extra_pass) { const struct bpf_prog *prog = ctx->prog; @@ -1288,6 +1360,8 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog) goto out_off; } + ctx.fpb_offset = find_fpb_offset(prog); + /* * 1. Initial fake pass to compute ctx->idx and ctx->offset. *