[RESEND PATCH bpf-next 1/2] bpf, arm64: Jit BPF_CALL to direct call when possible

[Xu Kuohai]

From: Xu Kuohai <xukuohai@xxxxxxxxxx>

Currently BPF_CALL is always jited to indirect call, but when target is
in the range of direct call, BPF_CALL can be jited to direct call.

For example, the following BPF_CALL

    call __htab_map_lookup_elem

is always jited to an indirect call:

    mov     x10, #0xffffffffffff18f4
    movk    x10, #0x821, lsl #16
    movk    x10, #0x8000, lsl #32
    blr     x10

When the target is in the range of direct call, it can be jited to:

    bl      0xfffffffffd33bc98

This patch does such jit when possible.

1. First pass, get the maximum jited image size. Since the jited image
   memory is not allocated yet, the distance between jited BPF_CALL
   instructon and call target is unknown, so jit all BPF_CALL to indirect
   call to get the maximum image size.

2. Allocate image memory with the size caculated in step 1.

3. Second pass, determine the jited address and size for every bpf instruction.
   Since image memory is now allocated and there is only one jit method for
   bpf instructions other than BPF_CALL, so the jited address for the first
   BPF_CALL is determined, so the distance to call target is determined, so
   the first BPF_CALL is determined to be jited to direct or indirect call,
   so the jited image size after the first BPF_CALL is determined. By analogy,
   the jited addresses and sizes for all subsequent BPF instructions are
   determined.

4. Last pass, generate the final image. The jump offset of jump instruction
   whose target is within the jited image is determined in this pass, since
   the target instruction address may be changed in step 3.

Tested with test_bpf.ko and some arm64 working selftests, nothing failed.

Signed-off-by: Xu Kuohai <xukuohai@xxxxxxxxxx>
---
 arch/arm64/net/bpf_jit_comp.c | 71 ++++++++++++++++++++++++++++-------
 1 file changed, 58 insertions(+), 13 deletions(-)

diff --git a/arch/arm64/net/bpf_jit_comp.c b/arch/arm64/net/bpf_jit_comp.c
index 30f76178608b..06437e34614b 100644
--- a/arch/arm64/net/bpf_jit_comp.c
+++ b/arch/arm64/net/bpf_jit_comp.c
@@ -72,6 +72,7 @@ static const int bpf2a64[] = {
 struct jit_ctx {
 	const struct bpf_prog *prog;
 	int idx;
+	bool write;
 	int epilogue_offset;
 	int *offset;
 	int exentry_idx;
@@ -91,7 +92,7 @@ struct bpf_plt {
 
 static inline void emit(const u32 insn, struct jit_ctx *ctx)
 {
-	if (ctx->image != NULL)
+	if (ctx->image != NULL && ctx->write)
 		ctx->image[ctx->idx] = cpu_to_le32(insn);
 
 	ctx->idx++;
@@ -178,10 +179,29 @@ static inline void emit_addr_mov_i64(const int reg, const u64 val,
 
 static inline void emit_call(u64 target, struct jit_ctx *ctx)
 {
-	u8 tmp = bpf2a64[TMP_REG_1];
+	u8 tmp;
+	long offset;
+	unsigned long pc;
+	u32 insn = AARCH64_BREAK_FAULT;
+
+	/* if ctx->image == NULL or target == 0, the jump distance is unknown,
+	 * emit indirect call.
+	 */
+	if (ctx->image && target) {
+		pc = (unsigned long)&ctx->image[ctx->idx];
+		offset = (long)target - (long)pc;
+		if (offset >= -SZ_128M && offset < SZ_128M)
+			insn = aarch64_insn_gen_branch_imm(pc, target,
+					AARCH64_INSN_BRANCH_LINK);
+	}
 
-	emit_addr_mov_i64(tmp, target, ctx);
-	emit(A64_BLR(tmp), ctx);
+	if (insn == AARCH64_BREAK_FAULT) {
+		tmp = bpf2a64[TMP_REG_1];
+		emit_addr_mov_i64(tmp, target, ctx);
+		emit(A64_BLR(tmp), ctx);
+	} else {
+		emit(insn, ctx);
+	}
 }
 
 static inline int bpf2a64_offset(int bpf_insn, int off,
@@ -1392,13 +1412,11 @@ static int build_body(struct jit_ctx *ctx, bool extra_pass)
 		const struct bpf_insn *insn = &prog->insnsi[i];
 		int ret;
 
-		if (ctx->image == NULL)
-			ctx->offset[i] = ctx->idx;
+		ctx->offset[i] = ctx->idx;
 		ret = build_insn(insn, ctx, extra_pass);
 		if (ret > 0) {
 			i++;
-			if (ctx->image == NULL)
-				ctx->offset[i] = ctx->idx;
+			ctx->offset[i] = ctx->idx;
 			continue;
 		}
 		if (ret)
@@ -1409,8 +1427,7 @@ static int build_body(struct jit_ctx *ctx, bool extra_pass)
 	 * the last element with the offset after the last
 	 * instruction (end of program)
 	 */
-	if (ctx->image == NULL)
-		ctx->offset[i] = ctx->idx;
+	ctx->offset[i] = ctx->idx;
 
 	return 0;
 }
@@ -1461,6 +1478,8 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)
 	bool extra_pass = false;
 	struct jit_ctx ctx;
 	u8 *image_ptr;
+	int body_idx;
+	int exentry_idx;
 
 	if (!prog->jit_requested)
 		return orig_prog;
@@ -1515,6 +1534,7 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)
 		goto out_off;
 	}
 
+	/* Get the max image size */
 	if (build_body(&ctx, extra_pass)) {
 		prog = orig_prog;
 		goto out_off;
@@ -1528,7 +1548,7 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)
 	extable_size = prog->aux->num_exentries *
 		sizeof(struct exception_table_entry);
 
-	/* Now we know the actual image size. */
+	/* Now we know the max image size. */
 	prog_size = sizeof(u32) * ctx.idx;
 	/* also allocate space for plt target */
 	extable_offset = round_up(prog_size + PLT_TARGET_SIZE, extable_align);
@@ -1548,15 +1568,37 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)
 skip_init_ctx:
 	ctx.idx = 0;
 	ctx.exentry_idx = 0;
+	ctx.write = true;
 
 	build_prologue(&ctx, was_classic);
 
+	/* Record exentry_idx and ctx.idx before first build_body */
+	exentry_idx = ctx.exentry_idx;
+	body_idx = ctx.idx;
+	/* Don't write instruction to memory for now */
+	ctx.write = false;
+
+	/* Determine call distance and instruction position */
 	if (build_body(&ctx, extra_pass)) {
 		bpf_jit_binary_free(header);
 		prog = orig_prog;
 		goto out_off;
 	}
 
+	ctx.epilogue_offset = ctx.idx;
+
+	ctx.exentry_idx = exentry_idx;
+	ctx.idx = body_idx;
+	ctx.write = true;
+
+	/* Determine jump offset and write result to memory */
+	if (build_body(&ctx, extra_pass) ||
+		WARN_ON_ONCE(ctx.idx != ctx.epilogue_offset)) {
+		bpf_jit_binary_free(header);
+		prog = orig_prog;
+		goto out_off;
+	}
+
 	build_epilogue(&ctx);
 	build_plt(&ctx);
 
@@ -1567,6 +1609,8 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)
 		goto out_off;
 	}
 
+	/* Update prog size */
+	prog_size = sizeof(u32) * ctx.idx;
 	/* And we're done. */
 	if (bpf_jit_enable > 1)
 		bpf_jit_dump(prog->len, prog_size, 2, ctx.image);
@@ -1574,8 +1618,8 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)
 	bpf_flush_icache(header, ctx.image + ctx.idx);
 
 	if (!prog->is_func || extra_pass) {
-		if (extra_pass && ctx.idx != jit_data->ctx.idx) {
-			pr_err_once("multi-func JIT bug %d != %d\n",
+		if (extra_pass && ctx.idx > jit_data->ctx.idx) {
+			pr_err_once("multi-func JIT bug %d > %d\n",
 				    ctx.idx, jit_data->ctx.idx);
 			bpf_jit_binary_free(header);
 			prog->bpf_func = NULL;
@@ -1976,6 +2020,7 @@ int arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *image,
 	struct jit_ctx ctx = {
 		.image = NULL,
 		.idx = 0,
+		.write = true,
 	};
 
 	/* the first 8 arguments are passed by registers */
-- 
2.30.2