LoongArch: BPF: Fix tailcall hierarchy

In specific use cases combining tailcalls and BPF-to-BPF calls，
MAX_TAIL_CALL_CNT won't work because of missing tail_call_cnt
back-propagation from callee to caller。This patch fixes this
tailcall issue caused by abusing the tailcall in bpf2bpf feature
on LoongArch like the way of "bpf, x64: Fix tailcall hierarchy".

push tail_call_cnt_ptr and tail_call_cnt into the stack,
tail_call_cnt_ptr is passed between tailcall and bpf2bpf,
uses tail_call_cnt_ptr to increment tail_call_cnt.

Signed-off-by: Haoran Jiang <[email protected]>

Author: Haoran Jiang

arch/loongarch/net/bpf_jit.c

@@ -12,6 +12,9 @@
 #define SAVE_RA		BIT(0)
 #define SAVE_TCC	BIT(1)
 
+#define BPF_TAIL_CALL_CNT_PTR_STACK_OFF(stack) (round_up(stack, 16) - 80)
+
+
 static const int regmap[] = {
 	/* return value from in-kernel function, and exit value for eBPF program */
 	[BPF_REG_0] = LOONGARCH_GPR_A5,
@@ -32,32 +35,37 @@ static const int regmap[] = {
 	[BPF_REG_AX] = LOONGARCH_GPR_T0,
 };
 
-static void mark_call(struct jit_ctx *ctx)
+static void prepare_bpf_tail_call_cnt(struct jit_ctx *ctx, int *store_offset)
 {
-	ctx->flags |= SAVE_RA;
-}
+	const struct bpf_prog *prog = ctx->prog;
+	const bool is_main_prog = !bpf_is_subprog(prog);
 
-static void mark_tail_call(struct jit_ctx *ctx)
-{
-	ctx->flags |= SAVE_TCC;
-}
+	if (is_main_prog) {
+		emit_insn(ctx, addid, LOONGARCH_GPR_T3, LOONGARCH_GPR_ZERO, MAX_TAIL_CALL_CNT);
+		*store_offset -= sizeof(long);
 
-static bool seen_call(struct jit_ctx *ctx)
-{
-	return (ctx->flags & SAVE_RA);
-}
+		emit_tailcall_jmp(ctx, BPF_JGT, REG_TCC, LOONGARCH_GPR_T3, 4);
 
-static bool seen_tail_call(struct jit_ctx *ctx)
-{
-	return (ctx->flags & SAVE_TCC);
-}
+		/* If REG_TCC < MAX_TAIL_CALL_CNT, push REG_TCC into stack */
+		emit_insn(ctx, std, REG_TCC, LOONGARCH_GPR_SP, *store_offset);
 
-static u8 tail_call_reg(struct jit_ctx *ctx)
-{
-	if (seen_call(ctx))
-		return TCC_SAVED;
+		/* Calculate the pointer to REG_TCC in the stack and assign it to REG_TCC */
+		emit_insn(ctx, addid, REG_TCC, LOONGARCH_GPR_SP, *store_offset);
+
+		emit_uncond_jmp(ctx, 2);
+
+		emit_insn(ctx, std, REG_TCC, LOONGARCH_GPR_SP, *store_offset);
 
-	return REG_TCC;
+		*store_offset -= sizeof(long);
+		emit_insn(ctx, std, REG_TCC, LOONGARCH_GPR_SP, *store_offset);
+
+	} else {
+		*store_offset -= sizeof(long);
+		emit_insn(ctx, std, REG_TCC, LOONGARCH_GPR_SP, *store_offset);
+
+		*store_offset -= sizeof(long);
+		emit_insn(ctx, std, REG_TCC, LOONGARCH_GPR_SP, *store_offset);
+	}
 }
 
 /*
@@ -80,6 +88,10 @@ static u8 tail_call_reg(struct jit_ctx *ctx)
  *                            |           $s4           |
  *                            +-------------------------+
  *                            |           $s5           |
+ *                            +-------------------------+
+ *                            |          reg_tcc        |
+ *                            +-------------------------+
+ *                            |          reg_tcc_ptr    |
  *                            +-------------------------+ <--BPF_REG_FP
  *                            |  prog->aux->stack_depth |
  *                            |        (optional)       |
@@ -89,21 +101,24 @@ static u8 tail_call_reg(struct jit_ctx *ctx)
 static void build_prologue(struct jit_ctx *ctx)
 {
 	int stack_adjust = 0, store_offset, bpf_stack_adjust;
+	const struct bpf_prog *prog = ctx->prog;
+	const bool is_main_prog = !bpf_is_subprog(prog);
 
 	bpf_stack_adjust = round_up(ctx->prog->aux->stack_depth, 16);
 
-	/* To store ra, fp, s0, s1, s2, s3, s4 and s5. */
-	stack_adjust += sizeof(long) * 8;
+	/* To store ra, fp, s0, s1, s2, s3, s4, s5, reg_tcc and reg_tcc_ptr */
+	stack_adjust += sizeof(long) * 10;
 
 	stack_adjust = round_up(stack_adjust, 16);
 	stack_adjust += bpf_stack_adjust;
 
 	/*
-	 * First instruction initializes the tail call count (TCC).
-	 * On tail call we skip this instruction, and the TCC is
+	 * First instruction initializes the tail call count (TCC) register
+	 * to zero. On tail call we skip this instruction, and the TCC is
 	 * passed in REG_TCC from the caller.
 	 */
-	emit_insn(ctx, addid, REG_TCC, LOONGARCH_GPR_ZERO, MAX_TAIL_CALL_CNT);
+	if (is_main_prog)
+		emit_insn(ctx, addid, REG_TCC, LOONGARCH_GPR_ZERO, 0);
 
 	emit_insn(ctx, addid, LOONGARCH_GPR_SP, LOONGARCH_GPR_SP, -stack_adjust);
 
@@ -131,20 +146,13 @@ static void build_prologue(struct jit_ctx *ctx)
 	store_offset -= sizeof(long);
 	emit_insn(ctx, std, LOONGARCH_GPR_S5, LOONGARCH_GPR_SP, store_offset);
 
+	prepare_bpf_tail_call_cnt(ctx, &store_offset);
+
 	emit_insn(ctx, addid, LOONGARCH_GPR_FP, LOONGARCH_GPR_SP, stack_adjust);
 
 	if (bpf_stack_adjust)
 		emit_insn(ctx, addid, regmap[BPF_REG_FP], LOONGARCH_GPR_SP, bpf_stack_adjust);
 
-	/*
-	 * Program contains calls and tail calls, so REG_TCC need
-	 * to be saved across calls.
-	 */
-	if (seen_tail_call(ctx) && seen_call(ctx))
-		move_reg(ctx, TCC_SAVED, REG_TCC);
-	else
-		emit_insn(ctx, nop);
-
 	ctx->stack_size = stack_adjust;
 }
 
@@ -177,6 +185,17 @@ static void __build_epilogue(struct jit_ctx *ctx, bool is_tail_call)
 	load_offset -= sizeof(long);
 	emit_insn(ctx, ldd, LOONGARCH_GPR_S5, LOONGARCH_GPR_SP, load_offset);
 
+	/*
+	 *  When push into the stack, follow the order of tcc then tcc_ptr.
+	 *  When pop from the stack, first pop tcc_ptr followed by tcc
+	 */
+	load_offset -= 2*sizeof(long);
+	emit_insn(ctx, ldd, REG_TCC, LOONGARCH_GPR_SP, load_offset);
+
+	/* pop tcc_ptr to REG_TCC */
+	load_offset += sizeof(long);
+	emit_insn(ctx, ldd, REG_TCC, LOONGARCH_GPR_SP, load_offset);
+
 	emit_insn(ctx, addid, LOONGARCH_GPR_SP, LOONGARCH_GPR_SP, stack_adjust);
 
 	if (!is_tail_call) {
@@ -211,7 +230,8 @@ bool bpf_jit_supports_far_kfunc_call(void)
 static int emit_bpf_tail_call(int insn, struct jit_ctx *ctx)
 {
 	int off;
-	u8 tcc = tail_call_reg(ctx);
+	int tcc_ptr_off = BPF_TAIL_CALL_CNT_PTR_STACK_OFF(ctx->stack_size);
+
 	u8 a1 = LOONGARCH_GPR_A1;
 	u8 a2 = LOONGARCH_GPR_A2;
 	u8 t1 = LOONGARCH_GPR_T1;
@@ -239,12 +259,17 @@ static int emit_bpf_tail_call(int insn, struct jit_ctx *ctx)
 		goto toofar;
 
 	/*
-	 * if (--TCC < 0)
-	 *	 goto out;
+	 * if ((*tcc_ptr)++ >= MAX_TAIL_CALL_CNT)
+	 *      goto out;
 	 */
-	emit_insn(ctx, addid, REG_TCC, tcc, -1);
+	emit_insn(ctx, ldd, REG_TCC, LOONGARCH_GPR_SP, tcc_ptr_off);
+	emit_insn(ctx, ldd, t3, REG_TCC, 0);
+	emit_insn(ctx, addid, t3, t3, 1);
+	emit_insn(ctx, std, t3, REG_TCC, 0);
+	emit_insn(ctx, addid, t2, LOONGARCH_GPR_ZERO, MAX_TAIL_CALL_CNT);
+
 	jmp_offset = tc_ninsn - (ctx->idx - idx0);
-	if (emit_tailcall_jmp(ctx, BPF_JSLT, REG_TCC, LOONGARCH_GPR_ZERO, jmp_offset) < 0)
+	if (emit_tailcall_jmp(ctx, BPF_JSGT, t3, t2, jmp_offset) < 0)
 		goto toofar;
 
 	/*
@@ -464,6 +489,7 @@ static int build_insn(const struct bpf_insn *insn, struct jit_ctx *ctx, bool ext
 	const s16 off = insn->off;
 	const s32 imm = insn->imm;
 	const bool is32 = BPF_CLASS(insn->code) == BPF_ALU || BPF_CLASS(insn->code) == BPF_JMP32;
+	int tcc_ptr_off;
 
 	switch (code) {
 	/* dst = src */
@@ -890,12 +916,17 @@ static int build_insn(const struct bpf_insn *insn, struct jit_ctx *ctx, bool ext
 
 	/* function call */
 	case BPF_JMP | BPF_CALL:
-		mark_call(ctx);
 		ret = bpf_jit_get_func_addr(ctx->prog, insn, extra_pass,
 					    &func_addr, &func_addr_fixed);
 		if (ret < 0)
 			return ret;
 
+		if (insn->src_reg == BPF_PSEUDO_CALL) {
+			tcc_ptr_off = BPF_TAIL_CALL_CNT_PTR_STACK_OFF(ctx->stack_size);
+			emit_insn(ctx, ldd, REG_TCC, LOONGARCH_GPR_SP, tcc_ptr_off);
+		}
+
+
 		move_addr(ctx, t1, func_addr);
 		emit_insn(ctx, jirl, LOONGARCH_GPR_RA, t1, 0);
 
@@ -906,7 +937,6 @@ static int build_insn(const struct bpf_insn *insn, struct jit_ctx *ctx, bool ext
 
 	/* tail call */
 	case BPF_JMP | BPF_TAIL_CALL:
-		mark_tail_call(ctx);
 		if (emit_bpf_tail_call(i, ctx) < 0)
 			return -EINVAL;
 		break;