LoongArch: BPF: Fix the 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.

Fixes: bb035ef ("LoongArch: BPF: Support mixing bpf2bpf and tailcalls")
Reviewed-by: Geliang Tang <geliang@kernel.org>
Reviewed-by: Hengqi Chen <hengqi.chen@gmail.com>
Signed-off-by: Haoran Jiang <jianghaoran@kylinos.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

Author: Haoran Jiang

arch/loongarch/net/bpf_jit.c

@@ -17,10 +17,7 @@
 #define LOONGARCH_BPF_FENTRY_NBYTES (LOONGARCH_LONG_JUMP_NINSNS * 4)
 
 #define REG_TCC		LOONGARCH_GPR_A6
-#define TCC_SAVED	LOONGARCH_GPR_S5
-
-#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 */
@@ -42,32 +39,57 @@ 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) {
+		/*
+		 * LOONGARCH_GPR_T3 = MAX_TAIL_CALL_CNT
+		 * if (REG_TCC > T3 )
+		 *	std REG_TCC -> LOONGARCH_GPR_SP + store_offset
+		 * else
+		 *	std REG_TCC -> LOONGARCH_GPR_SP + store_offset
+		 *	REG_TCC = LOONGARCH_GPR_SP + store_offset
+		 *
+		 * std REG_TCC -> LOONGARCH_GPR_SP + store_offset
+		 *
+		 * The purpose of this code is to first push the TCC into stack,
+		 * and then push the address of TCC into stack.
+		 * In cases where bpf2bpf and tailcall are used in combination,
+		 * the value in REG_TCC may be a count or an address,
+		 * these two cases need to be judged and handled separately.
+		 */
+		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_cond_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, the value in REG_TCC is a count,
+		 * push 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;
+		/* Push the address of TCC into the REG_TCC */
+		emit_insn(ctx, addid, REG_TCC, LOONGARCH_GPR_SP, *store_offset);
 
-	return REG_TCC;
+		emit_uncond_jmp(ctx, 2);
+
+		/*
+		 * If REG_TCC > MAX_TAIL_CALL_CNT, the value in REG_TCC is an address,
+		 * push tcc_ptr into stack
+		 */
+		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);
+	}
+
+	/* Push tcc_ptr into stack */
+	*store_offset -= sizeof(long);
+	emit_insn(ctx, std, REG_TCC, LOONGARCH_GPR_SP, *store_offset);
 }
 
 /*
@@ -90,6 +112,10 @@ static u8 tail_call_reg(struct jit_ctx *ctx)
  *                            |           $s4           |
  *                            +-------------------------+
  *                            |           $s5           |
+ *                            +-------------------------+
+ *                            |           tcc           |
+ *                            +-------------------------+
+ *                            |           tcc_ptr       |
  *                            +-------------------------+ <--BPF_REG_FP
  *                            |  prog->aux->stack_depth |
  *                            |        (optional)       |
@@ -99,12 +125,17 @@ static u8 tail_call_reg(struct jit_ctx *ctx)
 static void build_prologue(struct jit_ctx *ctx)
 {
 	int i, 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. */
+	/* To store ra, fp, s0, s1, s2, s3, s4, s5 */
 	stack_adjust += sizeof(long) * 8;
 
+	/* To store tcc and tcc_ptr */
+	stack_adjust += sizeof(long) * 2;
+
 	stack_adjust = round_up(stack_adjust, 16);
 	stack_adjust += bpf_stack_adjust;
 
@@ -113,11 +144,12 @@ static void build_prologue(struct jit_ctx *ctx)
 		emit_insn(ctx, nop);
 
 	/*
-	 * First instruction initializes the tail call count (TCC).
-	 * On tail call we skip this instruction, and the TCC is
-	 * passed in REG_TCC from the caller.
+	 * 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);
 
@@ -145,20 +177,13 @@ 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;
 }
 
@@ -191,6 +216,16 @@ 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 then followed by tcc.
+	 */
+	load_offset -= 2 * sizeof(long);
+	emit_insn(ctx, ldd, REG_TCC, LOONGARCH_GPR_SP, load_offset);
+
+	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) {
@@ -203,7 +238,7 @@ static void __build_epilogue(struct jit_ctx *ctx, bool is_tail_call)
 		 * Call the next bpf prog and skip the first instruction
 		 * of TCC initialization.
 		 */
-		emit_insn(ctx, jirl, LOONGARCH_GPR_ZERO, LOONGARCH_GPR_T3, 1);
+		emit_insn(ctx, jirl, LOONGARCH_GPR_ZERO, LOONGARCH_GPR_T3, 6);
 	}
 }
 
@@ -225,7 +260,7 @@ bool bpf_jit_supports_far_kfunc_call(void)
 static int emit_bpf_tail_call(struct jit_ctx *ctx, int insn)
 {
 	int off, tc_ninsn = 0;
-	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;
@@ -252,11 +287,15 @@ static int emit_bpf_tail_call(struct jit_ctx *ctx, int insn)
 		goto toofar;
 
 	/*
-	 * if (--TCC < 0)
-	 *	 goto out;
+	 * if ((*tcc_ptr)++ >= MAX_TAIL_CALL_CNT)
+	 *      goto out;
 	 */
-	emit_insn(ctx, addid, REG_TCC, tcc, -1);
-	if (emit_tailcall_jmp(ctx, BPF_JSLT, REG_TCC, LOONGARCH_GPR_ZERO, jmp_offset) < 0)
+	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);
+	if (emit_tailcall_jmp(ctx, BPF_JSGT, t3, t2, jmp_offset) < 0)
 		goto toofar;
 
 	/*
@@ -467,7 +506,7 @@ static int build_insn(const struct bpf_insn *insn, struct jit_ctx *ctx, bool ext
 	u64 func_addr;
 	bool func_addr_fixed, sign_extend;
 	int i = insn - ctx->prog->insnsi;
-	int ret, jmp_offset;
+	int ret, jmp_offset, tcc_ptr_off;
 	const u8 code = insn->code;
 	const u8 cond = BPF_OP(code);
 	const u8 t1 = LOONGARCH_GPR_T1;
@@ -903,12 +942,16 @@ 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);
 
@@ -919,7 +962,6 @@ 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(ctx, i) < 0)
 			return -EINVAL;
 		break;
@@ -1412,7 +1454,7 @@ static int __arch_prepare_bpf_trampoline(struct jit_ctx *ctx, struct bpf_tramp_i
 {
 	int i, ret, save_ret;
 	int stack_size = 0, nargs = 0;
-	int retval_off, args_off, nargs_off, ip_off, run_ctx_off, sreg_off;
+	int retval_off, args_off, nargs_off, ip_off, run_ctx_off, sreg_off, tcc_ptr_off;
 	bool is_struct_ops = flags & BPF_TRAMP_F_INDIRECT;
 	void *orig_call = func_addr;
 	struct bpf_tramp_links *fentry = &tlinks[BPF_TRAMP_FENTRY];
@@ -1447,6 +1489,7 @@ static int __arch_prepare_bpf_trampoline(struct jit_ctx *ctx, struct bpf_tramp_i
 	 *
 	 * FP - sreg_off    [ callee saved reg  ]
 	 *
+	 * FP - tcc_ptr_off [ tail_call_cnt_ptr ]
 	 */
 
 	if (m->nr_args > LOONGARCH_MAX_REG_ARGS)
@@ -1489,6 +1532,12 @@ static int __arch_prepare_bpf_trampoline(struct jit_ctx *ctx, struct bpf_tramp_i
 	stack_size += 8;
 	sreg_off = stack_size;
 
+	/* Room of trampoline frame to store tail_call_cnt_ptr */
+	if (flags & BPF_TRAMP_F_TAIL_CALL_CTX) {
+		stack_size += 8;
+		tcc_ptr_off = stack_size;
+	}
+
 	stack_size = round_up(stack_size, 16);
 
 	if (is_struct_ops) {
@@ -1519,6 +1568,9 @@ static int __arch_prepare_bpf_trampoline(struct jit_ctx *ctx, struct bpf_tramp_i
 		emit_insn(ctx, addid, LOONGARCH_GPR_FP, LOONGARCH_GPR_SP, stack_size);
 	}
 
+	if (flags & BPF_TRAMP_F_TAIL_CALL_CTX)
+		emit_insn(ctx, std, REG_TCC, LOONGARCH_GPR_FP, -tcc_ptr_off);
+
 	/* callee saved register S1 to pass start time */
 	emit_insn(ctx, std, LOONGARCH_GPR_S1, LOONGARCH_GPR_FP, -sreg_off);
 
@@ -1565,6 +1617,10 @@ static int __arch_prepare_bpf_trampoline(struct jit_ctx *ctx, struct bpf_tramp_i
 
 	if (flags & BPF_TRAMP_F_CALL_ORIG) {
 		restore_args(ctx, m->nr_args, args_off);
+
+		if (flags & BPF_TRAMP_F_TAIL_CALL_CTX)
+			emit_insn(ctx, ldd, REG_TCC, LOONGARCH_GPR_FP, -tcc_ptr_off);
+
 		ret = emit_call(ctx, (const u64)orig_call);
 		if (ret)
 			goto out;
@@ -1605,6 +1661,9 @@ static int __arch_prepare_bpf_trampoline(struct jit_ctx *ctx, struct bpf_tramp_i
 
 	emit_insn(ctx, ldd, LOONGARCH_GPR_S1, LOONGARCH_GPR_FP, -sreg_off);
 
+	if (flags & BPF_TRAMP_F_TAIL_CALL_CTX)
+		emit_insn(ctx, ldd, REG_TCC, LOONGARCH_GPR_FP, -tcc_ptr_off);
+
 	if (is_struct_ops) {
 		/* trampoline called directly */
 		emit_insn(ctx, ldd, LOONGARCH_GPR_RA, LOONGARCH_GPR_SP, stack_size - 8);