[PATCH bpf-next v2] bpf: Change value of MAX_TAIL_CALL_CNT from 32 to 33

In the current code, the actual max tail call count is 33 which is greater
than MAX_TAIL_CALL_CNT (defined as 32), the actual limit is not consistent
with the meaning of MAX_TAIL_CALL_CNT, there is some confusion and need to
spend some time to think about the reason at the first glance.

We can see the historical evolution from commit 04fd61ab36ec ("bpf: allow
bpf programs to tail-call other bpf programs") and commit f9dabe016b63
("bpf: Undo off-by-one in interpreter tail call count limit").

In order to avoid changing existing behavior, the actual limit is 33 now,
this is reasonable.

After commit 874be05f525e ("bpf, tests: Add tail call test suite"), we can
see there exists failed testcase.

On all archs when CONFIG_BPF_JIT_ALWAYS_ON is not set:
 # echo 0 > /proc/sys/net/core/bpf_jit_enable
 # modprobe test_bpf
 # dmesg | grep -w FAIL
 Tail call error path, max count reached jited:0 ret 34 != 33 FAIL

On some archs:
 # echo 1 > /proc/sys/net/core/bpf_jit_enable
 # modprobe test_bpf
 # dmesg | grep -w FAIL
 Tail call error path, max count reached jited:1 ret 34 != 33 FAIL

So it is necessary to change the value of MAX_TAIL_CALL_CNT from 32 to 33,
then do some small changes of the related code.

With this patch, it does not change the current limit 33, MAX_TAIL_CALL_CNT
can reflect the actual max tail call count, the tailcall selftests can work
well, and also the above failed testcase in test_bpf can be fixed for the
interpreter (all archs) and the JIT (all archs except for x86).

 # uname -m
 x86_64
 # echo 1 > /proc/sys/net/core/bpf_jit_enable
 # modprobe test_bpf
 # dmesg | grep -w FAIL
 Tail call error path, max count reached jited:1 ret 33 != 34 FAIL

Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
---

v2:
  -- fix the typos in the commit message and update the commit message.
  -- fix the failed tailcall selftests for x86 jit.
     I am not quite sure the change on x86 is proper, with this change,
     tailcall selftests passed, but tailcall limit test in test_bpf.ko
     failed, I do not know the reason now, I think this is another issue,
     maybe someone more versed in x86 jit could take a look.

 arch/arm/net/bpf_jit_32.c         | 11 ++++++-----
 arch/arm64/net/bpf_jit_comp.c     |  7 ++++---
 arch/mips/net/ebpf_jit.c          |  4 ++--
 arch/powerpc/net/bpf_jit_comp32.c |  4 ++--
 arch/powerpc/net/bpf_jit_comp64.c | 12 ++++++------
 arch/riscv/net/bpf_jit_comp32.c   |  4 ++--
 arch/riscv/net/bpf_jit_comp64.c   |  4 ++--
 arch/sparc/net/bpf_jit_comp_64.c  |  8 ++++----
 arch/x86/net/bpf_jit_comp.c       | 10 +++++-----
 include/linux/bpf.h               |  2 +-
 kernel/bpf/core.c                 |  4 ++--
 11 files changed, 36 insertions(+), 34 deletions(-)

diff --git a/arch/arm/net/bpf_jit_32.c b/arch/arm/net/bpf_jit_32.c
index a951276..39d9ae9 100644
--- a/arch/arm/net/bpf_jit_32.c
+++ b/arch/arm/net/bpf_jit_32.c

@@ -1180,18 +1180,19 @@ static int emit_bpf_tail_call(struct jit_ctx *ctx)
 
 	/* tmp2[0] = array, tmp2[1] = index */
 
-	/* if (tail_call_cnt > MAX_TAIL_CALL_CNT)
-	 *	goto out;
+	/*
 	 * tail_call_cnt++;
+	 * if (tail_call_cnt > MAX_TAIL_CALL_CNT)
+	 *	goto out;
 	 */
+	tc = arm_bpf_get_reg64(tcc, tmp, ctx);
+	emit(ARM_ADDS_I(tc[1], tc[1], 1), ctx);
+	emit(ARM_ADC_I(tc[0], tc[0], 0), ctx);
 	lo = (u32)MAX_TAIL_CALL_CNT;
 	hi = (u32)((u64)MAX_TAIL_CALL_CNT >> 32);
-	tc = arm_bpf_get_reg64(tcc, tmp, ctx);
 	emit(ARM_CMP_I(tc[0], hi), ctx);
 	_emit(ARM_COND_EQ, ARM_CMP_I(tc[1], lo), ctx);
 	_emit(ARM_COND_HI, ARM_B(jmp_offset), ctx);
-	emit(ARM_ADDS_I(tc[1], tc[1], 1), ctx);
-	emit(ARM_ADC_I(tc[0], tc[0], 0), ctx);
 	arm_bpf_put_reg64(tcc, tmp, ctx);
 
 	/* prog = array->ptrs[index]

diff --git a/arch/arm64/net/bpf_jit_comp.c b/arch/arm64/net/bpf_jit_comp.c
index 41c23f4..5d6c843 100644
--- a/arch/arm64/net/bpf_jit_comp.c
+++ b/arch/arm64/net/bpf_jit_comp.c

@@ -286,14 +286,15 @@ static int emit_bpf_tail_call(struct jit_ctx *ctx)
 	emit(A64_CMP(0, r3, tmp), ctx);
 	emit(A64_B_(A64_COND_CS, jmp_offset), ctx);
 
-	/* if (tail_call_cnt > MAX_TAIL_CALL_CNT)
-	 *     goto out;
+	/*
 	 * tail_call_cnt++;
+	 * if (tail_call_cnt > MAX_TAIL_CALL_CNT)
+	 *     goto out;
 	 */
+	emit(A64_ADD_I(1, tcc, tcc, 1), ctx);
 	emit_a64_mov_i64(tmp, MAX_TAIL_CALL_CNT, ctx);
 	emit(A64_CMP(1, tcc, tmp), ctx);
 	emit(A64_B_(A64_COND_HI, jmp_offset), ctx);
-	emit(A64_ADD_I(1, tcc, tcc, 1), ctx);
 
 	/* prog = array->ptrs[index];
 	 * if (prog == NULL)

diff --git a/arch/mips/net/ebpf_jit.c b/arch/mips/net/ebpf_jit.c
index 3a73e93..029fc34 100644
--- a/arch/mips/net/ebpf_jit.c
+++ b/arch/mips/net/ebpf_jit.c

@@ -617,14 +617,14 @@ static int emit_bpf_tail_call(struct jit_ctx *ctx, int this_idx)
 	b_off = b_imm(this_idx + 1, ctx);
 	emit_instr(ctx, bne, MIPS_R_AT, MIPS_R_ZERO, b_off);
 	/*
-	 * if (TCC-- < 0)
+	 * if (--TCC < 0)
 	 *     goto out;
 	 */
 	/* Delay slot */
 	tcc_reg = (ctx->flags & EBPF_TCC_IN_V1) ? MIPS_R_V1 : MIPS_R_S4;
 	emit_instr(ctx, daddiu, MIPS_R_T5, tcc_reg, -1);
 	b_off = b_imm(this_idx + 1, ctx);
-	emit_instr(ctx, bltz, tcc_reg, b_off);
+	emit_instr(ctx, bltz, MIPS_R_T5, b_off);
 	/*
 	 * prog = array->ptrs[index];
 	 * if (prog == NULL)

diff --git a/arch/powerpc/net/bpf_jit_comp32.c b/arch/powerpc/net/bpf_jit_comp32.c
index beb12cb..b5585ad 100644
--- a/arch/powerpc/net/bpf_jit_comp32.c
+++ b/arch/powerpc/net/bpf_jit_comp32.c

@@ -221,12 +221,12 @@ static void bpf_jit_emit_tail_call(u32 *image, struct codegen_context *ctx, u32
 	PPC_BCC(COND_GE, out);
 
 	/*
+	 * tail_call_cnt++;
 	 * if (tail_call_cnt > MAX_TAIL_CALL_CNT)
 	 *   goto out;
 	 */
-	EMIT(PPC_RAW_CMPLWI(_R0, MAX_TAIL_CALL_CNT));
-	/* tail_call_cnt++; */
 	EMIT(PPC_RAW_ADDIC(_R0, _R0, 1));
+	EMIT(PPC_RAW_CMPLWI(_R0, MAX_TAIL_CALL_CNT));
 	PPC_BCC(COND_GT, out);
 
 	/* prog = array->ptrs[index]; */

diff --git a/arch/powerpc/net/bpf_jit_comp64.c b/arch/powerpc/net/bpf_jit_comp64.c
index b87a63d..bb15cc4 100644
--- a/arch/powerpc/net/bpf_jit_comp64.c
+++ b/arch/powerpc/net/bpf_jit_comp64.c

@@ -227,6 +227,12 @@ static void bpf_jit_emit_tail_call(u32 *image, struct codegen_context *ctx, u32
 	PPC_BCC(COND_GE, out);
 
 	/*
+	 * tail_call_cnt++;
+	 */
+	EMIT(PPC_RAW_ADDI(b2p[TMP_REG_1], b2p[TMP_REG_1], 1));
+	PPC_BPF_STL(b2p[TMP_REG_1], 1, bpf_jit_stack_tailcallcnt(ctx));
+
+	/*
 	 * if (tail_call_cnt > MAX_TAIL_CALL_CNT)
 	 *   goto out;
 	 */

@@ -234,12 +240,6 @@ static void bpf_jit_emit_tail_call(u32 *image, struct codegen_context *ctx, u32
 	EMIT(PPC_RAW_CMPLWI(b2p[TMP_REG_1], MAX_TAIL_CALL_CNT));
 	PPC_BCC(COND_GT, out);
 
-	/*
-	 * tail_call_cnt++;
-	 */
-	EMIT(PPC_RAW_ADDI(b2p[TMP_REG_1], b2p[TMP_REG_1], 1));
-	PPC_BPF_STL(b2p[TMP_REG_1], 1, bpf_jit_stack_tailcallcnt(ctx));
-
 	/* prog = array->ptrs[index]; */
 	EMIT(PPC_RAW_MULI(b2p[TMP_REG_1], b2p_index, 8));
 	EMIT(PPC_RAW_ADD(b2p[TMP_REG_1], b2p[TMP_REG_1], b2p_bpf_array));

diff --git a/arch/riscv/net/bpf_jit_comp32.c b/arch/riscv/net/bpf_jit_comp32.c
index e649742..1608d94 100644
--- a/arch/riscv/net/bpf_jit_comp32.c
+++ b/arch/riscv/net/bpf_jit_comp32.c

@@ -800,12 +800,12 @@ static int emit_bpf_tail_call(int insn, struct rv_jit_context *ctx)
 
 	/*
 	 * temp_tcc = tcc - 1;
-	 * if (tcc < 0)
+	 * if (temp_tcc < 0)
 	 *   goto out;
 	 */
 	emit(rv_addi(RV_REG_T1, RV_REG_TCC, -1), ctx);
 	off = ninsns_rvoff(tc_ninsn - (ctx->ninsns - start_insn));
-	emit_bcc(BPF_JSLT, RV_REG_TCC, RV_REG_ZERO, off, ctx);
+	emit_bcc(BPF_JSLT, RV_REG_T1, RV_REG_ZERO, off, ctx);
 
 	/*
 	 * prog = array->ptrs[index];

diff --git a/arch/riscv/net/bpf_jit_comp64.c b/arch/riscv/net/bpf_jit_comp64.c
index 3af4131..6e9ba83 100644
--- a/arch/riscv/net/bpf_jit_comp64.c
+++ b/arch/riscv/net/bpf_jit_comp64.c

@@ -311,12 +311,12 @@ static int emit_bpf_tail_call(int insn, struct rv_jit_context *ctx)
 	off = ninsns_rvoff(tc_ninsn - (ctx->ninsns - start_insn));
 	emit_branch(BPF_JGE, RV_REG_A2, RV_REG_T1, off, ctx);
 
-	/* if (TCC-- < 0)
+	/* if (--TCC < 0)
 	 *     goto out;
 	 */
 	emit_addi(RV_REG_T1, tcc, -1, ctx);
 	off = ninsns_rvoff(tc_ninsn - (ctx->ninsns - start_insn));
-	emit_branch(BPF_JSLT, tcc, RV_REG_ZERO, off, ctx);
+	emit_branch(BPF_JSLT, RV_REG_T1, RV_REG_ZERO, off, ctx);
 
 	/* prog = array->ptrs[index];
 	 * if (!prog)

diff --git a/arch/sparc/net/bpf_jit_comp_64.c b/arch/sparc/net/bpf_jit_comp_64.c
index 9a2f20c..50d914c 100644
--- a/arch/sparc/net/bpf_jit_comp_64.c
+++ b/arch/sparc/net/bpf_jit_comp_64.c

@@ -863,6 +863,10 @@ static void emit_tail_call(struct jit_ctx *ctx)
 	emit_branch(BGEU, ctx->idx, ctx->idx + OFFSET1, ctx);
 	emit_nop(ctx);
 
+	emit_alu_K(ADD, tmp, 1, ctx);
+	off = BPF_TAILCALL_CNT_SP_OFF;
+	emit(ST32 | IMMED | RS1(SP) | S13(off) | RD(tmp), ctx);
+
 	off = BPF_TAILCALL_CNT_SP_OFF;
 	emit(LD32 | IMMED | RS1(SP) | S13(off) | RD(tmp), ctx);
 	emit_cmpi(tmp, MAX_TAIL_CALL_CNT, ctx);

@@ -870,10 +874,6 @@ static void emit_tail_call(struct jit_ctx *ctx)
 	emit_branch(BGU, ctx->idx, ctx->idx + OFFSET2, ctx);
 	emit_nop(ctx);
 
-	emit_alu_K(ADD, tmp, 1, ctx);
-	off = BPF_TAILCALL_CNT_SP_OFF;
-	emit(ST32 | IMMED | RS1(SP) | S13(off) | RD(tmp), ctx);
-
 	emit_alu3_K(SLL, bpf_index, 3, tmp, ctx);
 	emit_alu(ADD, bpf_array, tmp, ctx);
 	off = offsetof(struct bpf_array, ptrs);

diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
index 0fe6aac..74a9e61 100644
--- a/arch/x86/net/bpf_jit_comp.c
+++ b/arch/x86/net/bpf_jit_comp.c

@@ -402,7 +402,7 @@ static int get_pop_bytes(bool *callee_regs_used)
  * ... bpf_tail_call(void *ctx, struct bpf_array *array, u64 index) ...
  *   if (index >= array->map.max_entries)
  *     goto out;
- *   if (++tail_call_cnt > MAX_TAIL_CALL_CNT)
+ *   if (tail_call_cnt++ == MAX_TAIL_CALL_CNT)
  *     goto out;
  *   prog = array->ptrs[index];
  *   if (prog == NULL)

@@ -452,13 +452,13 @@ static void emit_bpf_tail_call_indirect(u8 **pprog, bool *callee_regs_used,
 	EMIT2(X86_JBE, OFFSET1);                  /* jbe out */
 
 	/*
-	 * if (tail_call_cnt > MAX_TAIL_CALL_CNT)
+	 * if (tail_call_cnt++ == MAX_TAIL_CALL_CNT)
 	 *	goto out;
 	 */
 	EMIT2_off32(0x8B, 0x85, tcc_off);         /* mov eax, dword ptr [rbp - tcc_off] */
 	EMIT3(0x83, 0xF8, MAX_TAIL_CALL_CNT);     /* cmp eax, MAX_TAIL_CALL_CNT */
 #define OFFSET2 (off2 + RETPOLINE_RCX_BPF_JIT_SIZE)
-	EMIT2(X86_JA, OFFSET2);                   /* ja out */
+	EMIT2(X86_JE, OFFSET2);                   /* je out */
 	EMIT3(0x83, 0xC0, 0x01);                  /* add eax, 1 */
 	EMIT2_off32(0x89, 0x85, tcc_off);         /* mov dword ptr [rbp - tcc_off], eax */
 

@@ -530,12 +530,12 @@ static void emit_bpf_tail_call_direct(struct bpf_jit_poke_descriptor *poke,
 	}
 
 	/*
-	 * if (tail_call_cnt > MAX_TAIL_CALL_CNT)
+	 * if (tail_call_cnt++ == MAX_TAIL_CALL_CNT)
 	 *	goto out;
 	 */
 	EMIT2_off32(0x8B, 0x85, tcc_off);             /* mov eax, dword ptr [rbp - tcc_off] */
 	EMIT3(0x83, 0xF8, MAX_TAIL_CALL_CNT);         /* cmp eax, MAX_TAIL_CALL_CNT */
-	EMIT2(X86_JA, off1);                          /* ja out */
+	EMIT2(X86_JE, off1);                          /* je out */
 	EMIT3(0x83, 0xC0, 0x01);                      /* add eax, 1 */
 	EMIT2_off32(0x89, 0x85, tcc_off);             /* mov dword ptr [rbp - tcc_off], eax */
 

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index f4c16f1..224cc7e 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h

@@ -1046,7 +1046,7 @@ struct bpf_array {
 };
 
 #define BPF_COMPLEXITY_LIMIT_INSNS      1000000 /* yes. 1M insns */
-#define MAX_TAIL_CALL_CNT 32
+#define MAX_TAIL_CALL_CNT 33
 
 #define BPF_F_ACCESS_MASK	(BPF_F_RDONLY |		\
 				 BPF_F_RDONLY_PROG |	\

diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
index 9f4636d..8edb1c3 100644
--- a/kernel/bpf/core.c
+++ b/kernel/bpf/core.c

@@ -1564,10 +1564,10 @@ static u64 ___bpf_prog_run(u64 *regs, const struct bpf_insn *insn)
 
 		if (unlikely(index >= array->map.max_entries))
 			goto out;
-		if (unlikely(tail_call_cnt > MAX_TAIL_CALL_CNT))
-			goto out;
 
 		tail_call_cnt++;
+		if (unlikely(tail_call_cnt > MAX_TAIL_CALL_CNT))
+			goto out;
 
 		prog = READ_ONCE(array->ptrs[index]);
 		if (!prog)

-- 
2.1.0

On 9/11/21 3:56 AM, Tiezhu Yang wrote:

In the current code, the actual max tail call count is 33 which is greater
than MAX_TAIL_CALL_CNT (defined as 32), the actual limit is not consistent
with the meaning of MAX_TAIL_CALL_CNT, there is some confusion and need to
spend some time to think about the reason at the first glance.

We can see the historical evolution from commit 04fd61ab36ec ("bpf: allow
bpf programs to tail-call other bpf programs") and commit f9dabe016b63
("bpf: Undo off-by-one in interpreter tail call count limit").

In order to avoid changing existing behavior, the actual limit is 33 now,
this is reasonable.

After commit 874be05f525e ("bpf, tests: Add tail call test suite"), we can
see there exists failed testcase.

On all archs when CONFIG_BPF_JIT_ALWAYS_ON is not set:
  # echo 0 > /proc/sys/net/core/bpf_jit_enable
  # modprobe test_bpf
  # dmesg | grep -w FAIL
  Tail call error path, max count reached jited:0 ret 34 != 33 FAIL

On some archs:
  # echo 1 > /proc/sys/net/core/bpf_jit_enable
  # modprobe test_bpf
  # dmesg | grep -w FAIL
  Tail call error path, max count reached jited:1 ret 34 != 33 FAIL

So it is necessary to change the value of MAX_TAIL_CALL_CNT from 32 to 33,
then do some small changes of the related code.

With this patch, it does not change the current limit 33, MAX_TAIL_CALL_CNT
can reflect the actual max tail call count, the tailcall selftests can work
well, and also the above failed testcase in test_bpf can be fixed for the
interpreter (all archs) and the JIT (all archs except for x86).

  # uname -m
  x86_64
  # echo 1 > /proc/sys/net/core/bpf_jit_enable
  # modprobe test_bpf
  # dmesg | grep -w FAIL
  Tail call error path, max count reached jited:1 ret 33 != 34 FAIL

Could you also state in here which archs you have tested with this change? I
presume /every/ arch which has a JIT?

Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
---

v2:
   -- fix the typos in the commit message and update the commit message.
   -- fix the failed tailcall selftests for x86 jit.
      I am not quite sure the change on x86 is proper, with this change,
      tailcall selftests passed, but tailcall limit test in test_bpf.ko
      failed, I do not know the reason now, I think this is another issue,
      maybe someone more versed in x86 jit could take a look.

There should be a series from Johan coming today with regards to test_bpf.ko
that will fix the "tail call error path, max count reached" test which had an
assumption in that R0 would always be valid for the fall-through and could be
passed to the bpf_exit insn whereas it is not guaranteed and verifier, for
example, forbids a subsequent access to R0 w/o reinit. For your testing, I
would suggested to recheck once this series is out.

  arch/arm/net/bpf_jit_32.c         | 11 ++++++-----
  arch/arm64/net/bpf_jit_comp.c     |  7 ++++---
  arch/mips/net/ebpf_jit.c          |  4 ++--
  arch/powerpc/net/bpf_jit_comp32.c |  4 ++--
  arch/powerpc/net/bpf_jit_comp64.c | 12 ++++++------
  arch/riscv/net/bpf_jit_comp32.c   |  4 ++--
  arch/riscv/net/bpf_jit_comp64.c   |  4 ++--
  arch/sparc/net/bpf_jit_comp_64.c  |  8 ++++----
  arch/x86/net/bpf_jit_comp.c       | 10 +++++-----
  include/linux/bpf.h               |  2 +-
  kernel/bpf/core.c                 |  4 ++--
  11 files changed, 36 insertions(+), 34 deletions(-)

[...]

[...]

Why such inconsistency to e.g. above with arm64 case but also compared to
x86 32 bit which uses JAE? If so, we should cleanly follow the reference
implementation (== interpreter) _everywhere_ and _not_ introduce additional
variants/implementations across JITs.

On 09/14/2021 03:30 PM, Daniel Borkmann wrote:

On 9/11/21 3:56 AM, Tiezhu Yang wrote:

quoted

[...]

quoted

With this patch, it does not change the current limit 33, 
MAX_TAIL_CALL_CNT
can reflect the actual max tail call count, the tailcall selftests 
can work
well, and also the above failed testcase in test_bpf can be fixed for 
the
interpreter (all archs) and the JIT (all archs except for x86).

  # uname -m
  x86_64
  # echo 1 > /proc/sys/net/core/bpf_jit_enable
  # modprobe test_bpf
  # dmesg | grep -w FAIL
  Tail call error path, max count reached jited:1 ret 33 != 34 FAIL

Could you also state in here which archs you have tested with this 
change? I
presume /every/ arch which has a JIT?

OK, will do it in v3.
I have tested on x86 and mips.

quoted

Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
---

v2:
   -- fix the typos in the commit message and update the commit message.
   -- fix the failed tailcall selftests for x86 jit.
      I am not quite sure the change on x86 is proper, with this change,
      tailcall selftests passed, but tailcall limit test in test_bpf.ko
      failed, I do not know the reason now, I think this is another 
issue,
      maybe someone more versed in x86 jit could take a look.

There should be a series from Johan coming today with regards to 
test_bpf.ko
that will fix the "tail call error path, max count reached" test which 
had an
assumption in that R0 would always be valid for the fall-through and 
could be
passed to the bpf_exit insn whereas it is not guaranteed and verifier, 
for
example, forbids a subsequent access to R0 w/o reinit. For your 
testing, I
would suggested to recheck once this series is out.

I will test the following patch on x86 and mips:

[PATCH bpf v4 13/14] bpf/tests: Fix error in tail call limit tests

[...]

quoted

diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
index 0fe6aac..74a9e61 100644
--- a/arch/x86/net/bpf_jit_comp.c
+++ b/arch/x86/net/bpf_jit_comp.c

@@ -402,7 +402,7 @@ static int get_pop_bytes(bool *callee_regs_used)
   * ... bpf_tail_call(void *ctx, struct bpf_array *array, u64 index) 

...
   *   if (index >= array->map.max_entries)
   *     goto out;
- *   if (++tail_call_cnt > MAX_TAIL_CALL_CNT)
+ *   if (tail_call_cnt++ == MAX_TAIL_CALL_CNT)

Why such inconsistency to e.g. above with arm64 case but also compared to
x86 32 bit which uses JAE? If so, we should cleanly follow the reference
implementation (== interpreter) _everywhere_ and _not_ introduce 
additional
variants/implementations across JITs.

In order tokeep consistencyand make as few changes as possible,
<javascript:void(0);>I will modify the check condition as follows:

#define MAX_TAIL_CALL_CNT 33
(1) for x86, arm64, ... (0 ~ 32)
tcc = 0;
if (tcc == MAX_TAIL_CALL_CNT)
     goto out;
tcc++;

(2) for mips, riscv (33 ~ 1)
tcc = MAX_TAIL_CALL_CNT;
if (tcc == 0)
     goto out;
tcc--;

[...]