diff mbox series

[bpf-next,v6] bpf: Change value of MAX_TAIL_CALL_CNT from 32 to 33

Message ID 1636075800-3264-1-git-send-email-yangtiezhu@loongson.cn (mailing list archive)
State Accepted
Commit ebf7f6f0a6cdcc17a3da52b81e4b3a98c4005028
Delegated to: BPF
Headers show
Series [bpf-next,v6] bpf: Change value of MAX_TAIL_CALL_CNT from 32 to 33 | expand

Checks

Context Check Description
netdev/tree_selection success Clearly marked for bpf-next
netdev/fixes_present success Fixes tag not required for -next series
netdev/subject_prefix success Link
netdev/cover_letter success Single patches do not need cover letters
netdev/patch_count success Link
netdev/header_inline success No static functions without inline keyword in header files
netdev/build_32bit success Errors and warnings before: 11541 this patch: 11541
netdev/cc_maintainers warning 32 maintainers not CCed: sparclinux@vger.kernel.org linux-riscv@lists.infradead.org linux-mips@vger.kernel.org dsahern@kernel.org linux-arm-kernel@lists.infradead.org linuxppc-dev@lists.ozlabs.org x86@kernel.org paulburton@kernel.org benh@kernel.crashing.org linux@armlinux.org.uk linux-s390@vger.kernel.org jackmanb@google.com tglx@linutronix.de liuhangbin@gmail.com catalin.marinas@arm.com mpe@ellerman.id.au paul.walmsley@sifive.com will@kernel.org mingo@redhat.com paulus@samba.org revest@chromium.org palmer@dabbelt.com borntraeger@de.ibm.com dave.hansen@linux.intel.com aou@eecs.berkeley.edu tsbogend@alpha.franken.de agordeev@linux.ibm.com joe@cilium.io brouer@redhat.com bp@alien8.de yoshfuji@linux-ipv6.org hpa@zytor.com
netdev/build_clang success Errors and warnings before: 2378 this patch: 2378
netdev/module_param success Was 0 now: 0
netdev/verify_signedoff success Signed-off-by tag matches author and committer
netdev/verify_fixes success No Fixes tag
netdev/build_allmodconfig_warn success Errors and warnings before: 11262 this patch: 11262
netdev/checkpatch warning WARNING: line length of 83 exceeds 80 columns
netdev/kdoc success Errors and warnings before: 0 this patch: 0
netdev/source_inline success Was 0 now: 0
bpf/vmtest-bpf-next fail VM_Test
bpf/vmtest-bpf-next-PR fail PR summary

Commit Message

Tiezhu Yang Nov. 5, 2021, 1:30 a.m. UTC 
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

Although the above failed testcase has been fixed in commit 18935a72eb25
("bpf/tests: Fix error in tail call limit tests"), it is still necessary
to change the value of MAX_TAIL_CALL_CNT from 32 to 33 to make the code
more readable, then do some small changes of the related code.

The 32-bit x86 JIT was using a limit of 32, just fix the wrong comments and
limit to 33 tail calls as the constant MAX_TAIL_CALL_CNT updated.

For the other implementations, no function changes, it does not change the
current limit 33, the new value of MAX_TAIL_CALL_CNT can reflect the actual
max tail call count, the related tail call testcases in test_bpf module and
selftests can work well for the interpreter and the JIT.

Here are the test results on x86_64:

 # uname -m
 x86_64
 # echo 0 > /proc/sys/net/core/bpf_jit_enable
 # modprobe test_bpf test_suite=test_tail_calls
 # dmesg | tail -1
 test_bpf: test_tail_calls: Summary: 8 PASSED, 0 FAILED, [0/8 JIT'ed]
 # rmmod test_bpf
 # echo 1 > /proc/sys/net/core/bpf_jit_enable
 # modprobe test_bpf test_suite=test_tail_calls
 # dmesg | tail -1
 test_bpf: test_tail_calls: Summary: 8 PASSED, 0 FAILED, [8/8 JIT'ed]
 # rmmod test_bpf
 # ./test_progs -t tailcalls
 #142 tailcalls:OK
 Summary: 1/11 PASSED, 0 SKIPPED, 0 FAILED

Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Acked-by: Björn Töpel <bjorn@kernel.org>
Tested-by: Johan Almbladh <johan.almbladh@anyfinetworks.com>
Acked-by: Johan Almbladh <johan.almbladh@anyfinetworks.com>
Tested-by: Ilya Leoshkevich <iii@linux.ibm.com>
Acked-by: Ilya Leoshkevich <iii@linux.ibm.com>
---

v6: Use "ori" instead of "addiu" for mips64, update the commit message,
    suggested by Johan Almbladh, thank you.

v5: Use RV_REG_TCC directly to save one move for RISC-V,
    suggested by Björn Töpel, thank you.

v4: Use >= as check condition,
    suggested by Daniel Borkmann, thank you.

 arch/arm/net/bpf_jit_32.c         |  5 +++--
 arch/arm64/net/bpf_jit_comp.c     |  5 +++--
 arch/mips/net/bpf_jit_comp32.c    |  3 +--
 arch/mips/net/bpf_jit_comp64.c    |  2 +-
 arch/powerpc/net/bpf_jit_comp32.c |  4 ++--
 arch/powerpc/net/bpf_jit_comp64.c |  4 ++--
 arch/riscv/net/bpf_jit_comp32.c   |  6 ++----
 arch/riscv/net/bpf_jit_comp64.c   |  7 +++----
 arch/s390/net/bpf_jit_comp.c      |  6 +++---
 arch/sparc/net/bpf_jit_comp_64.c  |  2 +-
 arch/x86/net/bpf_jit_comp.c       | 10 +++++-----
 arch/x86/net/bpf_jit_comp32.c     |  4 ++--
 include/linux/bpf.h               |  2 +-
 include/uapi/linux/bpf.h          |  2 +-
 kernel/bpf/core.c                 |  3 ++-
 lib/test_bpf.c                    |  4 ++--
 tools/include/uapi/linux/bpf.h    |  2 +-
 17 files changed, 35 insertions(+), 36 deletions(-)

Comments

patchwork-bot+netdevbpf@kernel.org Nov. 16, 2021, 1:40 p.m. UTC | #1 
Hello:

This patch was applied to bpf/bpf-next.git (master)
by Daniel Borkmann <daniel@iogearbox.net>:

On Fri,  5 Nov 2021 09:30:00 +0800 you 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").
> 
> [...]

Here is the summary with links:
  - [bpf-next,v6] bpf: Change value of MAX_TAIL_CALL_CNT from 32 to 33
    https://git.kernel.org/bpf/bpf-next/c/ebf7f6f0a6cd

You are awesome, thank you!
diff mbox series

Patch

diff --git a/arch/arm/net/bpf_jit_32.c b/arch/arm/net/bpf_jit_32.c
index eeb6dc0..e59b41e 100644
--- a/arch/arm/net/bpf_jit_32.c
+++ b/arch/arm/net/bpf_jit_32.c
@@ -1199,7 +1199,8 @@  static int emit_bpf_tail_call(struct jit_ctx *ctx)
 
 	/* tmp2[0] = array, tmp2[1] = index */
 
-	/* if (tail_call_cnt > MAX_TAIL_CALL_CNT)
+	/*
+	 * if (tail_call_cnt >= MAX_TAIL_CALL_CNT)
 	 *	goto out;
 	 * tail_call_cnt++;
 	 */
@@ -1208,7 +1209,7 @@  static int emit_bpf_tail_call(struct jit_ctx *ctx)
 	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_COND_CS, 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);
diff --git a/arch/arm64/net/bpf_jit_comp.c b/arch/arm64/net/bpf_jit_comp.c
index 3a8a714..356fb21 100644
--- a/arch/arm64/net/bpf_jit_comp.c
+++ b/arch/arm64/net/bpf_jit_comp.c
@@ -287,13 +287,14 @@  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)
+	/*
+	 * if (tail_call_cnt >= MAX_TAIL_CALL_CNT)
 	 *     goto out;
 	 * tail_call_cnt++;
 	 */
 	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_B_(A64_COND_CS, jmp_offset), ctx);
 	emit(A64_ADD_I(1, tcc, tcc, 1), ctx);
 
 	/* prog = array->ptrs[index];
diff --git a/arch/mips/net/bpf_jit_comp32.c b/arch/mips/net/bpf_jit_comp32.c
index bd996ed..044b11b 100644
--- a/arch/mips/net/bpf_jit_comp32.c
+++ b/arch/mips/net/bpf_jit_comp32.c
@@ -1381,8 +1381,7 @@  void build_prologue(struct jit_context *ctx)
 	 * 16-byte area in the parent's stack frame. On a tail call, the
 	 * calling function jumps into the prologue after these instructions.
 	 */
-	emit(ctx, ori, MIPS_R_T9, MIPS_R_ZERO,
-	     min(MAX_TAIL_CALL_CNT + 1, 0xffff));
+	emit(ctx, ori, MIPS_R_T9, MIPS_R_ZERO, min(MAX_TAIL_CALL_CNT, 0xffff));
 	emit(ctx, sw, MIPS_R_T9, 0, MIPS_R_SP);
 
 	/*
diff --git a/arch/mips/net/bpf_jit_comp64.c b/arch/mips/net/bpf_jit_comp64.c
index 815ade7..6475828 100644
--- a/arch/mips/net/bpf_jit_comp64.c
+++ b/arch/mips/net/bpf_jit_comp64.c
@@ -552,7 +552,7 @@  void build_prologue(struct jit_context *ctx)
 	 * On a tail call, the calling function jumps into the prologue
 	 * after this instruction.
 	 */
-	emit(ctx, addiu, tc, MIPS_R_ZERO, min(MAX_TAIL_CALL_CNT + 1, 0xffff));
+	emit(ctx, ori, tc, MIPS_R_ZERO, min(MAX_TAIL_CALL_CNT, 0xffff));
 
 	/* === Entry-point for tail calls === */
 
diff --git a/arch/powerpc/net/bpf_jit_comp32.c b/arch/powerpc/net/bpf_jit_comp32.c
index 0da31d4..8a4faa0 100644
--- a/arch/powerpc/net/bpf_jit_comp32.c
+++ b/arch/powerpc/net/bpf_jit_comp32.c
@@ -221,13 +221,13 @@  static int bpf_jit_emit_tail_call(u32 *image, struct codegen_context *ctx, u32 o
 	PPC_BCC(COND_GE, out);
 
 	/*
-	 * if (tail_call_cnt > MAX_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));
-	PPC_BCC(COND_GT, out);
+	PPC_BCC(COND_GE, out);
 
 	/* prog = array->ptrs[index]; */
 	EMIT(PPC_RAW_RLWINM(_R3, b2p_index, 2, 0, 29));
diff --git a/arch/powerpc/net/bpf_jit_comp64.c b/arch/powerpc/net/bpf_jit_comp64.c
index 8b5157c..8571aaf 100644
--- a/arch/powerpc/net/bpf_jit_comp64.c
+++ b/arch/powerpc/net/bpf_jit_comp64.c
@@ -228,12 +228,12 @@  static int bpf_jit_emit_tail_call(u32 *image, struct codegen_context *ctx, u32 o
 	PPC_BCC(COND_GE, out);
 
 	/*
-	 * if (tail_call_cnt > MAX_TAIL_CALL_CNT)
+	 * if (tail_call_cnt >= MAX_TAIL_CALL_CNT)
 	 *   goto out;
 	 */
 	PPC_BPF_LL(b2p[TMP_REG_1], 1, bpf_jit_stack_tailcallcnt(ctx));
 	EMIT(PPC_RAW_CMPLWI(b2p[TMP_REG_1], MAX_TAIL_CALL_CNT));
-	PPC_BCC(COND_GT, out);
+	PPC_BCC(COND_GE, out);
 
 	/*
 	 * tail_call_cnt++;
diff --git a/arch/riscv/net/bpf_jit_comp32.c b/arch/riscv/net/bpf_jit_comp32.c
index e649742..529a83b 100644
--- a/arch/riscv/net/bpf_jit_comp32.c
+++ b/arch/riscv/net/bpf_jit_comp32.c
@@ -799,11 +799,10 @@  static int emit_bpf_tail_call(int insn, struct rv_jit_context *ctx)
 	emit_bcc(BPF_JGE, lo(idx_reg), RV_REG_T1, off, ctx);
 
 	/*
-	 * temp_tcc = tcc - 1;
-	 * if (tcc < 0)
+	 * if (--tcc < 0)
 	 *   goto out;
 	 */
-	emit(rv_addi(RV_REG_T1, RV_REG_TCC, -1), ctx);
+	emit(rv_addi(RV_REG_TCC, 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);
 
@@ -829,7 +828,6 @@  static int emit_bpf_tail_call(int insn, struct rv_jit_context *ctx)
 	if (is_12b_check(off, insn))
 		return -1;
 	emit(rv_lw(RV_REG_T0, off, RV_REG_T0), ctx);
-	emit(rv_addi(RV_REG_TCC, RV_REG_T1, 0), ctx);
 	/* Epilogue jumps to *(t0 + 4). */
 	__build_epilogue(true, ctx);
 	return 0;
diff --git a/arch/riscv/net/bpf_jit_comp64.c b/arch/riscv/net/bpf_jit_comp64.c
index 2ca345c..f4466b7 100644
--- a/arch/riscv/net/bpf_jit_comp64.c
+++ b/arch/riscv/net/bpf_jit_comp64.c
@@ -327,12 +327,12 @@  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);
+	emit_addi(RV_REG_TCC, 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_TCC, RV_REG_ZERO, off, ctx);
 
 	/* prog = array->ptrs[index];
 	 * if (!prog)
@@ -352,7 +352,6 @@  static int emit_bpf_tail_call(int insn, struct rv_jit_context *ctx)
 	if (is_12b_check(off, insn))
 		return -1;
 	emit_ld(RV_REG_T3, off, RV_REG_T2, ctx);
-	emit_mv(RV_REG_TCC, RV_REG_T1, ctx);
 	__build_epilogue(true, ctx);
 	return 0;
 }
diff --git a/arch/s390/net/bpf_jit_comp.c b/arch/s390/net/bpf_jit_comp.c
index 1a374d0..3553cfc 100644
--- a/arch/s390/net/bpf_jit_comp.c
+++ b/arch/s390/net/bpf_jit_comp.c
@@ -1369,7 +1369,7 @@  static noinline int bpf_jit_insn(struct bpf_jit *jit, struct bpf_prog *fp,
 				 jit->prg);
 
 		/*
-		 * if (tail_call_cnt++ > MAX_TAIL_CALL_CNT)
+		 * if (tail_call_cnt++ >= MAX_TAIL_CALL_CNT)
 		 *         goto out;
 		 */
 
@@ -1381,9 +1381,9 @@  static noinline int bpf_jit_insn(struct bpf_jit *jit, struct bpf_prog *fp,
 		EMIT4_IMM(0xa7080000, REG_W0, 1);
 		/* laal %w1,%w0,off(%r15) */
 		EMIT6_DISP_LH(0xeb000000, 0x00fa, REG_W1, REG_W0, REG_15, off);
-		/* clij %w1,MAX_TAIL_CALL_CNT,0x2,out */
+		/* clij %w1,MAX_TAIL_CALL_CNT-1,0x2,out */
 		patch_2_clij = jit->prg;
-		EMIT6_PCREL_RIEC(0xec000000, 0x007f, REG_W1, MAX_TAIL_CALL_CNT,
+		EMIT6_PCREL_RIEC(0xec000000, 0x007f, REG_W1, MAX_TAIL_CALL_CNT - 1,
 				 2, jit->prg);
 
 		/*
diff --git a/arch/sparc/net/bpf_jit_comp_64.c b/arch/sparc/net/bpf_jit_comp_64.c
index 9a2f20c..0bfe1c7 100644
--- a/arch/sparc/net/bpf_jit_comp_64.c
+++ b/arch/sparc/net/bpf_jit_comp_64.c
@@ -867,7 +867,7 @@  static void emit_tail_call(struct jit_ctx *ctx)
 	emit(LD32 | IMMED | RS1(SP) | S13(off) | RD(tmp), ctx);
 	emit_cmpi(tmp, MAX_TAIL_CALL_CNT, ctx);
 #define OFFSET2 13
-	emit_branch(BGU, ctx->idx, ctx->idx + OFFSET2, ctx);
+	emit_branch(BGEU, ctx->idx, ctx->idx + OFFSET2, ctx);
 	emit_nop(ctx);
 
 	emit_alu_K(ADD, tmp, 1, ctx);
diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
index 726700f..6318479 100644
--- a/arch/x86/net/bpf_jit_comp.c
+++ b/arch/x86/net/bpf_jit_comp.c
@@ -412,7 +412,7 @@  static void emit_indirect_jump(u8 **pprog, int reg, u8 *ip)
  * ... 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)
@@ -446,14 +446,14 @@  static void emit_bpf_tail_call_indirect(u8 **pprog, bool *callee_regs_used,
 	EMIT2(X86_JBE, offset);                   /* 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 */
 
 	offset = ctx->tail_call_indirect_label - (prog + 2 - start);
-	EMIT2(X86_JA, offset);                    /* ja out */
+	EMIT2(X86_JAE, offset);                   /* jae out */
 	EMIT3(0x83, 0xC0, 0x01);                  /* add eax, 1 */
 	EMIT2_off32(0x89, 0x85, tcc_off);         /* mov dword ptr [rbp - tcc_off], eax */
 
@@ -504,14 +504,14 @@  static void emit_bpf_tail_call_direct(struct bpf_jit_poke_descriptor *poke,
 	int offset;
 
 	/*
-	 * 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 */
 
 	offset = ctx->tail_call_direct_label - (prog + 2 - start);
-	EMIT2(X86_JA, offset);                        /* ja out */
+	EMIT2(X86_JAE, offset);                       /* jae out */
 	EMIT3(0x83, 0xC0, 0x01);                      /* add eax, 1 */
 	EMIT2_off32(0x89, 0x85, tcc_off);             /* mov dword ptr [rbp - tcc_off], eax */
 
diff --git a/arch/x86/net/bpf_jit_comp32.c b/arch/x86/net/bpf_jit_comp32.c
index da9b7cf..429a89c 100644
--- a/arch/x86/net/bpf_jit_comp32.c
+++ b/arch/x86/net/bpf_jit_comp32.c
@@ -1323,7 +1323,7 @@  static void emit_bpf_tail_call(u8 **pprog, u8 *ip)
 	EMIT2(IA32_JBE, jmp_label(jmp_label1, 2));
 
 	/*
-	 * if (tail_call_cnt > MAX_TAIL_CALL_CNT)
+	 * if (tail_call_cnt++ >= MAX_TAIL_CALL_CNT)
 	 *     goto out;
 	 */
 	lo = (u32)MAX_TAIL_CALL_CNT;
@@ -1337,7 +1337,7 @@  static void emit_bpf_tail_call(u8 **pprog, u8 *ip)
 	/* cmp ecx,lo */
 	EMIT3(0x83, add_1reg(0xF8, IA32_ECX), lo);
 
-	/* ja out */
+	/* jae out */
 	EMIT2(IA32_JAE, jmp_label(jmp_label1, 2));
 
 	/* add eax,0x1 */
diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 2be6dfd..4bce7ee 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -1075,7 +1075,7 @@  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/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index ba5af15..b12cfce 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -1744,7 +1744,7 @@  union bpf_attr {
  * 		if the maximum number of tail calls has been reached for this
  * 		chain of programs. This limit is defined in the kernel by the
  * 		macro **MAX_TAIL_CALL_CNT** (not accessible to user space),
- * 		which is currently set to 32.
+ *		which is currently set to 33.
  * 	Return
  * 		0 on success, or a negative error in case of failure.
  *
diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
index 327e399..870881d 100644
--- a/kernel/bpf/core.c
+++ b/kernel/bpf/core.c
@@ -1567,7 +1567,8 @@  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))
+
+		if (unlikely(tail_call_cnt >= MAX_TAIL_CALL_CNT))
 			goto out;
 
 		tail_call_cnt++;
diff --git a/lib/test_bpf.c b/lib/test_bpf.c
index adae395..0c5cb2d 100644
--- a/lib/test_bpf.c
+++ b/lib/test_bpf.c
@@ -14683,7 +14683,7 @@  static struct tail_call_test tail_call_tests[] = {
 			BPF_EXIT_INSN(),
 		},
 		.flags = FLAG_NEED_STATE | FLAG_RESULT_IN_STATE,
-		.result = (MAX_TAIL_CALL_CNT + 1 + 1) * MAX_TESTRUNS,
+		.result = (MAX_TAIL_CALL_CNT + 1) * MAX_TESTRUNS,
 	},
 	{
 		"Tail call count preserved across function calls",
@@ -14705,7 +14705,7 @@  static struct tail_call_test tail_call_tests[] = {
 		},
 		.stack_depth = 8,
 		.flags = FLAG_NEED_STATE | FLAG_RESULT_IN_STATE,
-		.result = (MAX_TAIL_CALL_CNT + 1 + 1) * MAX_TESTRUNS,
+		.result = (MAX_TAIL_CALL_CNT + 1) * MAX_TESTRUNS,
 	},
 	{
 		"Tail call error path, NULL target",
diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
index ba5af15..b12cfce 100644
--- a/tools/include/uapi/linux/bpf.h
+++ b/tools/include/uapi/linux/bpf.h
@@ -1744,7 +1744,7 @@  union bpf_attr {
  * 		if the maximum number of tail calls has been reached for this
  * 		chain of programs. This limit is defined in the kernel by the
  * 		macro **MAX_TAIL_CALL_CNT** (not accessible to user space),
- * 		which is currently set to 32.
+ *		which is currently set to 33.
  * 	Return
  * 		0 on success, or a negative error in case of failure.
  *