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 and thus confusing at 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 would still be good
to change the value of MAX_TAIL_CALL_CNT from 32 to 33 to make the code
more readable.

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
mips64 JIT, use "ori" instead of "addiu" as suggested by Johan Almbladh.
For the riscv JIT, use RV_REG_TCC directly to save one register move as
suggested by Björn Töpel. 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>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Tested-by: Johan Almbladh <johan.almbladh@anyfinetworks.com>
Tested-by: Ilya Leoshkevich <iii@linux.ibm.com>
Acked-by: Björn Töpel <bjorn@kernel.org>
Acked-by: Johan Almbladh <johan.almbladh@anyfinetworks.com>
Acked-by: Ilya Leoshkevich <iii@linux.ibm.com>
Link: https://lore.kernel.org/bpf/1636075800-3264-1-git-send-email-yangtiezhu@loongson.cn

17 files changed, 35 insertions, 36 deletions

diff --git a/arch/arm/net/bpf_jit_32.c b/arch/arm/net/bpf_jit_32.c
index eeb6dc0ecf46..e59b41e9ab0c 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 86c9dc0681cc..07c12c42b751 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 bd996ede12f8..044b11b65bca 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 815ade724227..6475828ffb36 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 0da31d41d413..8a4faa05f9e4 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 8b5157ccfeba..8571aafcc9e1 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 e6497424cbf6..529a83b85c1c 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 f2a779c7e225..603630b6f3c5 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 233cc9bcd652..9ff2bd83aad7 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 9a2f20cbd48b..0bfe1c72a0c9 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 726700fabca6..631847907786 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 da9b7cfa4632..429a89c5468b 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 56098c866704..cc7a0c36e7df 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -1081,7 +1081,7 @@ 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 6297eafdc40f..a69e4b04ffeb 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 2405e39d800f..b52dc845ecea 100644
--- a/kernel/bpf/core.c
+++ b/kernel/bpf/core.c
@@ -1574,7 +1574,8 @@ select_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 adae39567264..0c5cb2d6436a 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 6297eafdc40f..a69e4b04ffeb 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.
  *