[PATCH bpf-next v4] bpf: Change value of MAX_TAIL_CALL_CNT from 32 to 33

From: Tiezhu Yang
Date: Tue Nov 02 2021 - 04:52:00 EST

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.

With this patch, it does not change the current limit 33, MAX_TAIL_CALL_CNT
can reflect the actual max tail call count, the related tailcall testcases
in test_bpf 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
#141 tailcalls:OK
Summary: 1/11 PASSED, 0 SKIPPED, 0 FAILED

Signed-off-by: Tiezhu Yang <yangtiezhu@xxxxxxxxxxx>
---
arch/arm/net/bpf_jit_32.c | 5 +++--
arch/arm64/net/bpf_jit_comp.c | 5 +++--
arch/mips/net/bpf_jit_comp32.c | 2 +-
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 | 5 ++---
arch/riscv/net/bpf_jit_comp64.c | 4 ++--
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 | 6 +++---
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(+), 33 deletions(-)

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 803e777..b2f6efb 100644
--- a/arch/arm64/net/bpf_jit_comp.c
+++ b/arch/arm64/net/bpf_jit_comp.c
@@ -286,13 +286,14 @@ 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..a7b424f 100644
--- a/arch/mips/net/bpf_jit_comp32.c
+++ b/arch/mips/net/bpf_jit_comp32.c
@@ -1382,7 +1382,7 @@ void build_prologue(struct jit_context *ctx)
* 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));
+ 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..5ef8778 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, addiu, 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..ead9733 100644
--- a/arch/riscv/net/bpf_jit_comp32.c
+++ b/arch/riscv/net/bpf_jit_comp32.c
@@ -799,13 +799,12 @@ 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);
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 2ca345c..9822f58 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);
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/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 e474718..adb2999 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_JAE, OFFSET2); /* jae 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_JAE, off1); /* 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 3bfda5f..fd37c21 100644
--- a/arch/x86/net/bpf_jit_comp32.c
+++ b/arch/x86/net/bpf_jit_comp32.c
@@ -1272,7 +1272,7 @@ static void emit_epilogue(u8 **pprog, u32 stack_depth)
* ... 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)
@@ -1307,7 +1307,7 @@ static void emit_bpf_tail_call(u8 **pprog)
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;
@@ -1321,7 +1321,7 @@ static void emit_bpf_tail_call(u8 **pprog)
/* 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.
*
--
2.1.0