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 <[email protected]>
---
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 <[email protected]>
> ---
>
> 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(-)
[...]
> /* 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/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.
> * 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)
>
On 09/14/2021 03:30 PM, Daniel Borkmann wrote:
> On 9/11/21 3:56 AM, Tiezhu Yang wrote:
>>
[...]
>> 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.
>
>> Signed-off-by: Tiezhu Yang <[email protected]>
>> ---
>>
>> 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
[...]
>> 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--;
[...]