[PATCH bpf-next v2] bpf: Change value of MAX_TAIL_CALL_CNT from 32 to 33
Daniel Borkmann
daniel at iogearbox.net
Tue Sep 14 17:30:59 AEST 2021
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 at 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(-)
[...]
> /* 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)
>
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