Re: [PATCH bpf-next v2] bpf: Add bpf_check_attach_target_with_klog method to output failure logs to kernel
From: Manjusaka
Date: Thu Jul 25 2024 - 02:33:10 EST
On 2024/7/25 14:09, Yonghong Song wrote:
>
> On 7/24/24 11:05 PM, Leon Hwang wrote:
>>
>> On 25/7/24 13:54, Yonghong Song wrote:
>>> On 7/24/24 10:15 PM, Zheao Li wrote:
>>>> This is a v2 patch, previous Link:
>>>> https://lore.kernel.org/bpf/20240724152521.20546-1-me@xxxxxxxxxxxx/T/#u
>>>>
>>>> Compare with v1:
>>>>
>>>> 1. Format the code style and signed-off field
>>>> 2. Use a shorter name bpf_check_attach_target_with_klog instead of
>>>> original name bpf_check_attach_target_with_kernel_log
>>>>
>>>> When attaching a freplace hook, failures can occur,
>>>> but currently, no output is provided to help developers diagnose the
>>>> root cause.
>>>>
>>>> This commit adds a new method, bpf_check_attach_target_with_klog,
>>>> which outputs the verifier log to the kernel.
>>>> Developers can then use dmesg to obtain more detailed information
>>>> about the failure.
>>>>
>>>> For an example of eBPF code,
>>>> Link:
>>>> https://github.com/Asphaltt/learn-by-example/blob/main/ebpf/freplace/main.go
>>>>
>>>> Co-developed-by: Leon Hwang <hffilwlqm@xxxxxxxxx>
>>>> Signed-off-by: Leon Hwang <hffilwlqm@xxxxxxxxx>
>>>> Signed-off-by: Zheao Li <me@xxxxxxxxxxxx>
>>>> ---
>>>> include/linux/bpf_verifier.h | 5 +++++
>>>> kernel/bpf/syscall.c | 5 +++--
>>>> kernel/bpf/trampoline.c | 6 +++---
>>>> kernel/bpf/verifier.c | 19 +++++++++++++++++++
>>>> 4 files changed, 30 insertions(+), 5 deletions(-)
>>>>
>>>> diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h
>>>> index 5cea15c81b8a..8eddba62c194 100644
>>>> --- a/include/linux/bpf_verifier.h
>>>> +++ b/include/linux/bpf_verifier.h
>>>> @@ -848,6 +848,11 @@ static inline void bpf_trampoline_unpack_key(u64
>>>> key, u32 *obj_id, u32 *btf_id)
>>>> *btf_id = key & 0x7FFFFFFF;
>>>> }
>>>> +int bpf_check_attach_target_with_klog(const struct bpf_prog *prog,
>>>> + const struct bpf_prog *tgt_prog,
>>>> + u32 btf_id,
>>>> + struct bpf_attach_target_info *tgt_info);
>>> format issue in the above. Same code alignment is needed for arguments
>>> in different lines.
>>>
>>>> +
>>>> int bpf_check_attach_target(struct bpf_verifier_log *log,
>>>> const struct bpf_prog *prog,
>>>> const struct bpf_prog *tgt_prog,
>>>> diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
>>>> index 869265852d51..bf826fcc8cf4 100644
>>>> --- a/kernel/bpf/syscall.c
>>>> +++ b/kernel/bpf/syscall.c
>>>> @@ -3464,8 +3464,9 @@ static int bpf_tracing_prog_attach(struct
>>>> bpf_prog *prog,
>>>> */
>>>> struct bpf_attach_target_info tgt_info = {};
>>>> - err = bpf_check_attach_target(NULL, prog, tgt_prog, btf_id,
>>>> - &tgt_info);
>>>> + err = bpf_check_attach_target_with_klog(prog, NULL,
>>>> + prog->aux->attach_btf_id,
>>>> + &tgt_info);
>>> code alignment issue here as well.
>>> Also, the argument should be 'prog, tgt_prog, btf_id, &tgt_info', right?
>>>
>>>> if (err)
>>>> goto out_unlock;
>>>> diff --git a/kernel/bpf/trampoline.c b/kernel/bpf/trampoline.c
>>>> index f8302a5ca400..8862adaa7302 100644
>>>> --- a/kernel/bpf/trampoline.c
>>>> +++ b/kernel/bpf/trampoline.c
>>>> @@ -699,9 +699,9 @@ int bpf_trampoline_link_cgroup_shim(struct
>>>> bpf_prog *prog,
>>>> u64 key;
>>>> int err;
>>>> - err = bpf_check_attach_target(NULL, prog, NULL,
>>>> - prog->aux->attach_btf_id,
>>>> - &tgt_info);
>>>> + err = bpf_check_attach_target_with_klog(prog, NULL,
>>>> + prog->aux->attach_btf_id,
>>>> + &tgt_info);
>>> code alignment issue here
>>>
>>>> if (err)
>>>> return err;
>>>> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
>>>> index 1f5302fb0957..4873b72f5a9a 100644
>>>> --- a/kernel/bpf/verifier.c
>>>> +++ b/kernel/bpf/verifier.c
>>>> @@ -21643,6 +21643,25 @@ static int
>>>> check_non_sleepable_error_inject(u32 btf_id)
>>>> return btf_id_set_contains(&btf_non_sleepable_error_inject,
>>>> btf_id);
>>>> }
>>>> +int bpf_check_attach_target_with_klog(const struct bpf_prog *prog,
>>>> + const struct bpf_prog *tgt_prog,
>>>> + u32 btf_id,
>>>> + struct bpf_attach_target_info *tgt_info);
>>> code alignment issue here.
>>>
>>>> +{
>>>> + struct bpf_verifier_log *log;
>>>> + int err;
>>>> +
>>>> + log = kzalloc(sizeof(*log), GFP_KERNEL | __GFP_NOWARN);
>>> __GFP_NOWARN is unnecessary here.
>>>
>>>> + if (!log) {
>>>> + err = -ENOMEM;
>>>> + return err;
>>>> + }
>>>> + log->level = BPF_LOG_KERNEL;
>>>> + err = bpf_check_attach_target(log, prog, tgt_prog, btf_id,
>>>> tgt_info);
>>>> + kfree(log);
>>>> + return err;
>>>> +}
>>>> +
>>>> int bpf_check_attach_target(struct bpf_verifier_log *log,
>>>> const struct bpf_prog *prog,
>>>> const struct bpf_prog *tgt_prog,
>>> More importantly, Andrii has implemented retsnoop, which intends to locate
>>> precise location in the kernel where err happens. The link is
>>> https://github.com/anakryiko/retsnoop
>>>
>>> Maybe you want to take a look and see whether it can resolve your issue.
>>> We should really avoid putting more stuff in dmesg whenever possible.
>>>
>> retsnoop is really cool.
>>
>> However, when something wrong in bpf_check_attach_target(), retsnoop
>> only gets its return value -EINVAL, without any bpf_log() in it. It's
>> hard to figure out the reason why bpf_check_attach_target() returns -EINVAL.
>
> It should have line number like below in https://github.com/anakryiko/retsnoop
>
> |$ sudo ./retsnoop -e '*sys_bpf' -a ':kernel/bpf/*.c' Receiving data... 20:19:36.372607 -> 20:19:36.372682 TID/PID 8346/8346 (simfail/simfail): entry_SYSCALL_64_after_hwframe+0x63 (arch/x86/entry/entry_64.S:120:0) do_syscall_64+0x35 (arch/x86/entry/common.c:80:7) . do_syscall_x64 (arch/x86/entry/common.c:50:12) 73us [-ENOMEM] __x64_sys_bpf+0x1a (kernel/bpf/syscall.c:5067:1) 70us [-ENOMEM] __sys_bpf+0x38b (kernel/bpf/syscall.c:4947:9) . map_create (kernel/bpf/syscall.c:1106:8) . find_and_alloc_map (kernel/bpf/syscall.c:132:5) ! 50us [-ENOMEM] array_map_alloc !* 2us [NULL] bpf_map_alloc_percpu Could you double check? It does need corresponding kernel source though. |
>
>>
>> How about adding a tracepoint in bpf_check_attach_target_with_klog()?
>> It's to avoid putting stuff in dmesg.
>>
>> Thanks,
>> Leon
>>
>>
Thanks for the review and reply. I will update the patch later when we reach the same point
Actually, for personally, I think it would be better to get the error message from dmesg.
I can get enough information without installing extra dependency
Thanks
Zheao Li/Nadeshiko Manju