Re: [PATCH] libbpf: resolve kernel function name optimization for kprobe

[Yonghong Song <yhs@meta.com>]

On 1/9/23 7:11 PM, Menglong Dong wrote:
> On Tue, Jan 10, 2023 at 4:29 AM Yonghong Song <yhs@meta.com> wrote:
>>
>>
>>
>> On 1/9/23 1:42 AM, menglong8.dong@gmail.com wrote:
>>> From: Menglong Dong <imagedong@tencent.com>
>>>
>>> The function name in kernel may be changed by the compiler. For example,
>>> the function 'ip_rcv_core' can be compiled to 'ip_rcv_core.isra.0'.
>>>
>>> This kind optimization can happen in any kernel function. Therefor, we
>>> should conside this case.
>>>
>>> If we failed to attach kprobe with a '-ENOENT', then we can lookup the
>>> kallsyms and check if there is a similar function end with '.xxx', and
>>> retry.
>>
>> This might produce incorrect result, so this approach won't work
>> for all .isra.0 cases. When a function name is changed from
>> <func> to <func>.isra.<num>, it is possible that compiler may have
>> make some changes to the arguments, e.g., removing one argument,
>> chaning a semantics of argument, etc. if bpf program still
>> uses the original function signature, the bpf program may
>> produce unexpected result.
> 
> Oops, I wasn't aware of this part. Can we make this function disabled
> by default and offer an option to users to enable it? Such as:
> 
>      bpf_object_adapt_sym(struct bpf_object *obj)
> 
> In my case, kernel function rename is common, and I have to
> check all functions and do such adaptation before attaching
> my kprobe programs, which makes me can't use auto-attach.
> 
> What's more, I haven't seen the arguments change so far, and
> maybe it's not a common case?

I don't have statistics, but it happens. In general, if you
want to attach to a function like <foo>, but it has a variant
<foo>.isra.<num>, you probably should check assembly code
to ensure the parameter semantics not changed, and then
you can attach to kprobe function <foo>.isra.<num>, which
I assume current libbpf infrastructure should support it.
After you investigate all these <foo>.isra.<num> functions
and confirm their argument semantics won't change, you
could use kprobe multi to do attachment.

> 
> (Please just ignore this reply if it doesn't work :/ )
> 
> Thanks!
> Menglong Dong
>>
>>>
>>> Signed-off-by: Menglong Dong <imagedong@tencent.com>
>>> ---
>>>    tools/lib/bpf/libbpf.c | 37 ++++++++++++++++++++++++++++++++++++-
>>>    1 file changed, 36 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
>>> index a5c67a3c93c5..fdfb1ca34ced 100644
>>> --- a/tools/lib/bpf/libbpf.c
>>> +++ b/tools/lib/bpf/libbpf.c
>>> @@ -10375,12 +10375,30 @@ bpf_program__attach_kprobe_multi_opts(const struct bpf_program *prog,
>>>        return libbpf_err_ptr(err);
>>>    }
>>>
>>> +struct kprobe_resolve {
>>> +     char pattern[128];
>>> +     char name[128];
>>> +};
>>> +
>>> +static int kprobe_kallsyms_cb(unsigned long long sym_addr, char sym_type,
>>> +                           const char *sym_name, void *ctx)
>>> +{
>>> +     struct kprobe_resolve *res = ctx;
>>> +
>>> +     if (!glob_match(sym_name, res->pattern))
>>> +             return 0;
>>> +     strcpy(res->name, sym_name);
>>> +     return 1;
>>> +}
>>> +
>>>    static int attach_kprobe(const struct bpf_program *prog, long cookie, struct bpf_link **link)
>>>    {
>>>        DECLARE_LIBBPF_OPTS(bpf_kprobe_opts, opts);
>>> +     struct kprobe_resolve res = {};
>>>        unsigned long offset = 0;
>>>        const char *func_name;
>>>        char *func;
>>> +     int err;
>>>        int n;
>>>
>>>        *link = NULL;
>>> @@ -10408,8 +10426,25 @@ static int attach_kprobe(const struct bpf_program *prog, long cookie, struct bpf
>>>
>>>        opts.offset = offset;
>>>        *link = bpf_program__attach_kprobe_opts(prog, func, &opts);
>>> +     err = libbpf_get_error(*link);
>>> +
>>> +     if (!err || err != -ENOENT)
>>> +             goto out;
>>> +
>>> +     sprintf(res.pattern, "%s.*", func);
>>> +     if (!libbpf_kallsyms_parse(kprobe_kallsyms_cb, &res))
>>> +             goto out;
>>> +
>>> +     pr_warn("prog '%s': trying to create %s '%s+0x%zx' perf event instead\n",
>>> +             prog->name, opts.retprobe ? "kretprobe" : "kprobe",
>>> +             res.name, offset);
>>> +
>>> +     *link = bpf_program__attach_kprobe_opts(prog, res.name, &opts);
>>> +     err = libbpf_get_error(*link);
>>> +
>>> +out:
>>>        free(func);
>>> -     return libbpf_get_error(*link);
>>> +     return err;
>>>    }
>>>
>>>    static int attach_ksyscall(const struct bpf_program *prog, long cookie, struct bpf_link **link)