Re: [PATCH 11/19] bpf: Add support to load multi func tracing program

Yonghong Song
2021-06-07T19:35:59Z

On 6/7/21 11:18 AM, Jiri Olsa wrote:
> On Sun, Jun 06, 2021 at 08:56:47PM -0700, Yonghong Song wrote:
>>
>>
>> On 6/5/21 4:10 AM, Jiri Olsa wrote:
>>> Adding support to load tracing program with new BPF_F_MULTI_FUNC flag,
>>> that allows the program to be loaded without specific function to be
>>> attached to.
>>>
>>> The verifier assumes the program is using all (6) available arguments
>>
>> Is this a verifier failure or it is due to the check in the
>> beginning of function arch_prepare_bpf_trampoline()?
>>
>> /* x86-64 supports up to 6 arguments. 7+ can be added in the future
>> */
>> if (nr_args > 6)
>> return -ENOTSUPP;
>
> yes, that's the limit.. it allows the traced program to
> touch 6 arguments, because it's the maximum for JIT
>
>>
>> If it is indeed due to arch_prepare_bpf_trampoline() maybe we
>> can improve it instead of specially processing the first argument
>> "ip" in quite some places?
>
> do you mean to teach JIT to process more than 6 arguments?

Yes. Not sure how hard it is. If it is doable with reasonable
complexity, I think it will be worth it as it will benefit this
case to avoid special tweaks of the first argument, but also
benefit other cases e.g., attaching to a kernel function with
7 or more arguments.

>
>>
>>> as unsigned long values. We can't add extra ip argument at this time,
>>> because JIT on x86 would fail to process this function. Instead we
>>> allow to access extra first 'ip' argument in btf_ctx_access.
>>>
>>> Such program will be allowed to be attached to multiple functions
>>> in following patches.
>>>
>>> Signed-off-by: Jiri Olsa <jolsa@kernel.org>
>>> ---
>>> include/linux/bpf.h | 1 +
>>> include/uapi/linux/bpf.h | 7 +++++++
>>> kernel/bpf/btf.c | 5 +++++
>>> kernel/bpf/syscall.c | 35 +++++++++++++++++++++++++++++-----
>>> kernel/bpf/verifier.c | 3 ++-
>>> tools/include/uapi/linux/bpf.h | 7 +++++++
>>> 6 files changed, 52 insertions(+), 6 deletions(-)
>>> …