linux-kernel - Re: [PATCH bpf-next v1 03/10] bpf: Verifier support for KF_IMPLICIT

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Message-ID: <e796df2b-6dd9-4b45-b8fb-9ef9f7fceab6@linux.dev>
Date: Tue, 13 Jan 2026 20:08:02 -0800
From: Ihor Solodrai <ihor.solodrai@...ux.dev>
To: Eduard Zingerman <eddyz87@...il.com>, Alexei Starovoitov
 <ast@...nel.org>, Andrii Nakryiko <andrii@...nel.org>,
 Daniel Borkmann <daniel@...earbox.net>,
 Martin KaFai Lau <martin.lau@...ux.dev>
Cc: Mykyta Yatsenko <yatsenko@...a.com>, Tejun Heo <tj@...nel.org>,
 Alan Maguire <alan.maguire@...cle.com>,
 Benjamin Tissoires <bentiss@...nel.org>, Jiri Kosina <jikos@...nel.org>,
 bpf@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-input@...r.kernel.org, sched-ext@...ts.linux.dev
Subject: Re: [PATCH bpf-next v1 03/10] bpf: Verifier support for
 KF_IMPLICIT_ARGS



On 1/13/26 5:06 PM, Eduard Zingerman wrote:
> On Tue, 2026-01-13 at 16:03 -0800, Ihor Solodrai wrote:
>> On 1/13/26 1:59 PM, Eduard Zingerman wrote:
>>> On Fri, 2026-01-09 at 10:48 -0800, Ihor Solodrai wrote:
>>>
>>> [...]
>>>
>>>> --- a/kernel/bpf/verifier.c
>>>> +++ b/kernel/bpf/verifier.c
>>>> @@ -3271,6 +3271,38 @@ static struct btf *find_kfunc_desc_btf(struct bpf_verifier_env *env, s16 offset)
>>>>  	return btf_vmlinux ?: ERR_PTR(-ENOENT);
>>>>  }
>>>>  
>>>> +#define KF_IMPL_SUFFIX "_impl"
>>>> +
>>>> +static const struct btf_type *find_kfunc_impl_proto(struct bpf_verifier_env *env,
>>>> +						    struct btf *btf,
>>>> +						    const char *func_name)
>>>> +{
>>>> +	char impl_name[KSYM_SYMBOL_LEN];
>>>
>>> Oh, as we discussed already, this should use env->tmp_str_buf.
>>
>> The env->tmp_str_buf size is smaller:
>>
>> 	#define TMP_STR_BUF_LEN 320
>>
>> *And* there is already a local char buffer of size KSYM_SYMBOL_LEN
>> already in use in verifier.c:
>>
>> 	int bpf_check_attach_target(...) {
>> 		bool prog_extension = prog->type == BPF_PROG_TYPE_EXT;
>> 		bool prog_tracing = prog->type == BPF_PROG_TYPE_TRACING;
>> 		char trace_symbol[KSYM_SYMBOL_LEN];
>> 	[...]
>>
>> Since these are function names, the real limit is KSYM_SYMBOL_LEN,
>> right?
>>
>> Sure >320 chars long kfunc name is unlikely, but technically possible.
> 
> 320 is good enough, you'll be able to cover this:
> 
> kfunc_trace_long_descriptive_kernel_symbol_for_tracing_scheduler_memory_io_and_interrupt_paths_during_runtime_analysis_of_latency_throughput_and_resource_contention_on_large_scale_multiprocessor_linux_systems_using_bpf_and_kprobes_without_requiring_kernel_recompilation_or_system_restart_for_production_use_cases_v2x
> 
> But not this:
> 
> kfunc_trace_kernel_scheduler_and_memory_management_path_for_observing_task_lifecycle_events_context_switches_page_fault_handling_and_io_wait_states_while_debugging_performance_regressions_on_large_multiprocessor_systems_running_preemptible_linux_kernels_with_bpf_tracing_and_dynamic_instrumentation_enabled_for_deep_visibility_into_runtime_behavior_and_latency_sensitive_code_paths_without_recompilation.
> 
> Should suffice, I think.

I will laugh for at least 321 seconds when the size of this buffer
will have to be increased, and will make sure you hear it :)

They thought 640K of memory is enough, you know.