Re: [PATCH v3 4/6] perf bpf-filter: Extend lock_held() to support RCU, Tasks Trace RCU, and SRCU

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] perf bpf-filter: btf_has_task_field() misses anon struct under RANDSTRUCT
--

commit 5ff446ff868169d426bb172f1b13dc459b646352
Author: Ian Rogers <irogers@google.com>

perf bpf-filter: Extend lock_held() to support RCU, Tasks Trace RCU, and SRCU

This commit extends the lock_held(<lock>) BPF sample filter to support RCU,
Tasks Trace RCU, and SRCU locks. For RCU and Tasks Trace RCU read-side
critical sections, it inspects task_struct nesting fields via CO-RE, while
attaching fentry/fexit hooks for SRCU and RCU synchronize/barrier waits.

> diff --git a/tools/perf/util/bpf-filter.c b/tools/perf/util/bpf-filter.c
> index 0c133b31b5d54..4a5ef78e26900 100644
> --- a/tools/perf/util/bpf-filter.c
> +++ b/tools/perf/util/bpf-filter.c

[ ... ]

> @@ -847,16 +913,54 @@ static bool find_slab_cache(struct sample_filter_bpf *skel, const char *name,
>  	return false;
>  }
>  
> +static u64 rcu_min_depth(struct evsel *evsel)
> +{
> +	if (evsel && evsel->core.attr.type == PERF_TYPE_SOFTWARE &&
> +	    evsel->core.attr.config != PERF_COUNT_SW_CPU_CLOCK &&
> +	    evsel->core.attr.config != PERF_COUNT_SW_TASK_CLOCK)
> +		return 2;
> +	return 1;
> +}
> +
> +static bool btf_has_task_field(struct btf *btf, const char *field_name)
> +{
> +	const struct btf_type *t;
> +	const struct btf_member *m;
> +	s32 id;
> +	u16 i, vlen;
> +
> +	if (!btf)
> +		return false;
> +
> +	id = btf__find_by_name_kind(btf, "task_struct", BTF_KIND_STRUCT);
> +	if (id < 0)
> +		return false;
> +
> +	t = btf__type_by_id(btf, id);
> +	if (!t)
> +		return false;
> +
> +	m = btf_members(t);
> +	vlen = btf_vlen(t);
> +	for (i = 0; i < vlen; i++, m++) {
> +		const char *name = btf__name_by_offset(btf, m->name_off);
> +
> +		if (name && !strcmp(name, field_name))
> +			return true;
> +	}
> +	return false;
> +}

[Severity: Medium]
In include/linux/sched.h, both rcu_read_lock_nesting and trc_reader_nesting
are declared inside the randomized_struct_fields_start /
randomized_struct_fields_end block of struct task_struct:

include/linux/sched.h:
    struct task_struct {
        ...
        randomized_struct_fields_start
        ...
    #ifdef CONFIG_PREEMPT_RCU
        int rcu_read_lock_nesting;
        ...
    #endif
    #ifdef CONFIG_TASKS_TRACE_RCU
        int trc_reader_nesting;
        ...
    #endif
        ...
        randomized_struct_fields_end
    };

When CONFIG_RANDSTRUCT is enabled, randomized_struct_fields_start wraps those
fields inside an anonymous struct member of task_struct, which appears in
vmlinux BTF as a member with m->name_off == 0 pointing to an inner
BTF_KIND_STRUCT.

While libbpf CO-RE in sample_filter.bpf.c recursively descends into anonymous
structs and unions, btf_has_task_field() only iterates over the direct
top-level members of task_struct.

Will btf_has_task_field() return false on CONFIG_RANDSTRUCT kernels, causing
resolve_lock_expr() to reject lock_held(rcu) and lock_held(rcu-trace) with
-EOPNOTSUPP?

[ ... ]

> @@ -929,6 +1033,48 @@ static int resolve_lock_expr(struct sample_filter_bpf *skel,
>  		return 0;
>  	}
>  
> +	if (!strcmp(base, "rcu_read_lock")) {
> +		if (mode == PBF_LOCK_MODE_WRITE) {
> +			pr_err("Error: Lock '%s' only supports read/shared mode\n", name);
> +			return -EINVAL;
> +		}
> +		if (btf && !btf_has_task_field(btf, "rcu_read_lock_nesting")) {
> +			pr_err("Error: lock_held(rcu) requires CONFIG_PREEMPT_RCU\n");
> +			return -EOPNOTSUPP;
> +		}
> +		entry->part = PBF_LOCK_RCU | PBF_LOCK_MODE_READ;
> +		entry->target = rcu_min_depth(evsel);
> +		return 0;
> +	}

[ ... ]

—
🤖 Generated by Linux Kernel Patch Reviewer (Model: gemini-3.1-pro-preview)

 Prompts used:
 - bpf.md (https://raw.githubusercontent.com/masoncl/review-prompts/main/bpf.md)
 - btf.md (https://raw.githubusercontent.com/masoncl/review-prompts/main/btf.md)
 - inline-template.md (https://raw.githubusercontent.com/masoncl/review-prompts/main/inline-template.md)
 - libbpf.md (https://raw.githubusercontent.com/masoncl/review-prompts/main/libbpf.md)
 - perf.md (https://raw.githubusercontent.com/masoncl/review-prompts/main/perf.md)
 - rcu.md (https://raw.githubusercontent.com/masoncl/review-prompts/main/rcu.md)
 - tracing.md (https://raw.githubusercontent.com/masoncl/review-prompts/main/tracing.md)

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261018053710.2572636-1-irogers@google.com?part=4