linux-kernel - Re: [PATCH bpf-next v5 1/3] perf: Refactor get_perf

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Message-Id: <886b17f3dd9f9836dd87b9249a01e737088fe52174b8d513ad6350c6bb19ff87@mail.kernel.org>
Date: Sun,  9 Nov 2025 16:58:15 +0000 (UTC)
From: bot+bpf-ci@...nel.org
To: chen.dylane@...ux.dev,peterz@...radead.org,mingo@...hat.com,acme@...nel.org,namhyung@...nel.org,mark.rutland@....com,alexander.shishkin@...ux.intel.com,jolsa@...nel.org,irogers@...gle.com,adrian.hunter@...el.com,kan.liang@...ux.intel.com
Cc: linux-perf-users@...r.kernel.org,linux-kernel@...r.kernel.org,bpf@...r.kernel.org,chen.dylane@...ux.dev,ast@...nel.org,andrii@...nel.org,daniel@...earbox.net,martin.lau@...nel.org,eddyz87@...il.com,yonghong.song@...ux.dev,clm@...a.com,ihor.solodrai@...ux.dev
Subject: Re: [PATCH bpf-next v5 1/3] perf: Refactor get_perf_callchain

> diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
> index fd1d91017..edd3058e4 100644
> --- a/include/linux/perf_event.h
> +++ b/include/linux/perf_event.h
> @@ -67,6 +67,7 @@ struct perf_callchain_entry_ctx {
>  	u32				nr;
>  	short				contexts;
>  	bool				contexts_maxed;
> +	bool				add_mark;
>  };

[ ... ]

> diff --git a/kernel/events/callchain.c b/kernel/events/callchain.c
> index 808c0d7a3..fb1f26be2 100644
> --- a/kernel/events/callchain.c
> +++ b/kernel/events/callchain.c

[ ... ]

> +void __get_perf_callchain_user(struct perf_callchain_entry_ctx *ctx, struct pt_regs *regs)
> +{
> +	int start_entry_idx;
> +
> +	if (!user_mode(regs)) {
> +		if (current->flags & (PF_KTHREAD | PF_USER_WORKER))
> +			return;
> +		regs = task_pt_regs(current);
> +	}
> +
> +	if (ctx->add_mark)
> +		perf_callchain_store_context(ctx, PERF_CONTEXT_USER);
> +
> +	start_entry_idx = ctx->nr;
                          ^^^^^^^^
> +	perf_callchain_user(ctx, regs);
> +	fixup_uretprobe_trampoline_entries(ctx->entry, start_entry_idx);
> +}

Can this break uprobe trampoline fixups? The original code used
entry->nr for start_entry_idx, but the refactored version uses ctx->nr.

These two counters track different things:
- entry->nr counts ALL entries (incremented by both
  perf_callchain_store_context() and perf_callchain_store())
- ctx->nr only counts regular callchain entries (incremented only by
  perf_callchain_store(), not by perf_callchain_store_context())

When add_mark is true, perf_callchain_store_context() increments
entry->nr but not ctx->nr, causing them to become out of sync.

For example, if kernel callchain collection runs first with add_mark
set:

  Initial: ctx->nr = entry->nr = 0

  In __get_perf_callchain_kernel():
    perf_callchain_store_context() stores PERF_CONTEXT_KERNEL marker
      -> entry->nr becomes 1, ctx->nr still 0
    perf_callchain_kernel() stores N kernel frames
      -> entry->nr becomes 1+N, ctx->nr becomes N

  In __get_perf_callchain_user():
    perf_callchain_store_context() stores PERF_CONTEXT_USER marker
      -> entry->nr becomes 2+N, ctx->nr still N
    start_entry_idx = ctx->nr assigns N (should be 2+N)
    fixup_uretprobe_trampoline_entries() scans from wrong index

This causes fixup_uretprobe_trampoline_entries() to scan kernel
callchain entries instead of user callchain entries, missing the
uretprobe trampolines that need to be fixed up.

Should this use ctx->entry->nr instead of ctx->nr?


---
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See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md

CI run summary: https://github.com/kernel-patches/bpf/actions/runs/19211481431