From nobody Sat Feb 7 21:08:11 2026 Received: from out-179.mta1.migadu.com (out-179.mta1.migadu.com [95.215.58.179]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 98D5E286415 for ; Tue, 28 Oct 2025 16:26:47 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=95.215.58.179 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1761668810; cv=none; b=XqeHeDN1D1Mqj3c3ANHAcKa5YHY1v8xbQYmWpJKuVvWdZqm1A1Zs8GG9dXrMFCOzZhVEhYKWjpPXlObXAvk9EYras0LiVhj6Hj5OHn5lVGEbd86tPMVvlsTzkt1cmyUbUoiOrShz9T3o644ygSLPd9p/O6daU1O/9qmpR0DnqeY= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1761668810; c=relaxed/simple; bh=C77mCOcYKZDT1tjjWTbER/wZyL1dVdsIKBcn719SqSw=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=bA0mIOmTtWDemGbT836Mh5vpnSorojQbSm8tMdyIId11KaQJ/wVUKoL6n0OMZ+JvdDnOheErugtjFkasHqcc5pJDmbv/0uR3MjICm7QN6hRjZa3l8CClHuSUuc/fYA2EiQluBINS7qmRZmWC+pvqfz2mb8pxojtLpYwFvY9yd2g= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=a0PCiNY5; arc=none smtp.client-ip=95.215.58.179 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="a0PCiNY5" X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1761668805; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=K3lqjGd6Xn2XO6sjJZ+0OWHHK1rsFpO56tXY2t0cYhs=; b=a0PCiNY5lQxcHytYeRlIhR7nevUi+ijJoAYHK+qLd1TLrzdawejupjN6sogsllFKiSQeAn 4bULc/bSvMz6MnUftVg13hbmMM9VqgOjoH8gIYqMxd9ae7QuTEVljlbvPjZy19zS5qtV+A YP7mC8PAUdlB/KflPDUOvUWtd9XNsxo= From: Tao Chen To: peterz@infradead.org, mingo@redhat.com, acme@kernel.org, namhyung@kernel.org, mark.rutland@arm.com, alexander.shishkin@linux.intel.com, jolsa@kernel.org, irogers@google.com, adrian.hunter@intel.com, kan.liang@linux.intel.com, song@kernel.org, ast@kernel.org, daniel@iogearbox.net, andrii@kernel.org, martin.lau@linux.dev, eddyz87@gmail.com, yonghong.song@linux.dev, john.fastabend@gmail.com, kpsingh@kernel.org, sdf@fomichev.me, haoluo@google.com Cc: linux-perf-users@vger.kernel.org, linux-kernel@vger.kernel.org, bpf@vger.kernel.org, Tao Chen Subject: [PATCH bpf-next v4 2/2] bpf: Hold the perf callchain entry until used completely Date: Wed, 29 Oct 2025 00:25:02 +0800 Message-ID: <20251028162502.3418817-3-chen.dylane@linux.dev> In-Reply-To: <20251028162502.3418817-1-chen.dylane@linux.dev> References: <20251028162502.3418817-1-chen.dylane@linux.dev> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable X-Migadu-Flow: FLOW_OUT Content-Type: text/plain; charset="utf-8" As Alexei noted, get_perf_callchain() return values may be reused if a task is preempted after the BPF program enters migrate disable mode. The perf_callchain_entres has a small stack of entries, and we can reuse it as follows: 1. get the perf callchain entry 2. BPF use... 3. put the perf callchain entry Signed-off-by: Tao Chen --- kernel/bpf/stackmap.c | 61 ++++++++++++++++++++++++++++++++++--------- 1 file changed, 48 insertions(+), 13 deletions(-) diff --git a/kernel/bpf/stackmap.c b/kernel/bpf/stackmap.c index e28b35c7e0b..70d38249083 100644 --- a/kernel/bpf/stackmap.c +++ b/kernel/bpf/stackmap.c @@ -188,13 +188,12 @@ static void stack_map_get_build_id_offset(struct bpf_= stack_build_id *id_offs, } =20 static struct perf_callchain_entry * -get_callchain_entry_for_task(struct task_struct *task, u32 max_depth) +get_callchain_entry_for_task(int *rctx, struct task_struct *task, u32 max_= depth) { #ifdef CONFIG_STACKTRACE struct perf_callchain_entry *entry; - int rctx; =20 - entry =3D get_callchain_entry(&rctx); + entry =3D get_callchain_entry(rctx); =20 if (!entry) return NULL; @@ -216,8 +215,6 @@ get_callchain_entry_for_task(struct task_struct *task, = u32 max_depth) to[i] =3D (u64)(from[i]); } =20 - put_callchain_entry(rctx); - return entry; #else /* CONFIG_STACKTRACE */ return NULL; @@ -297,6 +294,31 @@ static long __bpf_get_stackid(struct bpf_map *map, return id; } =20 +static struct perf_callchain_entry * +bpf_get_perf_callchain(int *rctx, struct pt_regs *regs, bool kernel, bool = user, + int max_stack, bool crosstask) +{ + struct perf_callchain_entry_ctx ctx; + struct perf_callchain_entry *entry; + + entry =3D get_callchain_entry(rctx); + if (unlikely(!entry)) + return NULL; + + __init_perf_callchain_ctx(&ctx, entry, max_stack, false); + if (kernel) + __get_perf_callchain_kernel(&ctx, regs); + if (user && !crosstask) + __get_perf_callchain_user(&ctx, regs); + + return entry; +} + +static void bpf_put_callchain_entry(int rctx) +{ + put_callchain_entry(rctx); +} + BPF_CALL_3(bpf_get_stackid, struct pt_regs *, regs, struct bpf_map *, map, u64, flags) { @@ -305,6 +327,7 @@ BPF_CALL_3(bpf_get_stackid, struct pt_regs *, regs, str= uct bpf_map *, map, bool user =3D flags & BPF_F_USER_STACK; struct perf_callchain_entry *trace; bool kernel =3D !user; + int rctx, ret; =20 if (unlikely(flags & ~(BPF_F_SKIP_FIELD_MASK | BPF_F_USER_STACK | BPF_F_FAST_STACK_CMP | BPF_F_REUSE_STACKID))) @@ -314,14 +337,15 @@ BPF_CALL_3(bpf_get_stackid, struct pt_regs *, regs, s= truct bpf_map *, map, if (max_depth > sysctl_perf_event_max_stack) max_depth =3D sysctl_perf_event_max_stack; =20 - trace =3D get_perf_callchain(regs, kernel, user, max_depth, - false); - + trace =3D bpf_get_perf_callchain(&rctx, regs, kernel, user, max_depth, fa= lse); if (unlikely(!trace)) /* couldn't fetch the stack trace */ return -EFAULT; =20 - return __bpf_get_stackid(map, trace, flags); + ret =3D __bpf_get_stackid(map, trace, flags); + bpf_put_callchain_entry(rctx); + + return ret; } =20 const struct bpf_func_proto bpf_get_stackid_proto =3D { @@ -415,6 +439,7 @@ static long __bpf_get_stack(struct pt_regs *regs, struc= t task_struct *task, bool kernel =3D !user; int err =3D -EINVAL; u64 *ips; + int rctx; =20 if (unlikely(flags & ~(BPF_F_SKIP_FIELD_MASK | BPF_F_USER_STACK | BPF_F_USER_BUILD_ID))) @@ -449,17 +474,24 @@ static long __bpf_get_stack(struct pt_regs *regs, str= uct task_struct *task, if (trace_in) trace =3D trace_in; else if (kernel && task) - trace =3D get_callchain_entry_for_task(task, max_depth); + trace =3D get_callchain_entry_for_task(&rctx, task, max_depth); else - trace =3D get_perf_callchain(regs, kernel, user, max_depth, - crosstask); + trace =3D bpf_get_perf_callchain(&rctx, regs, kernel, user, max_depth, c= rosstask); =20 - if (unlikely(!trace) || trace->nr < skip) { + if (unlikely(!trace)) { if (may_fault) rcu_read_unlock(); goto err_fault; } =20 + if (trace->nr < skip) { + if (may_fault) + rcu_read_unlock(); + if (!trace_in) + bpf_put_callchain_entry(rctx); + goto err_fault; + } + trace_nr =3D trace->nr - skip; trace_nr =3D (trace_nr <=3D num_elem) ? trace_nr : num_elem; copy_len =3D trace_nr * elem_size; @@ -479,6 +511,9 @@ static long __bpf_get_stack(struct pt_regs *regs, struc= t task_struct *task, if (may_fault) rcu_read_unlock(); =20 + if (!trace_in) + bpf_put_callchain_entry(rctx); + if (user_build_id) stack_map_get_build_id_offset(buf, trace_nr, user, may_fault); =20 --=20 2.48.1