From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-176.mta1.migadu.com (out-176.mta1.migadu.com [95.215.58.176]) (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 D13F9287272 for ; Sun, 28 Sep 2025 04:28:50 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=95.215.58.176 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1759033732; cv=none; b=KBI5VRsFvo1BBhulCWKTaq7wokTS/OIsWyIbzfTAt2UCQokEukTFBPHzl9miuv06Qb6JmS5wH/E9EkSPp+9t3al0aD7VJCnjkedMbYbetuCanr9DqMSaJ+QPp1/Sknvm7K6gOprPPESmVlPyfNUY2qvkTD66sodElFVjLrKfv8w= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1759033732; c=relaxed/simple; bh=aGJzrxX/6tyC66XM5+5CXfFahLXDJKs8U5VVSlR/nJM=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=oa/BNCQCIqfpHcAvc9P0yhWG6pqCfKhbWq9yEe35XWVREN2G7AW+bk/AnMGddFklLEZn1ORDCL2roYHQavR8vDVTlbChvrJJ5SComnaRvNvYyB5l5DiFaC+Q1GAzyWpx5z4ds4fcu/yVmKPuZSG8voF06jQNWhPcjzTlSvQCII8= 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=VKvJboHT; arc=none smtp.client-ip=95.215.58.176 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="VKvJboHT" Message-ID: <71799ce4-cc2e-41d6-a5fb-d4af5c445e43@linux.dev> DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1759033719; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=uBCSfd9thTLA3FjzbiGVoo7eVJR3xdivXq6kfTEcxEU=; b=VKvJboHTewBe8S6pxoc9+ncgfgD7CxJJDHoc19V6jDlvJ1gI1s4poS+le9OQ9GcIyejdjU G55JV7+kzZwH3zcOJIAyJmzRvKM0YTG/M9NB6BnU0i7bNVdHzOTjVRACU+1nD+OESejfp4 j3Wq/LUL5RIXMw6d1p9i580UCSN9NiQ= Date: Sun, 28 Sep 2025 12:28:26 +0800 Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Subject: Re: [PATCH bpf-next v2] bpf: Add preempt_disable to protect get_perf_callchain To: Andrii Nakryiko Cc: song@kernel.org, jolsa@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, bpf@vger.kernel.org, linux-kernel@vger.kernel.org References: <20250926153952.1661146-1-chen.dylane@linux.dev> X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Tao Chen In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT 在 2025/9/27 02:52, Andrii Nakryiko 写道: > On Fri, Sep 26, 2025 at 8:40 AM Tao Chen wrote: >> >> As Alexei noted, get_perf_callchain() return values may be reused >> if a task is preempted after the BPF program enters migrate disable >> mode. We therefore use bpf_perf_callchain_entries percpu entries >> similarly to bpf_try_get_buffers to preserve the current task's >> callchain and prevent overwriting by preempting tasks. And we also >> add preempt_disable to protect get_perf_callchain. >> >> Reported-by: Alexei Starovoitov >> Closes: https://lore.kernel.org/bpf/CAADnVQ+s8B7-fvR1TNO-bniSyKv57cH_ihRszmZV7pQDyV=VDQ@mail.gmail.com >> Signed-off-by: Tao Chen >> --- >> kernel/bpf/stackmap.c | 76 ++++++++++++++++++++++++++++++++++--------- >> 1 file changed, 61 insertions(+), 15 deletions(-) >> >> Change list: >> v1 -> v2: >> From Alexei >> - create percpu entris to preserve current task's callchain >> similarly to bpf_try_get_buffers. >> v1: https://lore.kernel.org/bpf/20250922075333.1452803-1-chen.dylane@linux.dev >> >> diff --git a/kernel/bpf/stackmap.c b/kernel/bpf/stackmap.c >> index 2e182a3ac4c..8788c219926 100644 >> --- a/kernel/bpf/stackmap.c >> +++ b/kernel/bpf/stackmap.c >> @@ -31,6 +31,55 @@ struct bpf_stack_map { >> struct stack_map_bucket *buckets[] __counted_by(n_buckets); >> }; >> >> +struct bpf_perf_callchain_entry { >> + u64 nr; >> + u64 ip[PERF_MAX_STACK_DEPTH]; >> +}; >> + >> +#define MAX_PERF_CALLCHAIN_PREEMPT 3 >> +static DEFINE_PER_CPU(struct bpf_perf_callchain_entry[MAX_PERF_CALLCHAIN_PREEMPT], >> + bpf_perf_callchain_entries); >> +static DEFINE_PER_CPU(int, bpf_perf_callchain_preempt_cnt); >> + >> +static int bpf_get_perf_callchain(struct bpf_perf_callchain_entry **entry, >> + struct pt_regs *regs, u32 init_nr, bool kernel, >> + bool user, u32 max_stack, bool crosstack, >> + bool add_mark) >> +{ >> + struct bpf_perf_callchain_entry *bpf_entry; >> + struct perf_callchain_entry *perf_entry; >> + int preempt_cnt; >> + >> + preempt_cnt = this_cpu_inc_return(bpf_perf_callchain_preempt_cnt); >> + if (WARN_ON_ONCE(preempt_cnt > MAX_PERF_CALLCHAIN_PREEMPT)) { >> + this_cpu_dec(bpf_perf_callchain_preempt_cnt); >> + return -EBUSY; >> + } >> + >> + bpf_entry = this_cpu_ptr(&bpf_perf_callchain_entries[preempt_cnt - 1]); >> + >> + preempt_disable(); >> + perf_entry = get_perf_callchain(regs, init_nr, kernel, user, max_stack, >> + crosstack, add_mark); >> + if (unlikely(!perf_entry)) { >> + preempt_enable(); >> + this_cpu_dec(bpf_perf_callchain_preempt_cnt); >> + return -EFAULT; >> + } >> + memcpy(bpf_entry, perf_entry, sizeof(u64) * (perf_entry->nr + 1)); > > N copies of a stack trace is not good enough, let's have N + 1 now :) > > If we are going with our own buffers, we need to teach > get_perf_callchain to let us pass that buffer directly to avoid that > unnecessary copy. > > Also, I know it's about 1KB, but it would be so simple and efficient > to just have this bpf_perf_callchain_entry on the stack. Kernel has a > 16KB stack, right? It feels like for something like this using 1KB of > the stack to simplify and speed up stack trace capture is a good > enough reason. > >> + *entry = bpf_entry; >> + preempt_enable(); >> + >> + return 0; >> +} >> + >> +static void bpf_put_perf_callchain(void) >> +{ >> + if (WARN_ON_ONCE(this_cpu_read(bpf_perf_callchain_preempt_cnt) == 0)) >> + return; >> + this_cpu_dec(bpf_perf_callchain_preempt_cnt); >> +} >> + >> static inline bool stack_map_use_build_id(struct bpf_map *map) >> { >> return (map->map_flags & BPF_F_STACK_BUILD_ID); >> @@ -303,8 +352,9 @@ BPF_CALL_3(bpf_get_stackid, struct pt_regs *, regs, struct bpf_map *, map, >> u32 max_depth = map->value_size / stack_map_data_size(map); >> u32 skip = flags & BPF_F_SKIP_FIELD_MASK; >> bool user = flags & BPF_F_USER_STACK; >> - struct perf_callchain_entry *trace; >> + struct bpf_perf_callchain_entry *trace; >> bool kernel = !user; >> + int err; >> >> if (unlikely(flags & ~(BPF_F_SKIP_FIELD_MASK | BPF_F_USER_STACK | >> BPF_F_FAST_STACK_CMP | BPF_F_REUSE_STACKID))) >> @@ -314,14 +364,15 @@ BPF_CALL_3(bpf_get_stackid, struct pt_regs *, regs, struct bpf_map *, map, >> if (max_depth > sysctl_perf_event_max_stack) >> max_depth = sysctl_perf_event_max_stack; >> >> - trace = get_perf_callchain(regs, 0, kernel, user, max_depth, >> - false, false); >> + err = bpf_get_perf_callchain(&trace, regs, 0, kernel, user, max_depth, >> + false, false); >> + if (err) >> + return err; >> >> - if (unlikely(!trace)) >> - /* couldn't fetch the stack trace */ >> - return -EFAULT; >> + err = __bpf_get_stackid(map, (struct perf_callchain_entry *)trace, flags); >> + bpf_put_perf_callchain(); >> >> - return __bpf_get_stackid(map, trace, flags); >> + return err; >> } >> >> const struct bpf_func_proto bpf_get_stackid_proto = { >> @@ -443,8 +494,7 @@ static long __bpf_get_stack(struct pt_regs *regs, struct task_struct *task, >> if (sysctl_perf_event_max_stack < max_depth) >> max_depth = sysctl_perf_event_max_stack; >> >> - if (may_fault) >> - rcu_read_lock(); /* need RCU for perf's callchain below */ >> + preempt_disable(); >> >> if (trace_in) >> trace = trace_in; >> @@ -455,8 +505,7 @@ static long __bpf_get_stack(struct pt_regs *regs, struct task_struct *task, >> crosstask, false); >> >> if (unlikely(!trace) || trace->nr < skip) { >> - if (may_fault) >> - rcu_read_unlock(); >> + preempt_enable(); >> goto err_fault; >> } >> >> @@ -474,10 +523,7 @@ static long __bpf_get_stack(struct pt_regs *regs, struct task_struct *task, >> } else { >> memcpy(buf, ips, copy_len); >> } >> - >> - /* trace/ips should not be dereferenced after this point */ >> - if (may_fault) >> - rcu_read_unlock(); >> + preempt_enable(); >> >> if (user_build_id) >> stack_map_get_build_id_offset(buf, trace_nr, user, may_fault); > > really it's just build_id resolution that can take a while, which is > why we are trying to avoid preemption around it. But for non-build_id > case, can we avoid extra copying? > Maybe possible, you mean optimize the memcpy(buf, ips, copy_len) for non-build_id? I'm trying to add an external entry in get_perf_callchain to see if the perf maintainers agree. If it's approved, everything seems manageable. >> -- >> 2.48.1 >> -- Best Regards Tao Chen