From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-184.mta0.migadu.com (out-184.mta0.migadu.com [91.218.175.184]) (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 B00D9230BDF for ; Mon, 25 Aug 2025 21:15:16 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.184 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1756156523; cv=none; b=sajmU1ElyGXaX1FLQOYWLd4CrcwokNWZl/NrcDLi38YhqEkZKwqY0S8H+GVR/UsQ0GHAJZYL0p7r3q5VpxzBrA5EnVN6mvzDzcGFV0uAzJ9Xh0fIWuTWJWl5nW8dmyBSw30wkLTCN7QSy/3quhC6fHFgXEyHa5GvCAlZBr8DEfU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1756156523; c=relaxed/simple; bh=AbLB2eRMtTZnNOt/j5On2L0B7d0nlrQWpgV2/bU/veA=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=UQNzxI+3jfPjZfY9joF5OJP5C+cUe1Hxieze8mUn/EN5qdeIgqmynuvDclNupLh2BtPf4mSGTlz0iTlgLJKfpSMceQiGegnqe1LzLATUK/orbrgcRuaemeEGG6qxoRZFilEtoYdTq/9besY9saCPro6QlrzSm9nfAnn6gWL3BX0= 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=JIoa1C0+; arc=none smtp.client-ip=91.218.175.184 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="JIoa1C0+" Message-ID: <2feb48ea-4846-4030-b9fc-cc9300bab57f@linux.dev> DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1756156514; 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=nnNxmZ2TVEDKw8UzTdWgJgQWMgrvXznrwWTltZCEw2g=; b=JIoa1C0+ao82/CXIozGCXasa+n/yN0qY0I5fGAKQcrhZPiiq1HarSCnZ9b1ZUbsA9VD5K4 EyDR9PqfeDdhHw1K5dnNer6TW2vT/8LbjQqxH8G/rbvH/BtxoK2eLfsOokGeFpCBpDAbBL VydWLFZykROR6UCB+jDibCpO20rnqF4= Date: Mon, 25 Aug 2025 14:15:07 -0700 Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Subject: Re: [PATCH bpf-next RESEND v4 1/2] bpf: refactor max_depth computation in bpf_get_stack() Content-Language: en-GB To: "Lecomte, Arnaud" , Martin KaFai Lau Cc: andrii@kernel.org, ast@kernel.org, bpf@vger.kernel.org, daniel@iogearbox.net, eddyz87@gmail.com, haoluo@google.com, john.fastabend@gmail.com, jolsa@kernel.org, kpsingh@kernel.org, linux-kernel@vger.kernel.org, sdf@fomichev.me, syzbot+c9b724fbb41cf2538b7b@syzkaller.appspotmail.com, syzkaller-bugs@googlegroups.com, song@kernel.org References: <20250819162652.8776-1-contact@arnaud-lcm.com> <3d8fe484-2889-4367-9405-91aeee7d2ef0@linux.dev> <43b9d0ff-9922-490a-ac6b-7e8e7baa2247@linux.dev> <0165bf55-4a46-4e75-91df-644b0281b247@arnaud-lcm.com> X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Yonghong Song In-Reply-To: <0165bf55-4a46-4e75-91df-644b0281b247@arnaud-lcm.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT On 8/25/25 1:07 PM, Lecomte, Arnaud wrote: > > On 25/08/2025 19:27, Yonghong Song wrote: >> >> >> On 8/25/25 9:39 AM, Lecomte, Arnaud wrote: >>> >>> On 19/08/2025 22:15, Martin KaFai Lau wrote: >>>> On 8/19/25 9:26 AM, Arnaud Lecomte wrote: >>>>> A new helper function stack_map_calculate_max_depth() that >>>>> computes the max depth for a stackmap. >>>>> >>>>> Changes in v2: >>>>>   - Removed the checking 'map_size % map_elem_size' from >>>>>     stack_map_calculate_max_depth >>>>>   - Changed stack_map_calculate_max_depth params name to be more >>>>> generic >>>>> >>>>> Changes in v3: >>>>>   - Changed map size param to size in max depth helper >>>>> >>>>> Changes in v4: >>>>>   - Fixed indentation in max depth helper for args >>>>> >>>>> Link to v3: >>>>> https://lore.kernel.org/all/09dc40eb-a84e-472a-8a68-36a2b1835308@linux.dev/ >>>>> >>>>> Signed-off-by: Arnaud Lecomte >>>>> Acked-by: Yonghong Song >>>>> --- >>>>>   kernel/bpf/stackmap.c | 30 ++++++++++++++++++++++++------ >>>>>   1 file changed, 24 insertions(+), 6 deletions(-) >>>>> >>>>> diff --git a/kernel/bpf/stackmap.c b/kernel/bpf/stackmap.c >>>>> index 3615c06b7dfa..b9cc6c72a2a5 100644 >>>>> --- a/kernel/bpf/stackmap.c >>>>> +++ b/kernel/bpf/stackmap.c >>>>> @@ -42,6 +42,27 @@ static inline int stack_map_data_size(struct >>>>> bpf_map *map) >>>>>           sizeof(struct bpf_stack_build_id) : sizeof(u64); >>>>>   } >>>>>   +/** >>>>> + * stack_map_calculate_max_depth - Calculate maximum allowed >>>>> stack trace depth >>>>> + * @size:  Size of the buffer/map value in bytes >>>>> + * @elem_size:  Size of each stack trace element >>>>> + * @flags:  BPF stack trace flags (BPF_F_USER_STACK, >>>>> BPF_F_USER_BUILD_ID, ...) >>>>> + * >>>>> + * Return: Maximum number of stack trace entries that can be >>>>> safely stored >>>>> + */ >>>>> +static u32 stack_map_calculate_max_depth(u32 size, u32 elem_size, >>>>> u64 flags) >>>>> +{ >>>>> +    u32 skip = flags & BPF_F_SKIP_FIELD_MASK; >>>>> +    u32 max_depth; >>>>> + >>>>> +    max_depth = size / elem_size; >>>>> +    max_depth += skip; >>>>> +    if (max_depth > sysctl_perf_event_max_stack) >>>>> +        return sysctl_perf_event_max_stack; >>>> >>>> hmm... this looks a bit suspicious. Is it possible that >>>> sysctl_perf_event_max_stack is being changed to a larger value in >>>> parallel? >>>> >>> Hi Martin, this is a valid concern as sysctl_perf_event_max_stack >>> can be modified at runtime through >>> /proc/sys/kernel/perf_event_max_stack. >>> What we could maybe do instead is to create a copy: u32 current_max >>> = READ_ONCE(sysctl_perf_event_max_stack); >>> Any thoughts on this ? >> >> There is no need to have READ_ONCE. Jut do >>     int curr_sysctl_max_stack = sysctl_perf_event_max_stack; >>     if (max_depth > curr_sysctl_max_stack) >>       return curr_sysctl_max_stack; >> >> Because of the above change, the patch is not a refactoring change >> any more. >> > Why would you not consider it as a refactoring change anymore ? Sorry, I think I made a couple of mistakes in the above. First, yes, we do want READ_ONCE, other potentially compiler may optimization the above back to the original code with two references to sysctl_perf_event_max_stack. Second, yes, it is indeed a refactoring. >>> >>>>> + >>>>> +    return max_depth; >>>>> +} >>>>> + >>>>>   static int prealloc_elems_and_freelist(struct bpf_stack_map *smap) >>>>>   { >>>>>       u64 elem_size = sizeof(struct stack_map_bucket) + >>>>> @@ -406,7 +427,7 @@ static long __bpf_get_stack(struct pt_regs >>>>> *regs, struct task_struct *task, >>>>>                   struct perf_callchain_entry *trace_in, >>>>>                   void *buf, u32 size, u64 flags, bool may_fault) >>>>>   { >>>>> -    u32 trace_nr, copy_len, elem_size, num_elem, max_depth; >>>>> +    u32 trace_nr, copy_len, elem_size, max_depth; >>>>>       bool user_build_id = flags & BPF_F_USER_BUILD_ID; >>>>>       bool crosstask = task && task != current; >>>>>       u32 skip = flags & BPF_F_SKIP_FIELD_MASK; >>>>> @@ -438,10 +459,7 @@ static long __bpf_get_stack(struct pt_regs >>>>> *regs, struct task_struct *task, >>>>>           goto clear; >>>>>       } >>>>>   -    num_elem = size / elem_size; >>>>> -    max_depth = num_elem + skip; >>>>> -    if (sysctl_perf_event_max_stack < max_depth) >>>>> -        max_depth = sysctl_perf_event_max_stack; >>>>> +    max_depth = stack_map_calculate_max_depth(size, elem_size, >>>>> flags); >>>>>         if (may_fault) >>>>>           rcu_read_lock(); /* need RCU for perf's callchain below */ >>>>> @@ -461,7 +479,7 @@ static long __bpf_get_stack(struct pt_regs >>>>> *regs, struct task_struct *task, >>>>>       } >>>>>         trace_nr = trace->nr - skip; >>>>> -    trace_nr = (trace_nr <= num_elem) ? trace_nr : num_elem; >>>> >>>> I suspect it was fine because trace_nr was still bounded by num_elem. >>>> >>> We should bring back the num_elem bound as an additional safe net. >>>>> +    trace_nr = min(trace_nr, max_depth - skip); >>>> >>>> but now the min() is also based on max_depth which could be >>>> sysctl_perf_event_max_stack. >>>> >>>> beside, if I read it correctly, in "max_depth - skip", the >>>> max_depth could also be less than skip. I assume trace->nr is bound >>>> by max_depth, so should be less of a problem but still a bit >>>> unintuitive to read. >>>> >>>>>       copy_len = trace_nr * elem_size; >>>>>         ips = trace->ip + skip; >>>> >> >>