From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-188.mta0.migadu.com (out-188.mta0.migadu.com [91.218.175.188]) (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 0488D2EB873 for ; Tue, 19 Aug 2025 21:15:53 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.188 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1755638155; cv=none; b=HaUIb6suyOOV7botLIlULNW0Wix7i1YdmaYzghD0X1ZcFpA6PdMksuQaqh786zjsWe3zAhmnBcp6jMmikBzlJKCJJ6FkFw0oZBTLHRv1yLOrp+u3Kx9XeGJmmgLP4QuXwgk2LxDFebE4pJtFyRT0OuHNDERGlWa7o7BWQlGdfoU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1755638155; c=relaxed/simple; bh=Kcmtw1SZqqnrUX4Pm9hd7A8p6P84NFZYeq3s6aCruOU=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=MVszwqEkLXzA2gRe249QfzuP5hz8lQ1RHRQAsOvL5OL/dEB/U3TjIdz6zuoeRMxiMXc6QJMWaCodzsWj7UOyFPD+5BBSjNFRh/Jkm8LIppEWHYFc2RLWTuMP5UpG1mJS1+LdVvomz+ubjNX9iKmEg8Yd8zmuesN70dWSXX2Rq5E= 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=GGuM87G9; arc=none smtp.client-ip=91.218.175.188 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="GGuM87G9" Message-ID: <3d8fe484-2889-4367-9405-91aeee7d2ef0@linux.dev> DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1755638142; 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=ShCNXUlDFKixmBPum6gITWmou0RkPrZF8Uv21LjgDng=; b=GGuM87G9FuFa6XPQEfZh+gmLc5qCa33Tgu+LhABXESCl1slLrMjWOBKvH3sNrVHhUzJXCq URIlYCVxK5AG6tLDvKladglvQpdtbzMtpj4vmeZFpKjnPjqTYElqCqHn+bDe7uPnPjCDXf Zge4zQdhfeR7ncN/KnPr8T9UYtouhIc= Date: Tue, 19 Aug 2025 14:15:08 -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() To: Arnaud Lecomte , yonghong.song@linux.dev 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> Content-Language: en-US X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Martin KaFai Lau In-Reply-To: <20250819162652.8776-1-contact@arnaud-lcm.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Migadu-Flow: FLOW_OUT 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? > + > + 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. > + 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;