From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-177.mta1.migadu.com (out-177.mta1.migadu.com [95.215.58.177]) (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 1A1CF1F03FB for ; Thu, 7 Aug 2025 19:02:07 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=95.215.58.177 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1754593330; cv=none; b=VaojaOmFqgiNveFvEzPV96mTRy5sfDuw/VeHBc1tGMU3OObq6GKik5Y6stRKHyi31nJao2QUUhyeO0jxBTlyXlvhHXeZNQAeePgl97PdgMOQqAc69XEOkW5Zi/GQ8rZQZ7EEXjL4GzY+rj2fgm9w0m5Zeq3IyGl4BKVEBMKj2po= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1754593330; c=relaxed/simple; bh=YU3FWR8iG4THu3BwrW+8ZOs+Rk5Onr2YbeN26Hjq9gg=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=TkCNymGF9bnYVq8msJe5BoM+7uz63YKT53jMB3lP4dowinrKpZeiHIydbtzYJzsfTGFM1B4zVaiNBMcwjBvTtE6f2WxfgmrZy94HcAHMLug2/Yq+pEzDsaLZnX0ODGAjlrXY9LN5TS4KFWJmppjDW643Llx6dTQmKnw4ScSA72w= 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=eLUHsNvO; arc=none smtp.client-ip=95.215.58.177 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="eLUHsNvO" Message-ID: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1754593326; 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=jSPNJqkeusudPMefdSnHFALV74tRT6VfM3GcmBqG6YE=; b=eLUHsNvOLX6OA25UC9F604zsVRybNuc2c4EQTwVMyoLd4/dkf8UImV5/yCF927u77LepHW 9ms8HzK8HNSVKOUns4rv8bmqtj0ynGLni1LhkSv7F2Lpd+qJ2ligL2N9xsDcGVsW4hifDR 8RBWGUZDjJ/PdaGKdj+0Biw9tKnK4lk= Date: Thu, 7 Aug 2025 12:01:57 -0700 Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Subject: Re: [PATCH 1/2] bpf: refactor max_depth computation in bpf_get_stack() Content-Language: en-GB To: Arnaud Lecomte 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, martin.lau@linux.dev, sdf@fomichev.me, song@kernel.org, syzbot+c9b724fbb41cf2538b7b@syzkaller.appspotmail.com, syzkaller-bugs@googlegroups.com References: <6cc26e1f-6ad6-44cd-a049-c4e7af9a229a@linux.dev> <20250807175032.7381-1-contact@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: <20250807175032.7381-1-contact@arnaud-lcm.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Migadu-Flow: FLOW_OUT On 8/7/25 10:50 AM, Arnaud Lecomte wrote: > A new helper function stack_map_calculate_max_depth() that > computes the max depth for a stackmap. > > Signed-off-by: Arnaud Lecomte > --- > kernel/bpf/stackmap.c | 38 ++++++++++++++++++++++++++++++-------- > 1 file changed, 30 insertions(+), 8 deletions(-) > > diff --git a/kernel/bpf/stackmap.c b/kernel/bpf/stackmap.c > index 3615c06b7dfa..14e034045310 100644 > --- a/kernel/bpf/stackmap.c > +++ b/kernel/bpf/stackmap.c > @@ -42,6 +42,31 @@ 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 > + * @map_size: Size of the buffer/map value in bytes > + * @elem_size: Size of each stack trace element > + * @map_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, > + * or -EINVAL if size is not a multiple of elem_size -EINVAL is not needed here. See below. > + */ > +static u32 stack_map_calculate_max_depth(u32 map_size, u32 map_elem_size, u64 map_flags) map_elem_size -> elem_size > +{ > + u32 max_depth; > + u32 skip = map_flags & BPF_F_SKIP_FIELD_MASK; reverse Christmas tree? > + > + if (unlikely(map_size%map_elem_size)) > + return -EINVAL; The above should not be here. The checking 'map_size % map_elem_size' is only needed for bpf_get_stack(), not applicable for bpf_get_stackid(). > + > + max_depth = map_size / map_elem_size; > + max_depth += skip; > + if (max_depth > sysctl_perf_event_max_stack) > + return sysctl_perf_event_max_stack; > + > + return max_depth; > +} > + > static int prealloc_elems_and_freelist(struct bpf_stack_map *smap) > { > u64 elem_size = sizeof(struct stack_map_bucket) + > @@ -406,7 +431,7 @@ 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; > @@ -423,8 +448,6 @@ static long __bpf_get_stack(struct pt_regs *regs, struct task_struct *task, > goto clear; > > elem_size = user_build_id ? sizeof(struct bpf_stack_build_id) : sizeof(u64); > - if (unlikely(size % elem_size)) > - goto clear; Please keep this one. > > /* cannot get valid user stack for task without user_mode regs */ > if (task && user && !user_mode(regs)) > @@ -438,10 +461,9 @@ 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 (max_depth < 0) > + goto err_fault; max_depth is never less than 0. > > if (may_fault) > rcu_read_lock(); /* need RCU for perf's callchain below */ > @@ -461,7 +483,7 @@ 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; > + trace_nr = min(trace_nr, max_depth - skip); > copy_len = trace_nr * elem_size; > > ips = trace->ip + skip;