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Message-ID: <6c6ae616-d6ed-443a-a492-96081763883e@arnaud-lcm.com>
Date: Sat, 20 Sep 2025 21:02:49 +0100
From: "Lecomte, Arnaud" <contact@...aud-lcm.com>
To: Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc: alexei.starovoitov@...il.com, yonghong.song@...ux.dev, song@...nel.org,
andrii@...nel.org, ast@...nel.org, bpf@...r.kernel.org,
daniel@...earbox.net, eddyz87@...il.com, haoluo@...gle.com,
john.fastabend@...il.com, jolsa@...nel.org, kpsingh@...nel.org,
linux-kernel@...r.kernel.org, martin.lau@...ux.dev, sdf@...ichev.me,
syzbot+c9b724fbb41cf2538b7b@...kaller.appspotmail.com,
syzkaller-bugs@...glegroups.com
Subject: Re: [PATCH bpf-next v9 1/3] bpf: refactor max_depth computation in
bpf_get_stack()
On 19/09/2025 23:50, Andrii Nakryiko wrote:
> On Fri, Sep 12, 2025 at 4:34 PM Arnaud Lecomte <contact@...aud-lcm.com> wrote:
>> A new helper function stack_map_calculate_max_depth() that
>> computes the max depth for a stackmap.
>>
>> Acked-by: Yonghong Song <yonghong.song@...ux.dev>
>> Acked-by: Song Liu <song@...nel.org>
>> Signed-off-by: Arnaud Lecomte <contact@...aud-lcm.com>
>> ---
>> 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
>>
>> Changes in v5:
>> - Bound back trace_nr to num_elem in __bpf_get_stack
>> - Make a copy of sysctl_perf_event_max_stack
>> in stack_map_calculate_max_depth
>>
>> Changes in v6:
>> - Restrained max_depth computation only when required
>> - Additional cleanup from Song in __bpf_get_stack
>>
>> Changes in v7:
>> - Removed additional cleanup from v6
>>
>> Changes in v9:
>> - Fixed incorrect removal of num_elem in get stack
>>
>> Link to v8: https://lore.kernel.org/all/20250905134625.26531-1-contact@arnaud-lcm.com/
>> ---
>> ---
>> kernel/bpf/stackmap.c | 39 +++++++++++++++++++++++++++------------
>> 1 file changed, 27 insertions(+), 12 deletions(-)
>>
>> diff --git a/kernel/bpf/stackmap.c b/kernel/bpf/stackmap.c
>> index 3615c06b7dfa..a794e04f5ae9 100644
>> --- a/kernel/bpf/stackmap.c
>> +++ b/kernel/bpf/stackmap.c
>> @@ -42,6 +42,28 @@ 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;
>> + u32 curr_sysctl_max_stack = READ_ONCE(sysctl_perf_event_max_stack);
>> +
>> + max_depth = size / elem_size;
>> + max_depth += skip;
>> + if (max_depth > curr_sysctl_max_stack)
>> + return curr_sysctl_max_stack;
>> +
>> + return max_depth;
>> +}
>> +
>> static int prealloc_elems_and_freelist(struct bpf_stack_map *smap)
>> {
>> u64 elem_size = sizeof(struct stack_map_bucket) +
>> @@ -300,20 +322,17 @@ static long __bpf_get_stackid(struct bpf_map *map,
>> BPF_CALL_3(bpf_get_stackid, struct pt_regs *, regs, struct bpf_map *, map,
>> u64, flags)
>> {
>> - u32 max_depth = map->value_size / stack_map_data_size(map);
>> - u32 skip = flags & BPF_F_SKIP_FIELD_MASK;
>> + u32 elem_size = stack_map_data_size(map);
>> bool user = flags & BPF_F_USER_STACK;
>> struct perf_callchain_entry *trace;
>> bool kernel = !user;
>> + u32 max_depth;
>>
>> if (unlikely(flags & ~(BPF_F_SKIP_FIELD_MASK | BPF_F_USER_STACK |
>> BPF_F_FAST_STACK_CMP | BPF_F_REUSE_STACKID)))
>> return -EINVAL;
>>
>> - max_depth += skip;
>> - if (max_depth > sysctl_perf_event_max_stack)
>> - max_depth = sysctl_perf_event_max_stack;
>> -
>> + max_depth = stack_map_calculate_max_depth(map->value_size, elem_size, flags);
>> trace = get_perf_callchain(regs, 0, kernel, user, max_depth,
>> false, false);
>>
>> @@ -406,7 +425,7 @@ 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 +457,7 @@ 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 +477,6 @@ 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;
> Is this also part of refactoring? If yes, it deserves a mention on why
> it's ok to just drop this.
>
> pw-bot: cr
>
Yes it is also part of the refactoring as stack_map_calculate_max_depth
now already curtains the trace->nr to the max possible number of
elements, there is no need to do the clamping twice. This is valid
assuming that get_perf_callchain and get_callchain_entry_for_task
correctly set this limit.
>
>> copy_len = trace_nr * elem_size;
>>
>> ips = trace->ip + skip;
>> --
>> 2.43.0
>>
Thanks,
Arnaud
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