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Message-ID: <CAEf4BzZ-ovqXqLJ5oJ95n9prFnXsLOkO1UvdycUcON77=Akv-w@mail.gmail.com>
Date: Fri, 19 Sep 2025 15:50:27 -0700
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: Arnaud Lecomte <contact@...aud-lcm.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 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
> copy_len = trace_nr * elem_size;
>
> ips = trace->ip + skip;
> --
> 2.43.0
>
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