linux-kernel - Re: [PATCH bpf v3 3/4] bpf: Free special fields when update local storage maps

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Message-ID: <CAMB2axPhcYctJYz0bH032-Kc1h2LcJL74O5iS5g=8Qp74GPK_g@mail.gmail.com>
Date: Mon, 27 Oct 2025 08:44:53 -0700
From: Amery Hung <ameryhung@...il.com>
To: Leon Hwang <leon.hwang@...ux.dev>
Cc: bpf@...r.kernel.org, ast@...nel.org, andrii@...nel.org, 
	daniel@...earbox.net, martin.lau@...ux.dev, eddyz87@...il.com, 
	song@...nel.org, yonghong.song@...ux.dev, john.fastabend@...il.com, 
	kpsingh@...nel.org, sdf@...ichev.me, haoluo@...gle.com, jolsa@...nel.org, 
	memxor@...il.com, linux-kernel@...r.kernel.org, kernel-patches-bot@...com
Subject: Re: [PATCH bpf v3 3/4] bpf: Free special fields when update local
 storage maps

On Sun, Oct 26, 2025 at 8:41 AM Leon Hwang <leon.hwang@...ux.dev> wrote:
>
> When updating local storage maps with BPF_F_LOCK on the fast path, the
> special fields were not freed after being replaced. This could cause
> memory referenced by BPF_KPTR_{REF,PERCPU} fields to be held until the
> map gets freed.
>
> Similarly, on the other path, the old sdata's special fields were never
> freed regardless of whether BPF_F_LOCK was used, causing the same issue.
>
> Fix this by calling 'bpf_obj_free_fields()' after
> 'copy_map_value_locked()' to properly release the old fields.
>
> Fixes: 9db44fdd8105 ("bpf: Support kptrs in local storage maps")
> Signed-off-by: Leon Hwang <leon.hwang@...ux.dev>
> ---
>  kernel/bpf/bpf_local_storage.c | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/kernel/bpf/bpf_local_storage.c b/kernel/bpf/bpf_local_storage.c
> index b931fbceb54da..8e3aea4e07c50 100644
> --- a/kernel/bpf/bpf_local_storage.c
> +++ b/kernel/bpf/bpf_local_storage.c
> @@ -609,6 +609,7 @@ bpf_local_storage_update(void *owner, struct bpf_local_storage_map *smap,
>                 if (old_sdata && selem_linked_to_storage_lockless(SELEM(old_sdata))) {
>                         copy_map_value_locked(&smap->map, old_sdata->data,
>                                               value, false);
> +                       bpf_obj_free_fields(smap->map.record, old_sdata->data);

[ ... ]

>                         return old_sdata;
>                 }
>         }
> @@ -641,6 +642,7 @@ bpf_local_storage_update(void *owner, struct bpf_local_storage_map *smap,
>         if (old_sdata && (map_flags & BPF_F_LOCK)) {
>                 copy_map_value_locked(&smap->map, old_sdata->data, value,
>                                       false);
> +               bpf_obj_free_fields(smap->map.record, old_sdata->data);

The one above and this make sense. Thanks for fixing it.

>                 selem = SELEM(old_sdata);
>                 goto unlock;
>         }
> @@ -654,6 +656,7 @@ bpf_local_storage_update(void *owner, struct bpf_local_storage_map *smap,
>
>         /* Third, remove old selem, SELEM(old_sdata) */
>         if (old_sdata) {
> +               bpf_obj_free_fields(smap->map.record, old_sdata->data);

Is this really needed? bpf_selem_free_list() later should free special
fields in this selem.


>                 bpf_selem_unlink_map(SELEM(old_sdata));
>                 bpf_selem_unlink_storage_nolock(local_storage, SELEM(old_sdata),
>                                                 true, &old_selem_free_list);
> --
> 2.51.0
>
>