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

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Message-ID: <377791b5-2294-4ced-a0d3-918c7e078b2b@linux.dev>
Date: Tue, 28 Oct 2025 00:15:03 +0800
From: Leon Hwang <leon.hwang@...ux.dev>
To: Amery Hung <ameryhung@...il.com>
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

Hi Amery,

On 2025/10/27 23:44, Amery Hung wrote:
> 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.
> 

Thanks for your review.

>>                 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.
> 

Yes, it’s needed. The new selftest confirms that the special fields are
not freed when updating a local storage map.

Also, bpf_selem_unlink_storage_nolock() doesn’t invoke
bpf_selem_free_list(), unlike bpf_selem_unlink_storage(). So we need to
call bpf_obj_free_fields() here explicitly to free those fields.

Thanks,
Leon

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