Re: [PATCH net-next v1] net: sched: cls_u32: Avoid memcpy() false-positive warning in u32_init_knode()

["Gustavo A. R. Silva" <gustavo@embeddedor.com>]

On 3/5/26 17:09, Jiayuan Chen wrote:
> March 5, 2026 at 15:33, "Jiayuan Chen" <jiayuan.chen@linux.dev mailto:jiayuan.chen@linux.dev?to=%22Jiayuan%20Chen%22%20%3Cjiayuan.chen%40linux.dev%3E > wrote:
> 
> 
>>
>> From: Jiayuan Chen <jiayuan.chen@shopee.com>
>>
>> Syzbot reported a warning in u32_init_knode() [1].
>>
>> Similar to commit 7cba18332e36 ("net: sched: cls_u32: Avoid memcpy()
>> false-positive warning") which addressed the same issue in u32_change(),
>> use unsafe_memcpy() in u32_init_knode() to work around the compiler's
>> inability to see into composite flexible array structs.
>>
>> This silences the false-positive reported by syzbot:
>>
>>   memcpy: detected field-spanning write (size 32) of single field
>>   "&new->sel" at net/sched/cls_u32.c:855 (size 16)
>>
>> Since the memory is correctly allocated with kzalloc_flex() using
>> s->nkeys, this is purely a false positive and does not need a Fixes tag.
>>
>> [1] https://syzkaller.appspot.com/bug?extid=d5ace703ed883df56e42
>>
>> Reported-by: syzbot+d5ace703ed883df56e42@syzkaller.appspotmail.com
>> Closes: https://lore.kernel.org/all/69a811b9.a70a0220.b118c.0019.GAE@google.com/T/
>> Signed-off-by: Jiayuan Chen <jiayuan.chen@shopee.com>
>> Made-with: Cursor
>> ---
>>   net/sched/cls_u32.c | 5 ++++-
>>   1 file changed, 4 insertions(+), 1 deletion(-)
>>
>> diff --git a/net/sched/cls_u32.c b/net/sched/cls_u32.c
>> index 9241c025aa74..8f30cc82181d 100644
>> --- a/net/sched/cls_u32.c
>> +++ b/net/sched/cls_u32.c
>> @@ -852,7 +852,10 @@ static struct tc_u_knode *u32_init_knode(struct net *net, struct tcf_proto *tp,
>>   /* Similarly success statistics must be moved as pointers */
>>   new->pcpu_success = n->pcpu_success;
>>   #endif
>> - memcpy(&new->sel, s, struct_size(s, keys, s->nkeys));
>> + unsafe_memcpy(&new->sel, s, struct_size(s, keys, s->nkeys),
>> + /* A composite flex-array structure destination,
>> + * which was correctly sized with kzalloc_flex(),
>> + * above. */);
> 
> 
> 
> I'm thinking whether it's more appropriate:
> 
> new->sel.hdr = s->hdr;
> memcpy(new->sel.keys, s->keys, flex_array_size(s, keys, s->nkeys));

I think unsafe_memcpy() is "better" in this case, as it will mark code
that could be updated/reviewed once the compiler can correctly see into
composite flexible structures in the future.

-Gustavo

> 
>>   if (tcf_exts_init(&new->exts, net, TCA_U32_ACT, TCA_U32_POLICE)) {
>>   kfree(new);
>> -- 
>> 2.43.0
>>
>