Re: [PATCH] bpf: Annotate struct bpf_cand_cache with __counted_by()

[Thorsten Blum]

On 13. Aug 2024, at 18:28, Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx> wrote:
> On Tue, Aug 13, 2024 at 8:19 AM Thorsten Blum <thorsten.blum@xxxxxxxxxx> wrote:
>> 
>> Add the __counted_by compiler attribute to the flexible array member
>> cands to improve access bounds-checking via CONFIG_UBSAN_BOUNDS and
>> CONFIG_FORTIFY_SOURCE.
>> 
>> Increment cnt before adding a new struct to the cands array.
> 
> why? What happens otherwise?

If you try to access cands->cands[cands->cnt] without incrementing
cands->cnt first, you're essentially accessing the array out of bounds
which will fail during runtime.

You can read more about it at [1] and [2].

> Signed-off-by: Thorsten Blum <thorsten.blum@xxxxxxxxxx>
>> ---
>> kernel/bpf/btf.c | 6 +++---
>> 1 file changed, 3 insertions(+), 3 deletions(-)
>> 
>> diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
>> index 520f49f422fe..42bc70a56fcd 100644
>> --- a/kernel/bpf/btf.c
>> +++ b/kernel/bpf/btf.c
>> @@ -7240,7 +7240,7 @@ struct bpf_cand_cache {
>>        struct {
>>                const struct btf *btf;
>>                u32 id;
>> -       } cands[];
>> +       } cands[] __counted_by(cnt);
>> };
>> 
>> static DEFINE_MUTEX(cand_cache_mutex);
>> @@ -8784,9 +8784,9 @@ bpf_core_add_cands(struct bpf_cand_cache *cands, const struct btf *targ_btf,
>>                memcpy(new_cands, cands, sizeof_cands(cands->cnt));
>>                bpf_free_cands(cands);
>>                cands = new_cands;
>> -               cands->cands[cands->cnt].btf = targ_btf;
>> -               cands->cands[cands->cnt].id = i;
>>                cands->cnt++;
>> +               cands->cands[cands->cnt - 1].btf = targ_btf;
>> +               cands->cands[cands->cnt - 1].id = i;
>>        }
>>        return cands;
>> }
>> --
>> 2.46.0
>> 

[1] https://opensource.googleblog.com/2024/07/bounds-checking-flexible-array-members.html
[2] https://embeddedor.com/blog/2024/06/18/how-to-use-the-new-counted_by-attribute-in-c-and-linux/