On 13. Aug 2024, at 20:57, Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx> wrote:
> On Tue, Aug 13, 2024 at 10:59 AM Thorsten Blum <thorsten.blum@xxxxxxxxxx> wrote:
>> 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.
>
> What kind of error/warn do you see ?
> Is it runtime or compile time?
I get a runtime error with Clang 18 [3].
> Is this the only place?
I think so.
> what about:
> new_cands = kmemdup(cands, sizeof_cands(cands->cnt), GFP_KERNEL);
>
> cnt field gets copied with other fields.
> Can compiler/runtime catch that?
I think this is ok and there's nothing to catch.
> 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/
[3] https://godbolt.org/z/cKee95777
