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/
