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?
Is this the only place?
what about:
new_cands = kmemdup(cands, sizeof_cands(cands->cnt), GFP_KERNEL);
cnt field gets copied with other fields.
Can compiler/runtime catch that?
> 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/
