On Tue, Jun 23, 2020 at 10:14 AM John Fastabend
<john.fastabend@xxxxxxxxx> wrote:
>
> To ensure btf_ctx_access() is safe the verifier checks that the BTF
> arg type is an int, enum, or pointer. When the function does the
> BTF arg lookup it uses the calculation 'arg = off / 8' using the
> fact that registers are 8B. This requires that the first arg is
> in the first reg, the second in the second, and so on. However,
> for __int128 the arg will consume two registers by default LLVM
> implementation. So this will cause the arg layout assumed by the
> 'arg = off / 8' calculation to be incorrect.
>
> Because __int128 is uncommon this patch applies the easiest fix and
> will force int types to be sizeof(u64) or smaller so that they will
> fit in a single register.
>
> Fixes: 9e15db66136a1 ("bpf: Implement accurate raw_tp context access via BTF")
> Signed-off-by: John Fastabend <john.fastabend@xxxxxxxxx>
> ---
"small int" for u64 looks funny, but naming is hard :)
Acked-by: Andrii Nakryiko <andriin@xxxxxx>
> include/linux/btf.h | 5 +++++
> kernel/bpf/btf.c | 4 ++--
> 2 files changed, 7 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/btf.h b/include/linux/btf.h
> index 5c1ea99..35642f6 100644
> --- a/include/linux/btf.h
> +++ b/include/linux/btf.h
> @@ -82,6 +82,11 @@ static inline bool btf_type_is_int(const struct btf_type *t)
> return BTF_INFO_KIND(t->info) == BTF_KIND_INT;
> }
>
> +static inline bool btf_type_is_small_int(const struct btf_type *t)
> +{
> + return btf_type_is_int(t) && (t->size <= sizeof(u64));
nit: unnecessary (), () are usually used to disambiguate | and & vs
|| and &&; this is not the case, though.
> +}
> +
> static inline bool btf_type_is_enum(const struct btf_type *t)
> {
> return BTF_INFO_KIND(t->info) == BTF_KIND_ENUM;
> diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> index 58c9af1..9a1a98d 100644
> --- a/kernel/bpf/btf.c
> +++ b/kernel/bpf/btf.c
> @@ -3746,7 +3746,7 @@ bool btf_ctx_access(int off, int size, enum bpf_access_type type,
> return false;
>
> t = btf_type_skip_modifiers(btf, t->type, NULL);
> - if (!btf_type_is_int(t)) {
> + if (!btf_type_is_small_int(t)) {
> bpf_log(log,
> "ret type %s not allowed for fmod_ret\n",
> btf_kind_str[BTF_INFO_KIND(t->info)]);
> @@ -3768,7 +3768,7 @@ bool btf_ctx_access(int off, int size, enum bpf_access_type type,
> /* skip modifiers */
> while (btf_type_is_modifier(t))
> t = btf_type_by_id(btf, t->type);
> - if (btf_type_is_int(t) || btf_type_is_enum(t))
> + if (btf_type_is_small_int(t) || btf_type_is_enum(t))
> /* accessing a scalar */
> return true;
> if (!btf_type_is_ptr(t)) {
>
