On Thu, Jun 23, 2022 at 9:56 PM KP Singh <kpsingh@xxxxxxxxxx> wrote:
>
> kfuncs can handle pointers to memory when the next argument is
> the size of the memory that can be read and verify these as
> ARG_CONST_SIZE_OR_ZERO
>
> Similarly add support for string constants (const char *) and
> verify it similar to ARG_PTR_TO_CONST_STR.
>
> Signed-off-by: KP Singh <kpsingh@xxxxxxxxxx>
> ---
> include/linux/bpf_verifier.h | 2 +
> kernel/bpf/btf.c | 25 ++++++++++
> kernel/bpf/verifier.c | 89 +++++++++++++++++++++---------------
> 3 files changed, 78 insertions(+), 38 deletions(-)
>
> diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h
> index 81b19669efba..f6d8898270d5 100644
> --- a/include/linux/bpf_verifier.h
> +++ b/include/linux/bpf_verifier.h
> @@ -560,6 +560,8 @@ int check_kfunc_mem_size_reg(struct bpf_verifier_env *env, struct bpf_reg_state
> u32 regno);
> int check_mem_reg(struct bpf_verifier_env *env, struct bpf_reg_state *reg,
> u32 regno, u32 mem_size);
> +int check_const_str(struct bpf_verifier_env *env,
> + const struct bpf_reg_state *reg, int regno);
>
> /* this lives here instead of in bpf.h because it needs to dereference tgt_prog */
> static inline u64 bpf_trampoline_compute_key(const struct bpf_prog *tgt_prog,
> diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> index 668ecf61649b..b31e8d8f2d4d 100644
> --- a/kernel/bpf/btf.c
> +++ b/kernel/bpf/btf.c
> @@ -6162,6 +6162,23 @@ static bool is_kfunc_arg_mem_size(const struct btf *btf,
> return true;
> }
>
> +static bool btf_param_is_const_str_ptr(const struct btf *btf,
> + const struct btf_param *param)
> +{
> + const struct btf_type *t;
> +
> + t = btf_type_by_id(btf, param->type);
> + if (!btf_type_is_ptr(t))
> + return false;
> +
> + t = btf_type_by_id(btf, t->type);
> + if (BTF_INFO_KIND(t->info) != BTF_KIND_CONST)
> + return false;
> +
> + t = btf_type_skip_modifiers(btf, t->type, NULL);
nit: this looks a bit fragile, you assume CONST comes first and then
skip the rest of modifiers (including typedefs). Maybe either make it
more permissive and then check that CONST is somewhere there in the
chain (you'll have to open-code btf_type_skip_modifiers() loop), or
make it more restrictive and say that it has to be `const char *` and
nothing else (no volatile, no restrict, no typedefs)?
> + return !strcmp(btf_name_by_offset(btf, t->name_off), "char");
> +}
> +
[...]
