[PATCH 1/3] bpf: Allow BTF ctx access for string pointers

[Jiri Olsa <jolsa@kernel.org>]

When accessing the context we allow access to arguments with
scalar type and pointer to struct. But we deny access for
pointer to scalar type, which is the case for many functions.

Alexei suggested to take conservative approach and allow
currently only string pointer access, which is the case
for most functions now:

> Compilers have a long history special casing 'char *'. In particular signed
> char because it's a pointer to null terminated string. I think it's still a
> special pointer from pointer aliasing point of view. I think the verifier can
> treat it as scalar here too. In the future the verifier will get smarter and
> will recognize it as PTR_TO_NULL_STRING while 'u8 *', 'u32 *' will be
> PTR_TO_BTF_ID. I think it will solve this particular issue. I like conservative
> approach to the verifier improvements: start with strict checking and relax it
> on case-by-case. Instead of accepting wide range of cases and cause potential
> compatibility issues.

Adding check if the pointer is to string type and allow access to it.

Suggested-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
---
 kernel/bpf/btf.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
index 32963b6d5a9c..b7c1660fb594 100644
--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c
@@ -3669,6 +3669,19 @@ struct btf *bpf_prog_get_target_btf(const struct bpf_prog *prog)
 	}
 }
 
+static bool is_string_ptr(struct btf *btf, const struct btf_type *t)
+{
+	/* t comes in already as a pointer */
+	t = btf_type_by_id(btf, t->type);
+
+	/* allow const */
+	if (BTF_INFO_KIND(t->info) == BTF_KIND_CONST)
+		t = btf_type_by_id(btf, t->type);
+
+	/* char, signed char, unsigned char */
+	return btf_type_is_int(t) && t->size == 1;
+}
+
 bool btf_ctx_access(int off, int size, enum bpf_access_type type,
 		    const struct bpf_prog *prog,
 		    struct bpf_insn_access_aux *info)
@@ -3735,6 +3748,9 @@ bool btf_ctx_access(int off, int size, enum bpf_access_type type,
 		 */
 		return true;
 
+	if (is_string_ptr(btf, t))
+		return true;
+
 	/* this is a pointer to another type */
 	info->reg_type = PTR_TO_BTF_ID;
 
-- 
2.24.1