[PATCH bpf-next v4 1/2] bpf: Support new pointer param types via SCALAR_VALUE for trampolines

[Slava Imameev <slava.imameev@crowdstrike.com>]

Add BPF verifier support for single- and multi-level pointer
parameters and return values in BPF trampolines. The implementation
treats these parameters as SCALAR_VALUE.

The following new single level pointer support is added:
- pointers to enum, 32 and 64
- pointers to functions

The following multi-level pointer support is added:
- multi-level pointers to int
- multi-level pointers to void
- multi-level pointers to enum, 32 and 64
- multi-level pointers to function
- multi-level pointers to structure

This is consistent with the existing pointers to int and void
already treated as SCALAR.

This provides consistent logic for single and multi-level pointers
- if the type is treated as SCALAR for a single level pointer, the
same is applicable for multi-level pointers, except the pointer to
struct which is currently PTR_TO_BTF_ID, but in case of multi-level
pointer it is treated as scalar as the verifier lacks the context
to infer the size of their target memory regions.

Signed-off-by: Slava Imameev <slava.imameev@crowdstrike.com>
---
 kernel/bpf/btf.c | 31 +++++++++++++++++++++++--------
 1 file changed, 23 insertions(+), 8 deletions(-)

diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
index 4872d2a6c42d..c2d06d2597d6 100644
--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c
@@ -6508,11 +6508,30 @@ struct btf *bpf_prog_get_target_btf(const struct bpf_prog *prog)
 		return prog->aux->attach_btf;
 }
 
-static bool is_void_or_int_ptr(struct btf *btf, const struct btf_type *t)
+static bool is_ptr_treated_as_scalar(const struct btf *btf,
+	const struct btf_type *t)
 {
-	/* skip modifiers */
+	int depth = 1;
+
+	WARN_ON(!btf_type_is_ptr(t));
+
 	t = btf_type_skip_modifiers(btf, t->type, NULL);
-	return btf_type_is_void(t) || btf_type_is_int(t);
+	while (btf_type_is_ptr(t) && depth < MAX_RESOLVE_DEPTH) {
+		depth += 1;
+		t = btf_type_skip_modifiers(btf, t->type, NULL);
+	}
+
+	/*
+	 * If it's a single or multilevel pointer to void, int, enum,
+	 * or function, it's the same as scalar from the verifier
+	 * safety POV. Multilevel pointers to structures are treated as
+	 * scalars. The verifier lacks the context to infer the size of
+	 * their target memory regions. Either way, no further pointer
+	 * walking is allowed.
+	 */
+	return btf_type_is_void(t) || btf_type_is_int(t) ||
+		   btf_is_any_enum(t) || btf_type_is_func_proto(t) ||
+		   (btf_type_is_struct(t) && depth > 1);
 }
 
 u32 btf_ctx_arg_idx(struct btf *btf, const struct btf_type *func_proto,
@@ -6902,11 +6921,7 @@ bool btf_ctx_access(int off, int size, enum bpf_access_type type,
 		}
 	}
 
-	/*
-	 * If it's a pointer to void, it's the same as scalar from the verifier
-	 * safety POV. Either way, no futher pointer walking is allowed.
-	 */
-	if (is_void_or_int_ptr(btf, t))
+	if (is_ptr_treated_as_scalar(btf, t))
 		return true;
 
 	/* this is a pointer to another type */
-- 
2.34.1