[PATCH bpf v4 04/11] bpf: Apply CO-RE relocations before subprogram validation

[Kumar Kartikeya Dwivedi <memxor@gmail.com>]

check_subprogs() verifies that each subprogram ends in an exit or an
unconditional jump before in-kernel CO-RE relocations are applied. An
unresolved relocation can then replace that terminal instruction with an
invalid helper call. The resulting fall-through into another subprogram
breaks the CFG invariant used by postorder and stack liveness analysis,
which can write past their per-subprogram arrays.

Apply CO-RE relocations immediately after preparing the program BTF, before
subprogram discovery and validation. Keep func_info and line_info validation
after subprogram discovery because those records depend on the complete
subprogram layout.

Include core_relo_cnt when deciding whether to prepare program BTF. A load
that supplied only CO-RE relocation metadata previously skipped both BTF
setup and relocation processing.

Suggested-by: Andrii Nakryiko <andrii@kernel.org>
Suggested-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
---
 include/linux/bpf_verifier.h |  2 ++
 kernel/bpf/check_btf.c       | 12 ++++--------
 kernel/bpf/verifier.c        |  7 ++++++-
 3 files changed, 12 insertions(+), 9 deletions(-)

diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h
index 36b65797877d..bba5a727c651 100644
--- a/include/linux/bpf_verifier.h
+++ b/include/linux/bpf_verifier.h
@@ -1197,6 +1197,8 @@ static inline void bpf_trampoline_unpack_key(u64 key, u32 *obj_id, u32 *btf_id)
 
 int bpf_prepare_btf_info(struct bpf_verifier_env *env,
 			 const union bpf_attr *attr, bpfptr_t uattr);
+int bpf_check_core_relo(struct bpf_verifier_env *env,
+			const union bpf_attr *attr, bpfptr_t uattr);
 int bpf_check_btf_info(struct bpf_verifier_env *env,
 		       const union bpf_attr *attr, bpfptr_t uattr);
 
diff --git a/kernel/bpf/check_btf.c b/kernel/bpf/check_btf.c
index 0e8b3ccc7a5b..4c1ed842f661 100644
--- a/kernel/bpf/check_btf.c
+++ b/kernel/bpf/check_btf.c
@@ -338,9 +338,9 @@ static int check_btf_line(struct bpf_verifier_env *env,
 #define MIN_CORE_RELO_SIZE	sizeof(struct bpf_core_relo)
 #define MAX_CORE_RELO_SIZE	MAX_FUNCINFO_REC_SIZE
 
-static int check_core_relo(struct bpf_verifier_env *env,
-			   const union bpf_attr *attr,
-			   bpfptr_t uattr)
+int bpf_check_core_relo(struct bpf_verifier_env *env,
+			const union bpf_attr *attr,
+			bpfptr_t uattr)
 {
 	u32 i, nr_core_relo, ncopy, expected_size, rec_size;
 	struct bpf_core_relo core_relo = {};
@@ -414,7 +414,7 @@ int bpf_prepare_btf_info(struct bpf_verifier_env *env,
 	struct btf *btf;
 	int err;
 
-	if (!attr->func_info_cnt && !attr->line_info_cnt) {
+	if (!attr->func_info_cnt && !attr->line_info_cnt && !attr->core_relo_cnt) {
 		if (check_abnormal_return(env))
 			return -EINVAL;
 		return 0;
@@ -455,9 +455,5 @@ int bpf_check_btf_info(struct bpf_verifier_env *env,
 	if (err)
 		return err;
 
-	err = check_core_relo(env, attr, uattr);
-	if (err)
-		return err;
-
 	return 0;
 }
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index ddba53eaa333..68d1b67b8c1a 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -21271,6 +21271,11 @@ int bpf_check(struct bpf_prog **prog, union bpf_attr *attr, bpfptr_t uattr,
 	if (ret < 0)
 		goto skip_full_check;
 
+	/* Apply CO-RE before validating the program's instruction layout. */
+	ret = bpf_check_core_relo(env, attr, uattr);
+	if (ret < 0)
+		goto skip_full_check;
+
 	/* Discover all subprograms before validating their layout and BTF. */
 	ret = add_subprogs(env);
 	if (ret < 0)
@@ -21280,7 +21285,7 @@ int bpf_check(struct bpf_prog **prog, union bpf_attr *attr, bpfptr_t uattr,
 	if (ret < 0)
 		goto skip_full_check;
 
-	/* Validate BTF against the complete subprogram layout and apply CO-RE. */
+	/* Validate BTF against the complete subprogram layout. */
 	ret = bpf_check_btf_info(env, attr, uattr);
 	if (ret < 0)
 		goto skip_full_check;
-- 
2.53.0