[PATCH bpf-next 3/8] bpf: Refactor btf_nested_type_is_trusted().

Alexei Starovoitov
2023-04-04T04:50:54Z

From: Alexei Starovoitov <ast@kernel.org>

btf_nested_type_is_trusted() tries to find a struct member at corresponding offset.
It works for flat structures and falls apart in more complex structs with nested structs.
The offset->member search is already performed by btf_struct_walk() including nested structs.
Reuse this work and pass {field name, field btf id} into btf_nested_type_is_trusted()
instead of offset to make BTF_TYPE_SAFE*() logic more robust.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
---
include/linux/bpf.h | 7 ++++---
kernel/bpf/btf.c | 44 +++++++++++++++++--------------------------
kernel/bpf/verifier.c | 23 +++++++++++-----------
3 files changed, 33 insertions(+), 41 deletions(-)

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 4f689dda748f..002a811b6b90 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -2263,7 +2263,7 @@ static inline bool bpf_tracing_btf_ctx_access(int off, int size,
int btf_struct_access(struct bpf_verifier_log *log,
const struct bpf_reg_state *reg,
int off, int size, enum bpf_access_type atype,
- u32 *next_btf_id, enum bpf_type_flag *flag);
+ u32 *next_btf_id, enum bpf_type_flag *flag, const char **field_name);
bool btf_struct_ids_match(struct bpf_verifier_log *log,
const struct btf *btf, u32 id, int off,
const struct btf *need_btf, u32 need_type_id,
@@ -2302,7 +2302,7 @@ struct bpf_core_ctx {

bool btf_nested_type_is_trusted(struct bpf_verifier_log *log,
const struct bpf_reg_state *reg,
- int off, const char *suffix);
+ const char *field_name, u32 btf_id, const char *suffix);

bool btf_type_ids_nocast_alias(struct bpf_verifier_log *log,
const struct btf *reg_btf, u32 reg_id,
@@ -2517,7 +2517,8 @@ static inline struct bpf_prog *bpf_prog_by_id(u32 id)
static inline int btf_struct_access(struct bpf_verifier_log *log,
const struct bpf_reg_state *reg,
int off, int size, enum bpf_access_type atype,
- u32 *next_btf_id, enum bpf_type_flag *flag)
+ u32 *next_btf_id, enum bpf_type_flag *flag,
+ const char **field_name)
{
return -EACCES;
}
diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
index b7e5a5510b91..593c45a294d0 100644
--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c
@@ -6166,7 +6166,8 @@ enum bpf_struct_walk_result {

static int btf_struct_walk(struct bpf_verifier_log *log, const struct btf *btf,
const struct btf_type *t, int off, int size,
- u32 *next_btf_id, enum bpf_type_flag *flag)
+ u32 *next_btf_id, enum bpf_type_flag *flag,
+ const char **field_name)
{
u32 i, moff, mtrue_end, msize = 0, total_nelems = 0;
const struct btf_type *mtype, *elem_type = NULL;
@@ -6395,6 +6396,8 @@ static int btf_struct_walk(struct bpf_verifier_log *log, const struct btf *btf,
if (btf_type_is_struct(stype)) {
*next_btf_id = id;
*flag |= tmp_flag;
+ if (field_name)
+ *field_name = mname;
return WALK_PTR;
}
}
@@ -6421,7 +6424,8 @@ static int btf_struct_walk(struct bpf_verifier_log *log, const struct btf *btf,
int btf_struct_access(struct bpf_verifier_log *log,
const struct bpf_reg_state *reg,
int off, int size, enum bpf_access_type atype __maybe_unused,
- u32 *next_btf_id, enum bpf_type_flag *flag)
+ u32 *next_btf_id, enum bpf_type_flag *flag,
+ const char **field_name)
{
const struct btf *btf = reg->btf;
enum bpf_type_flag tmp_flag = 0;
@@ -6453,7 +6457,7 @@ int btf_struct_access(struct bpf_verifier_log *log,

t = btf_type_by_id(btf, id);
do {
- err = btf_struct_walk(log, btf, t, off, size, &id, &tmp_flag);
+ err = btf_struct_walk(log, btf, t, off, size, &id, &tmp_flag, field_name);

switch (err) {
case WALK_PTR:
@@ -6528,7 +6532,7 @@ bool btf_struct_ids_match(struct bpf_verifier_log *log,
type = btf_type_by_id(btf, id);
if (!type)
return false;
- err = btf_struct_walk(log, btf, type, off, 1, &id, &flag);
+ err = btf_struct_walk(log, btf, type, off, 1, &id, &flag, NULL);
if (err != WALK_STRUCT)
return false;

@@ -8488,16 +8492,15 @@ int bpf_core_apply(struct bpf_core_ctx *ctx, const struct bpf_core_relo *relo,

bool btf_nested_type_is_trusted(struct bpf_verifier_log *log,
const struct bpf_reg_state *reg,
- int off, const char *suffix)
+ const char *field_name, u32 btf_id, const char *suffix)
{
struct btf *btf = reg->btf;
const struct btf_type *walk_type, *safe_type;
const char *tname;
char safe_tname[64];
long ret, safe_id;
- const struct btf_member *member, *m_walk = NULL;
+ const struct btf_member *member;
u32 i;
- const char *walk_name;

walk_type = btf_type_by_id(btf, reg->btf_id);
if (!walk_type)
@@ -8517,30 +8520,17 @@ bool btf_nested_type_is_trusted(struct bpf_verifier_log *log,
if (!safe_type)
return false;

- for_each_member(i, walk_type, member) {
- u32 moff;
-
- /* We're looking for the PTR_TO_BTF_ID member in the struct
- * type we're walking which matches the specified offset.
- * Below, we'll iterate over the fields in the safe variant of
- * the struct and see if any of them has a matching type /
- * name.
- */
- moff = __btf_member_bit_offset(walk_type, member) / 8;
- if (off == moff) {
- m_walk = member;
- break;
- }
- }
- if (m_walk == NULL)
- return false;
-
- walk_name = __btf_name_by_offset(btf, m_walk->name_off);
for_each_member(i, safe_type, member) {
const char *m_name = __btf_name_by_offset(btf, member->name_off);
+ const struct btf_type *mtype = btf_type_by_id(btf, member->type);
+ u32 id;
+
+ if (!btf_type_is_ptr(mtype))
+ continue;

+ btf_type_skip_modifiers(btf, mtype->type, &id);
/* If we match on both type and name, the field is considered trusted. */
- if (m_walk->type == member->type && !strcmp(walk_name, m_name))
+ if (btf_id == id && !strcmp(field_name, m_name))
return true;
}

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 5ca520e5eddf..2cd2e0b725cd 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -5400,12 +5400,12 @@ BTF_TYPE_SAFE_RCU(struct css_set) {

/* full trusted: these fields are trusted even outside of RCU CS and never NULL */
BTF_TYPE_SAFE_TRUSTED(struct bpf_iter_meta) {
- __bpf_md_ptr(struct seq_file *, seq);
+ struct seq_file *seq;
};

BTF_TYPE_SAFE_TRUSTED(struct bpf_iter__task) {
- __bpf_md_ptr(struct bpf_iter_meta *, meta);
- __bpf_md_ptr(struct task_struct *, task);
+ struct bpf_iter_meta *meta;
+ struct task_struct *task;
};

BTF_TYPE_SAFE_TRUSTED(struct linux_binprm) {
@@ -5427,17 +5427,17 @@ BTF_TYPE_SAFE_TRUSTED(struct socket) {

static bool type_is_rcu(struct bpf_verifier_env *env,
struct bpf_reg_state *reg,
- int off)
+ const char *field_name, u32 btf_id)
{
BTF_TYPE_EMIT(BTF_TYPE_SAFE_RCU(struct task_struct));
BTF_TYPE_EMIT(BTF_TYPE_SAFE_RCU(struct css_set));

- return btf_nested_type_is_trusted(&env->log, reg, off, "__safe_rcu");
+ return btf_nested_type_is_trusted(&env->log, reg, field_name, btf_id, "__safe_rcu");
}

static bool type_is_trusted(struct bpf_verifier_env *env,
struct bpf_reg_state *reg,
- int off)
+ const char *field_name, u32 btf_id)
{
BTF_TYPE_EMIT(BTF_TYPE_SAFE_TRUSTED(struct bpf_iter_meta));
BTF_TYPE_EMIT(BTF_TYPE_SAFE_TRUSTED(struct bpf_iter__task));
@@ -5446,7 +5446,7 @@ static bool type_is_trusted(struct bpf_verifier_env *env,
BTF_TYPE_EMIT(BTF_TYPE_SAFE_TRUSTED(struct dentry));
BTF_TYPE_EMIT(BTF_TYPE_SAFE_TRUSTED(struct socket));

- return btf_nested_type_is_trusted(&env->log, reg, off, "__safe_trusted");
+ return btf_nested_type_is_trusted(&env->log, reg, field_name, btf_id, "__safe_trusted");
}

static int check_ptr_to_btf_access(struct bpf_verifier_env *env,
@@ -5458,6 +5458,7 @@ static int check_ptr_to_btf_access(struct bpf_verifier_env *env,
struct bpf_reg_state *reg = regs + regno;
const struct btf_type *t = btf_type_by_id(reg->btf, reg->btf_id);
const char *tname = btf_name_by_offset(reg->btf, t->name_off);
+ const char *field_name = NULL;
enum bpf_type_flag flag = 0;
u32 btf_id = 0;
int ret;
@@ -5526,7 +5527,7 @@ static int check_ptr_to_btf_access(struct bpf_verifier_env *env,
return -EFAULT;
}

- ret = btf_struct_access(&env->log, reg, off, size, atype, &btf_id, &flag);
+ ret = btf_struct_access(&env->log, reg, off, size, atype, &btf_id, &flag, &field_name);
}

if (ret < 0)
@@ -5554,10 +5555,10 @@ static int check_ptr_to_btf_access(struct bpf_verifier_env *env,
* A regular RCU-protected pointer with __rcu tag can also be deemed
* trusted if we are in an RCU CS. Such pointer can be NULL.
*/
- if (type_is_trusted(env, reg, off)) {
+ if (type_is_trusted(env, reg, field_name, btf_id)) {
flag |= PTR_TRUSTED;
} else if (in_rcu_cs(env) && !type_may_be_null(reg->type)) {
- if (type_is_rcu(env, reg, off)) {
+ if (type_is_rcu(env, reg, field_name, btf_id)) {
/* ignore __rcu tag and mark it MEM_RCU */
flag |= MEM_RCU;
} else if (flag & MEM_RCU) {
@@ -5640,7 +5641,7 @@ static int check_ptr_to_map_access(struct bpf_verifier_env *env,
/* Simulate access to a PTR_TO_BTF_ID */
memset(&map_reg, 0, sizeof(map_reg));
mark_btf_ld_reg(env, &map_reg, 0, PTR_TO_BTF_ID, btf_vmlinux, *map->ops->map_btf_id, 0);
- ret = btf_struct_access(&env->log, &map_reg, off, size, atype, &btf_id, &flag);
+ ret = btf_struct_access(&env->log, &map_reg, off, size, atype, &btf_id, &flag, NULL);
if (ret < 0)
return ret;

--
2.34.1