Add an ability to pass a pointer to a type with known size in arguments
of a global function. Such pointers may be used to overcome the limit on
the maximum number of arguments, avoid expensive and tricky workarounds
and to have multiple output arguments.
A referenced type may contain pointers but indirect access through them
isn't supported.
The implementation consists of two parts. If a global function has an
argument that is a pointer to a type with known size then:
1) In btf_check_func_arg_match(): check that the corresponding
register points to NULL or to a valid memory region that is large enough
to contain the expected argument's type.
2) In btf_prepare_func_args(): set the corresponding register type to
PTR_TO_MEM_OR_NULL and its size to the size of the expected type.
A call to a global function may change stack's memory slot type(via
check_helper_mem_access) similar to a helper function. That is why new
pointer arguments are allowed only for functions with global linkage.
Consider a case: stack's memory slot type is changed to STACK_MISC from
spilled PTR_TO_PACKET(btf_check_func_arg_match() -> check_mem_reg() ->
check_helper_mem_access() -> check_stack_boundary()) after a call to a
static function and later verifier cannot infer safety of indirect
accesses through the stack memory(check_stack_read() ->
__mark_reg_unknown ()). This will break existing code.
Signed-off-by: Dmitrii Banshchikov <me@xxxxxxxxxxxxx>
---
v1 -> v2:
- Allow pointers only for global functions
- Add support of any type with known size
- Use conventional way to generate reg id
- Use boolean true/false instead of int 1/0
- Fix formatting
include/linux/bpf_verifier.h | 2 ++
kernel/bpf/btf.c | 55 +++++++++++++++++++++++++++++-------
kernel/bpf/verifier.c | 30 ++++++++++++++++++++
3 files changed, 77 insertions(+), 10 deletions(-)
diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h
index dfe6f85d97dd..ce91c8651ad9 100644
--- a/include/linux/bpf_verifier.h
+++ b/include/linux/bpf_verifier.h
@@ -470,6 +470,8 @@ bpf_prog_offload_remove_insns(struct bpf_verifier_env *env, u32 off, u32 cnt);
int check_ctx_reg(struct bpf_verifier_env *env,
const struct bpf_reg_state *reg, int regno);
+int check_mem_reg(struct bpf_verifier_env *env, struct bpf_reg_state *reg,
+ u32 regno, u32 mem_size);
/* 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 bd5d2c563693..90019f92deec 100644
--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c
@@ -5297,9 +5297,10 @@ int btf_check_func_arg_match(struct bpf_verifier_env *env, int subprog,
struct bpf_prog *prog = env->prog;
struct btf *btf = prog->aux->btf;
const struct btf_param *args;
- const struct btf_type *t;
- u32 i, nargs, btf_id;
+ const struct btf_type *t, *ref_t;
+ u32 i, nargs, btf_id, type_size;
const char *tname;
+ bool is_global;
if (!prog->aux->func_info)
return -EINVAL;
@@ -5333,6 +5334,8 @@ int btf_check_func_arg_match(struct bpf_verifier_env *env, int subprog,
bpf_log(log, "Function %s has %d > 5 args\n", tname, nargs);
goto out;
}
+
+ is_global = prog->aux->func_info_aux[subprog].linkage == BTF_FUNC_GLOBAL;
/* check that BTF function arguments match actual types that the
* verifier sees.
*/
@@ -5349,10 +5352,6 @@ int btf_check_func_arg_match(struct bpf_verifier_env *env, int subprog,
goto out;
}
if (btf_type_is_ptr(t)) {
- if (reg->type == SCALAR_VALUE) {
- bpf_log(log, "R%d is not a pointer\n", i + 1);
- goto out;
- }
/* If function expects ctx type in BTF check that caller
* is passing PTR_TO_CTX.
*/
@@ -5367,6 +5366,25 @@ int btf_check_func_arg_match(struct bpf_verifier_env *env, int subprog,
goto out;
continue;
}
+
+ if (!is_global)
+ goto out;
+
+ t = btf_type_skip_modifiers(btf, t->type, NULL);
+
+ ref_t = btf_resolve_size(btf, t, &type_size);
+ if (IS_ERR(ref_t)) {
+ bpf_log(log,
+ "arg#%d reference type('%s %s') size cannot be determined: %ld\n",
+ i, btf_type_str(t), btf_name_by_offset(btf, t->name_off),
+ PTR_ERR(ref_t));
+ goto out;
+ }
+
+ if (check_mem_reg(env, reg, i + 1, type_size))
+ goto out;
+
+ continue;
}
bpf_log(log, "Unrecognized arg#%d type %s\n",
i, btf_kind_str[BTF_INFO_KIND(t->info)]);
@@ -5397,7 +5415,7 @@ int btf_prepare_func_args(struct bpf_verifier_env *env, int subprog,
enum bpf_prog_type prog_type = prog->type;
struct btf *btf = prog->aux->btf;
const struct btf_param *args;
- const struct btf_type *t;
+ const struct btf_type *t, *ref_t;
u32 i, nargs, btf_id;
const char *tname;
@@ -5470,9 +5488,26 @@ int btf_prepare_func_args(struct bpf_verifier_env *env, int subprog,
reg->type = SCALAR_VALUE;
continue;
}
- if (btf_type_is_ptr(t) &&
- btf_get_prog_ctx_type(log, btf, t, prog_type, i)) {
- reg->type = PTR_TO_CTX;
+ if (btf_type_is_ptr(t)) {
+ if (btf_get_prog_ctx_type(log, btf, t, prog_type, i)) {
+ reg->type = PTR_TO_CTX;
+ continue;
+ }
+
+ t = btf_type_skip_modifiers(btf, t->type, NULL);
+
+ ref_t = btf_resolve_size(btf, t, ®->mem_size);
+ if (IS_ERR(ref_t)) {
+ bpf_log(log,
+ "arg#%d reference type('%s %s') size cannot be determined: %ld\n",
+ i, btf_type_str(t), btf_name_by_offset(btf, t->name_off),
+ PTR_ERR(ref_t));
+ return -EINVAL;
+ }
+
+ reg->type = PTR_TO_MEM_OR_NULL;
+ reg->id = i + 1;
+
continue;
}
bpf_log(log, "Arg#%d type %s in %s() is not supported yet.\n",
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index d68ea6eb4f9b..fd423af1cc57 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -3942,6 +3942,29 @@ static int check_helper_mem_access(struct bpf_verifier_env *env, int regno,
}
}
+int check_mem_reg(struct bpf_verifier_env *env, struct bpf_reg_state *reg,
+ u32 regno, u32 mem_size)
+{
+ if (register_is_null(reg))
+ return 0;
+
+ if (reg_type_may_be_null(reg->type)) {
+ /* Assuming that the register contains a value check if the memory
+ * access is safe. Temorarily save and restore the register's state as
+ * the conversion shouldn't be visible to a caller.
+ */
+ const struct bpf_reg_state saved_reg = *reg;
+ int rv;
+
+ mark_ptr_not_null_reg(reg);
+ rv = check_helper_mem_access(env, regno, mem_size, true, NULL);
+ *reg = saved_reg;
+ return rv;
+ }
+
+ return check_helper_mem_access(env, regno, mem_size, true, NULL);
+}
+
/* Implementation details:
* bpf_map_lookup returns PTR_TO_MAP_VALUE_OR_NULL
* Two bpf_map_lookups (even with the same key) will have different reg->id.
@@ -11572,6 +11595,13 @@ static int do_check_common(struct bpf_verifier_env *env, int subprog)
mark_reg_known_zero(env, regs, i);
else if (regs[i].type == SCALAR_VALUE)
mark_reg_unknown(env, regs, i);
+ else if (regs[i].type == PTR_TO_MEM_OR_NULL) {
+ const u32 mem_size = regs[i].mem_size;
+
+ mark_reg_known_zero(env, regs, i);
+ regs[i].mem_size = mem_size;
+ regs[i].id = ++env->id_gen;
+ }
}
} else {
/* 1st arg to a function */
--
2.25.1
