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bpf: Mark direct ld of stashed bpf_{rb,list}_node as non-owning ref

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This patch enables the following pattern:

  /* mapval contains a __kptr pointing to refcounted local kptr */
  mapval = bpf_map_lookup_elem(&map, &idx);
  if (!mapval || !mapval->some_kptr) { /* omitted */ }

  p = bpf_refcount_acquire(&mapval->some_kptr);

Currently this doesn't work because bpf_refcount_acquire expects an
owning or non-owning ref. The verifier defines non-owning ref as a type:

  PTR_TO_BTF_ID | MEM_ALLOC | NON_OWN_REF

while mapval->some_kptr is PTR_TO_BTF_ID | PTR_UNTRUSTED. It's possible
to do the refcount_acquire by first bpf_kptr_xchg'ing mapval->some_kptr
into a temp kptr, refcount_acquiring that, and xchg'ing back into
mapval, but this is unwieldy and shouldn't be necessary.

This patch modifies btf_ld_kptr_type such that user-allocated types are
marked MEM_ALLOC and if those types have a bpf_{rb,list}_node they're
marked NON_OWN_REF as well. Additionally, due to changes to
bpf_obj_drop_impl earlier in this series, rcu_protected_object now
returns true for all user-allocated types, resulting in
mapval->some_kptr being marked MEM_RCU.

After this patch's changes, mapval->some_kptr is now:

  PTR_TO_BTF_ID | MEM_ALLOC | NON_OWN_REF | MEM_RCU

which results in it passing the non-owning ref test, and the motivating
example passing verification.

Future work will likely get rid of special non-owning ref lifetime logic
in the verifier, at which point we'll be able to delete the NON_OWN_REF
flag entirely.

Signed-off-by: Dave Marchevsky <davemarchevsky@fb.com>
Link: https://lore.kernel.org/r/20231107085639.3016113-6-davemarchevsky@fb.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
This commit is contained in:
Dave Marchevsky 2023-11-07 00:56:38 -08:00 committed by Alexei Starovoitov
parent 790ce3cfef
commit 1b12171533
1 changed files with 31 additions and 5 deletions
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36
kernel/bpf/verifier.c
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@ -5557,10 +5557,23 @@ BTF_SET_END(rcu_protected_types)
static bool rcu_protected_object(const struct btf *btf, u32 btf_id)
{
if (!btf_is_kernel(btf))
return false;
return true;
return btf_id_set_contains(&rcu_protected_types, btf_id);
}
static struct btf_record *kptr_pointee_btf_record(struct btf_field *kptr_field)
{
struct btf_struct_meta *meta;
if (btf_is_kernel(kptr_field->kptr.btf))
return NULL;
meta = btf_find_struct_meta(kptr_field->kptr.btf,
kptr_field->kptr.btf_id);
return meta ? meta->record : NULL;
}
static bool rcu_safe_kptr(const struct btf_field *field)
{
const struct btf_field_kptr *kptr = &field->kptr;
@ -5571,12 +5584,25 @@ static bool rcu_safe_kptr(const struct btf_field *field)
static u32 btf_ld_kptr_type(struct bpf_verifier_env *env, struct btf_field *kptr_field)
{
struct btf_record *rec;
u32 ret;
ret = PTR_MAYBE_NULL;
if (rcu_safe_kptr(kptr_field) && in_rcu_cs(env)) {
if (kptr_field->type != BPF_KPTR_PERCPU)
return PTR_MAYBE_NULL | MEM_RCU;
return PTR_MAYBE_NULL | MEM_RCU | MEM_PERCPU;
ret |= MEM_RCU;
if (kptr_field->type == BPF_KPTR_PERCPU)
ret |= MEM_PERCPU;
else if (!btf_is_kernel(kptr_field->kptr.btf))
ret |= MEM_ALLOC;
rec = kptr_pointee_btf_record(kptr_field);
if (rec && btf_record_has_field(rec, BPF_GRAPH_NODE))
ret |= NON_OWN_REF;
} else {
ret |= PTR_UNTRUSTED;
}
return PTR_MAYBE_NULL | PTR_UNTRUSTED;
return ret;
}
static int check_map_kptr_access(struct bpf_verifier_env *env, u32 regno,