On 10/25/23 2:40 PM, Dave Marchevsky wrote:
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@xxxxxx>
---
kernel/bpf/verifier.c | 36 +++++++++++++++++++++++++++++++-----
1 file changed, 31 insertions(+), 5 deletions(-)
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 857d76694517..bb098a4c8fd5 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -5396,10 +5396,23 @@ 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;
@@ -5410,12 +5423,25 @@ 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;
+ 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;
}
The CI reported a failure.
https://github.com/kernel-patches/bpf/actions/runs/6675467065/job/18143577936?pr=5886
Error: #162 percpu_alloc
Error:#162/1 percpu_alloc/array
Error:#162/2 percpu_alloc/array_sleepable
Error:#162/3 percpu_alloc/cgrp_local_storage
Error: #162 percpu_alloc
Error:#162/1 percpu_alloc/array
Error: #162/1 percpu_alloc/array
test_array:PASS:percpu_alloc_array__open 0 nsec
libbpf: prog 'test_array_map_2': BPF program load failed: Permission denied
libbpf: prog 'test_array_map_2': -- BEGIN PROG LOAD LOG --
reg type unsupported for arg#0 function test_array_map_2#25
0: R1=ctx(off=0,imm=0) R10=fp0
; int BPF_PROG(test_array_map_2)
0: (b4) w1 = 0 ; R1_w=0
; int index = 0;
1: (63) *(u32 *)(r10 -4) = r1 ; R1_w=0 R10=fp0 fp-8=0000????
2: (bf) r2 = r10 ; R2_w=fp0 R10=fp0
;
3: (07) r2 += -4 ; R2_w=fp-4
; e = bpf_map_lookup_elem(&array, &index);
4: (18) r1 = 0xffffa3bd813c5400 ; R1_w=map_ptr(off=0,ks=4,vs=16,imm=0)
6: (85) call bpf_map_lookup_elem#1 ;
R0_w=map_value_or_null(id=1,off=0,ks=4,vs=16,imm=0)
; if (!e)
7: (15) if r0 == 0x0 goto pc+9 ; R0_w=map_value(off=0,ks=4,vs=16,imm=0)
; p = e->pc;
8: (79) r1 = *(u64 *)(r0 +8) ; R0=map_value(off=0,ks=4,vs=16,imm=0)
R1=percpu_rcu_ptr_or_null_val_t(id=2,off=0,imm=0)
; if (!p)
9: (15) if r1 == 0x0 goto pc+7 ; R1=percpu_rcu_ptr_val_t(off=0,imm=0)
; v = bpf_per_cpu_ptr(p, 0);
10: (b4) w2 = 0 ; R2_w=0
11: (85) call bpf_per_cpu_ptr#153
R1 type=percpu_rcu_ptr_ expected=percpu_ptr_, percpu_rcu_ptr_,
percpu_trusted_ptr_
processed 11 insns (limit 1000000) max_states_per_insn 0 total_states 1
peak_states 1 mark_read 1
-- END PROG LOAD LOG --
libbpf: prog 'test_array_map_2': failed to load: -13
libbpf: failed to load object 'percpu_alloc_array'
libbpf: failed to load BPF skeleton 'percpu_alloc_array': -13
test_array:FAIL:percpu_alloc_array__load unexpected error: -13 (errno 13)
The following hack can fix the issue.
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index bb098a4c8fd5..2bbda1f5e858 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -5431,7 +5431,7 @@ static u32 btf_ld_kptr_type(struct
bpf_verifier_env *env, struct btf_field *kptr
ret |= MEM_RCU;
if (kptr_field->type == BPF_KPTR_PERCPU)
ret |= MEM_PERCPU;
- if (!btf_is_kernel(kptr_field->kptr.btf))
+ else if (!btf_is_kernel(kptr_field->kptr.btf))
ret |= MEM_ALLOC;
rec = kptr_pointee_btf_record(kptr_field);
Note in the current kernel, MEM_RCU | MEM_PERCPU implies non-kernel
kptr. The kernel PERCPU kptr has PTR_TRUSTED | MEM_PERCPU. So there is
no MEM_ALLOC. Adding MEM_ALLOC might need changes in other places
w.r.t., percpu non-kernel kptr.
Please take a look.
static int check_map_kptr_access(struct bpf_verifier_env *env, u32 regno,
