On 12/3/24 12:46 PM, Maciej Fijalkowski wrote:
; bpf_skb_release((struct __sk_buff *)tmp); @ bpf_bpf.c:161
36: (bf) r1 = r6 ; R1_w=ptr_sk_buff(ref_obj_id=3) R6=ptr_sk_buff(ref_obj_id=3) refs=3
37: (85) call bpf_skb_release#102037
arg#0 expected pointer to ctx, but got ptr_
ic. The bpf_skb_release() is hitting the KF_ARG_PTR_TO_CTX again.
processed 34 insns (limit 1000000) max_states_per_insn 0 total_states 3 peak_states 3 mark_read 1
-- END PROG LOAD LOG --
Still the same problem. Also even it would work it was not very convenient
to cast these types back and forth...I then tried to store __sk_buff as
kptr but I ended up with:
"map 'skb_map' has to have BTF in order to use bpf_kptr_xchg"
which got me lost:) I have a solution though which I'd like to discuss.
Please share the patch and the test case. It will be easier for others to help.
I have come up with rather simple way of achieving what I desired when
starting this thread, how about something like this:
From 0df7760330cccfe71235b56018d0a33d4a3b9863 Mon Sep 17 00:00:00 2001
From: Maciej Fijalkowski <maciej.fijalkowski@xxxxxxxxx>
Date: Tue, 3 Dec 2024 21:00:44 +0100
Subject: [PATCH RFC bpf-next] bpf: add __ctx_ref kfunc argument suffix
The use case is when user wants to use sk_buff pointers as kptrs against
program types that take in __sk_buff struct as context.
A pair of kfuncs for acquiring and releasing skb would look as follows:
__bpf_kfunc struct sk_buff *bpf_skb_acquire(struct sk_buff *skb__ctx_ref)
__bpf_kfunc void bpf_skb_release(struct sk_buff *skb__ctx_ref)
Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@xxxxxxxxx>
---
kernel/bpf/verifier.c | 8 ++++++++
1 file changed, 8 insertions(+)
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 60938b136365..b16a39d28f8a 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -11303,6 +11303,11 @@ static bool is_kfunc_arg_const_str(const struct btf *btf, const struct btf_param
return btf_param_match_suffix(btf, arg, "__str");
}
+static bool is_kfunc_arg_ctx_ref(const struct btf *btf, const struct btf_param *arg)
+{
+ return btf_param_match_suffix(btf, arg, "__ctx_ref");
imo, new tagging is not needed. It does not give new information to
the ptr type. I still think the verifier can be taught to handle
it better.
I took a closer look. I think the issue is btf_is_prog_ctx_type() selectively
treating some kfunc arg type as the uapi prog ctx instead of honoring what
has been written in the kernel source code.
The projection_of test was first added in
commit 2f4643934670 ("bpf: Support "sk_buff" and "xdp_buff" as valid kfunc arg types").
It was originally added to allow the kfunc to accept reg->type == PTR_TO_CTX
as its "struct sk_buff/xdp_buff *" argument.
After commit cce4c40b9606 ("bpf: treewide: Align kfunc signatures to prog point-of-view"),
the need of projection_of in btf_is_prog_ctx_type() should be gone.
However, this projection_of check has an unintentional side effect
for the other btf_is_prog_ctx_type() callers. It treats subprog
(in the bpf prog code) taking the "struct sk_buff *skb" arg as the uapi
prog ctx also. I don't think the bpf prog should do this though. I have a
"call_dynptr_skb" subprog to show this.
For the skb acquire/release kfunc, I think it is better to begin with
the "struct sk_buff *" as its arg type and return type
instead of "struct __sk_buff *" because it is the __kptr type stored
in the map. The kptr_xchg will also return the PTR_TO_BTF_ID.
It will need another cast call for acquire, like
"bpf_skb_acauire(bpf_cast_to_kern_ctx(ctx))" but this should be fine.
The "struct sk_buff __kptr *skb" stored in the map cannot
be passed to the bpf_dynptr_from_skb() also. It shouldn't be
allowed because bpf_dynptr_from_skb will allow skb write
in the tc bpf prog. The same goes for other tc bpf helpers which
takes ARG_PTR_TO_CTX.
I think we can remove the projection_of call from the
bpf_is_prog_ctx_type() such that it honors the exact argument
type written in the kernel source code. Add this particular projection_of
check (renamed to bpf_is_kern_ctx in the diff) to the other callers for
backward compat such that the caller can selectively translate
the argument of a subprog to the corresponding prog ctx type.
Lightly tested only:
diff --git i/kernel/bpf/btf.c w/kernel/bpf/btf.c
index e7a59e6462a9..2d39f91617fb 100644
--- i/kernel/bpf/btf.c
+++ w/kernel/bpf/btf.c
@@ -5914,6 +5914,26 @@ bool btf_is_projection_of(const char *pname, const char *tname)
return false;
}
+static bool btf_is_kern_ctx(const struct btf *btf,
+ const struct btf_type *t,
+ enum bpf_prog_type prog_type)
+{
+ const struct btf_type *ctx_type;
+ const char *tname, *ctx_tname;
+
+ t = btf_type_skip_modifiers(btf, t->type, NULL);
+ if (!btf_type_is_struct(t))
+ return false;
+
+ tname = btf_name_by_offset(btf, t->name_off);
+ if (!tname)
+ return false;
+
+ ctx_type = find_canonical_prog_ctx_type(prog_type);
+ ctx_tname = btf_name_by_offset(btf_vmlinux, ctx_type->name_off);
+ return btf_is_projection_of(ctx_tname, tname);
+}
+
bool btf_is_prog_ctx_type(struct bpf_verifier_log *log, const struct btf *btf,
const struct btf_type *t, enum bpf_prog_type prog_type,
int arg)
@@ -5976,8 +5996,6 @@ bool btf_is_prog_ctx_type(struct bpf_verifier_log *log, const struct btf *btf,
* int socket_filter_bpf_prog(struct __sk_buff *skb)
* { // no fields of skb are ever used }
*/
- if (btf_is_projection_of(ctx_tname, tname))
- return true;
if (strcmp(ctx_tname, tname)) {
/* bpf_user_pt_regs_t is a typedef, so resolve it to
* underlying struct and check name again
@@ -6140,7 +6158,8 @@ static int btf_translate_to_vmlinux(struct bpf_verifier_log *log,
enum bpf_prog_type prog_type,
int arg)
{
- if (!btf_is_prog_ctx_type(log, btf, t, prog_type, arg))
+ if (!btf_is_prog_ctx_type(log, btf, t, prog_type, arg) &&
+ !btf_is_kern_ctx(btf, t, prog_type))
return -ENOENT;
return find_kern_ctx_type_id(prog_type);
}
@@ -7505,7 +7524,8 @@ int btf_prepare_func_args(struct bpf_verifier_env *env, int subprog)
if (!btf_type_is_ptr(t))
goto skip_pointer;
- if ((tags & ARG_TAG_CTX) || btf_is_prog_ctx_type(log, btf, t, prog_type, i)) {
+ if ((tags & ARG_TAG_CTX) || btf_is_prog_ctx_type(log, btf, t, prog_type, i) ||
+ btf_is_kern_ctx(btf, t, prog_type)) {
if (tags & ~ARG_TAG_CTX) {
bpf_log(log, "arg#%d has invalid combination of tags\n", i);
return -EINVAL;
diff --git a/tools/testing/selftests/bpf/progs/skb_acquire.c b/tools/testing/selftests/bpf/progs/skb_acquire.c
new file mode 100644
index 000000000000..65d62fd97905
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/skb_acquire.c
@@ -0,0 +1,59 @@
+#include <vmlinux.h>
+#include <bpf/bpf_helpers.h>
+#include "bpf_misc.h"
+#include "bpf_kfuncs.h"
+#include "bpf_tracing_net.h"
+
+struct sk_buff *dummy_skb;
+
+struct sk_buff *bpf_skb_acquire(struct sk_buff *skb) __ksym;
+void bpf_skb_release(struct sk_buff *skb) __ksym;
+void *bpf_cast_to_kern_ctx(void *) __ksym;
+
+struct map_value {
+ int a;
+ struct sk_buff __kptr *skb;
+};
+
+struct {
+ __uint(type, BPF_MAP_TYPE_ARRAY);
+ __type(key, int);
+ __type(value, struct map_value);
+ __uint(max_entries, 16);
+} skb_map SEC(".maps");
+
+__noinline int call_dynptr_skb(struct sk_buff *skb)
+{
+ struct bpf_dynptr ptr;
+
+ bpf_dynptr_from_skb((struct __sk_buff *)skb, 0, &ptr);
+
+ return 0;
+}
+
+__success
+SEC("tc")
+int bpf_tc_egress(struct __sk_buff *ctx)
+{
+ struct sk_buff *skb;
+ struct map_value *map_entry;
+ u32 zero = 0;
+
+ call_dynptr_skb((struct sk_buff *)ctx);
+
+ map_entry = bpf_map_lookup_elem(&skb_map, &zero);
+ if (!map_entry)
+ return TC_ACT_SHOT;
+
+ skb = bpf_skb_acquire(bpf_cast_to_kern_ctx(ctx));
+ if (!skb)
+ return TC_ACT_SHOT;
+
+ skb = bpf_kptr_xchg(&map_entry->skb, skb);
+ if (skb)
+ bpf_skb_release(skb);
+
+ return TC_ACT_OK;
+}
+
+char LICENSE[] SEC("license") = "GPL";
