[PATCH v10 bpf-next 5/7] bpf: Use btf_kfunc_id_set.remap logic for bpf_dynptr_from_skb

Song Liu
2025-01-24T20:30:03Z

btf_kfunc_id_set.remap can pick proper version of a kfunc for the calling
context. Use this logic to select bpf_dynptr_from_skb or
bpf_dynptr_from_skb_rdonly. This will make the verifier simpler.

Unfortunately, btf_kfunc_id_set.remap cannot cover the DYNPTR_TYPE_SKB
logic in check_kfunc_args(). This can be addressed later.

Signed-off-by: Song Liu <song@kernel.org>
---
kernel/bpf/verifier.c | 25 ++++++----------------
net/core/filter.c | 49 +++++++++++++++++++++++++++++++++++++++----
2 files changed, 51 insertions(+), 23 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 2188b6674266..55e710e318e5 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -11750,6 +11750,7 @@ enum special_kfunc_type {
KF_bpf_rbtree_add_impl,
KF_bpf_rbtree_first,
KF_bpf_dynptr_from_skb,
+ KF_bpf_dynptr_from_skb_rdonly,
KF_bpf_dynptr_from_xdp,
KF_bpf_dynptr_slice,
KF_bpf_dynptr_slice_rdwr,
@@ -11785,6 +11786,7 @@ BTF_ID(func, bpf_rbtree_add_impl)
BTF_ID(func, bpf_rbtree_first)
#ifdef CONFIG_NET
BTF_ID(func, bpf_dynptr_from_skb)
+BTF_ID(func, bpf_dynptr_from_skb_rdonly)
BTF_ID(func, bpf_dynptr_from_xdp)
#endif
BTF_ID(func, bpf_dynptr_slice)
@@ -11816,10 +11818,12 @@ BTF_ID(func, bpf_rbtree_add_impl)
BTF_ID(func, bpf_rbtree_first)
#ifdef CONFIG_NET
BTF_ID(func, bpf_dynptr_from_skb)
+BTF_ID(func, bpf_dynptr_from_skb_rdonly)
BTF_ID(func, bpf_dynptr_from_xdp)
#else
BTF_ID_UNUSED
BTF_ID_UNUSED
+BTF_ID_UNUSED
#endif
BTF_ID(func, bpf_dynptr_slice)
BTF_ID(func, bpf_dynptr_slice_rdwr)
@@ -12741,7 +12745,8 @@ static int check_kfunc_args(struct bpf_verifier_env *env, struct bpf_kfunc_call_
if (is_kfunc_arg_uninit(btf, &args[i]))
dynptr_arg_type |= MEM_UNINIT;

- if (meta->func_id == special_kfunc_list[KF_bpf_dynptr_from_skb]) {
+ if (meta->func_id == special_kfunc_list[KF_bpf_dynptr_from_skb] ||
+ meta->func_id == special_kfunc_list[KF_bpf_dynptr_from_skb_rdonly]) {
dynptr_arg_type |= DYNPTR_TYPE_SKB;
} else if (meta->func_id == special_kfunc_list[KF_bpf_dynptr_from_xdp]) {
dynptr_arg_type |= DYNPTR_TYPE_XDP;
@@ -20898,9 +20903,7 @@ static void specialize_kfunc(struct bpf_verifier_env *env,
u32 func_id, u16 offset, unsigned long *addr)
{
struct bpf_prog *prog = env->prog;
- bool seen_direct_write;
void *xdp_kfunc;
- bool is_rdonly;

if (bpf_dev_bound_kfunc_id(func_id)) {
xdp_kfunc = bpf_dev_bound_resolve_kfunc(prog, func_id);
@@ -20910,22 +20913,6 @@ static void specialize_kfunc(struct bpf_verifier_env *env,
}
/* fallback to default kfunc when not supported by netdev */
}
-
- if (offset)
- return;
-
- if (func_id == special_kfunc_list[KF_bpf_dynptr_from_skb]) {
- seen_direct_write = env->seen_direct_write;
- is_rdonly = !may_access_direct_pkt_data(env, NULL, BPF_WRITE);
-
- if (is_rdonly)
- *addr = (unsigned long)bpf_dynptr_from_skb_rdonly;
-
- /* restore env->seen_direct_write to its original value, since
- * may_access_direct_pkt_data mutates it
- */
- env->seen_direct_write = seen_direct_write;
- }
}

static void __fixup_collection_insert_kfunc(struct bpf_insn_aux_data *insn_aux,
diff --git a/net/core/filter.c b/net/core/filter.c
index 2ec162dd83c4..6416689e3976 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -12062,10 +12062,8 @@ __bpf_kfunc int bpf_sk_assign_tcp_reqsk(struct __sk_buff *s, struct sock *sk,
#endif
}

-__bpf_kfunc_end_defs();
-
-int bpf_dynptr_from_skb_rdonly(struct __sk_buff *skb, u64 flags,
- struct bpf_dynptr *ptr__uninit)
+__bpf_kfunc int bpf_dynptr_from_skb_rdonly(struct __sk_buff *skb, u64 flags,
+ struct bpf_dynptr *ptr__uninit)
{
struct bpf_dynptr_kern *ptr = (struct bpf_dynptr_kern *)ptr__uninit;
int err;
@@ -12079,10 +12077,16 @@ int bpf_dynptr_from_skb_rdonly(struct __sk_buff *skb, u64 flags,
return 0;
}

+__bpf_kfunc_end_defs();
+
BTF_KFUNCS_START(bpf_kfunc_check_set_skb)
BTF_ID_FLAGS(func, bpf_dynptr_from_skb, KF_TRUSTED_ARGS)
BTF_KFUNCS_END(bpf_kfunc_check_set_skb)

+BTF_HIDDEN_KFUNCS_START(bpf_kfunc_check_hidden_set_skb)
+BTF_ID_FLAGS(func, bpf_dynptr_from_skb_rdonly, KF_TRUSTED_ARGS)
+BTF_KFUNCS_END(bpf_kfunc_check_hidden_set_skb)
+
BTF_KFUNCS_START(bpf_kfunc_check_set_xdp)
BTF_ID_FLAGS(func, bpf_dynptr_from_xdp)
BTF_KFUNCS_END(bpf_kfunc_check_set_xdp)
@@ -12095,9 +12099,46 @@ BTF_KFUNCS_START(bpf_kfunc_check_set_tcp_reqsk)
BTF_ID_FLAGS(func, bpf_sk_assign_tcp_reqsk, KF_TRUSTED_ARGS)
BTF_KFUNCS_END(bpf_kfunc_check_set_tcp_reqsk)

+BTF_ID_LIST(bpf_dynptr_from_skb_list)
+BTF_ID(func, bpf_dynptr_from_skb)
+BTF_ID(func, bpf_dynptr_from_skb_rdonly)
+
+static u32 bpf_kfunc_set_skb_remap(const struct bpf_prog *prog, u32 kfunc_id)
+{
+ if (kfunc_id != bpf_dynptr_from_skb_list[0])
+ return 0;
+
+ switch (resolve_prog_type(prog)) {
+ /* Program types only with direct read access go here! */
+ case BPF_PROG_TYPE_LWT_IN:
+ case BPF_PROG_TYPE_LWT_OUT:
+ case BPF_PROG_TYPE_LWT_SEG6LOCAL:
+ case BPF_PROG_TYPE_SK_REUSEPORT:
+ case BPF_PROG_TYPE_FLOW_DISSECTOR:
+ case BPF_PROG_TYPE_CGROUP_SKB:
+ return bpf_dynptr_from_skb_list[1];
+
+ /* Program types with direct read + write access go here! */
+ case BPF_PROG_TYPE_SCHED_CLS:
+ case BPF_PROG_TYPE_SCHED_ACT:
+ case BPF_PROG_TYPE_XDP:
+ case BPF_PROG_TYPE_LWT_XMIT:
+ case BPF_PROG_TYPE_SK_SKB:
+ case BPF_PROG_TYPE_SK_MSG:
+ case BPF_PROG_TYPE_CGROUP_SOCKOPT:
+ return kfunc_id;
+
+ default:
+ break;
+ }
+ return bpf_dynptr_from_skb_list[1];
+}
+
static const struct btf_kfunc_id_set bpf_kfunc_set_skb = {
.owner = THIS_MODULE,
.set = &bpf_kfunc_check_set_skb,
+ .hidden_set = &bpf_kfunc_check_hidden_set_skb,
+ .remap = &bpf_kfunc_set_skb_remap,
};

static const struct btf_kfunc_id_set bpf_kfunc_set_xdp = {
--
2.43.5