[PATCH v2 bpf-next 1/3] bpf: fix global subprog context argument resolution logic

Andrii Nakryiko
2023-02-16T05:00:19Z

KPROBE program's user-facing context type is defined as typedef
bpf_user_pt_regs_t. This leads to a problem when trying to passing
kprobe/uprobe/usdt context argument into global subprog, as kernel
always strip away mods and typedefs of user-supplied type, but takes
expected type from bpf_ctx_convert as is, which causes mismatch.

Current way to work around this is to define a fake struct with the same
name as expected typedef:

struct bpf_user_pt_regs_t {};

__noinline my_global_subprog(struct bpf_user_pt_regs_t *ctx) { ... }

This patch fixes the issue by resolving expected type, if it's not
a struct. It still leaves the above work-around working for backwards
compatibility.

Fixes: 91cc1a99740e ("bpf: Annotate context types")
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
---
kernel/bpf/btf.c | 13 +++++++++++--
1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
index 6582735ef1fc..fa22ec79ac0e 100644
--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c
@@ -5669,6 +5669,7 @@ btf_get_prog_ctx_type(struct bpf_verifier_log *log, const struct btf *btf,
if (!ctx_struct)
/* should not happen */
return NULL;
+again:
ctx_tname = btf_name_by_offset(btf_vmlinux, ctx_struct->name_off);
if (!ctx_tname) {
/* should not happen */
@@ -5682,8 +5683,16 @@ btf_get_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 (strcmp(ctx_tname, tname))
- return NULL;
+ if (strcmp(ctx_tname, tname)) {
+ /* bpf_user_pt_regs_t is a typedef, so resolve it to
+ * underlying struct and check name again
+ */
+ if (!btf_type_is_modifier(ctx_struct))
+ return NULL;
+ while (btf_type_is_modifier(ctx_struct))
+ ctx_struct = btf_type_by_id(btf_vmlinux, ctx_struct->type);
+ goto again;
+ }
return ctx_type;
}

--
2.30.2