[PATCH bpf-next v5 03/21] bpf: Add the bpf_unwind_resume() kfunc

[Yonghong Song <yonghong.song@linux.dev>]

A compiler-emitted cleanup landing pad ends with a call to
_Unwind_Resume(), which carries the unwind on once that frame's cleanups
have run. The kernel provides the same terminator as a kfunc, named
bpf_unwind_resume() to keep the 'bpf_' prefix kfunc convention; a later
libbpf patch resolves the compiler's name to it.

The kfunc is defined here but not registered with any program type yet. A
call to it is only ever valid inside a landing pad, and the verifier cannot
say that until it knows what a landing pad is, so registration waits for
the patch that adds the rule.

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
---
 kernel/bpf/helpers.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index 301b35bd85c8..249418e60ee4 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -3445,6 +3445,19 @@ __bpf_kfunc void bpf_throw(u64 cookie)
 	WARN(1, "A call to BPF exception callback should never return\n");
 }
 
+/*
+ * Terminator of a compiler-emitted cleanup landing pad. The compiler names
+ * this _Unwind_Resume, the base unwind ABI's entry point for carrying an
+ * unwind on once a frame's cleanups have run. To match kernel kfunc
+ * convention, the kernel calls it bpf_unwind_resume and libbpf maps the
+ * compiler's name onto it.
+ */
+__bpf_kfunc void bpf_unwind_resume(void)
+{
+	/* Never reached: a JIT emits the way out of a landing pad instead. */
+	WARN_ONCE(1, "A JIT should have replaced the exception cleanup resume\n");
+}
+
 __bpf_kfunc int bpf_wq_init(struct bpf_wq *wq, void *p__const_map, unsigned int flags)
 {
 	struct bpf_async_kern *async = (struct bpf_async_kern *)wq;
-- 
2.53.0-Meta