[PATCH bpf-next v9 03/23] bpf: Add the bpf_unwind() and bpf_unwind_resume() kfuncs

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

An exception cleanup needs two terminators:

 - bpf_unwind() begins an unwind: the frames it leaves run their landing
   pads on the way out to the program's exit. It is separate from
   bpf_throw(), which discards the frames in between.
 - bpf_unwind_resume() ends a landing pad, carrying the unwind on. The
   compiler calls it _Unwind_Resume, which a later libbpf patch resolves
   to this name, and passes it the exception object, which the kfunc
   takes as ptr__ign.

Neither is registered yet: that waits for the patch that dispatches pads
at run time.

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
---
 kernel/bpf/helpers.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index a284f20c97d5..4eccd6742eba 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -3424,6 +3424,10 @@ static bool bpf_stack_walker(void *cookie, u64 ip, u64 sp, u64 bp)
 	return false;
 }
 
+__bpf_kfunc void bpf_unwind(void)
+{
+}
+
 __bpf_kfunc void bpf_throw(u64 cookie)
 {
 	struct bpf_throw_ctx ctx = {};
@@ -3445,6 +3449,16 @@ __bpf_kfunc void bpf_throw(u64 cookie)
 	WARN(1, "A call to BPF exception callback should never return\n");
 }
 
+__bpf_kfunc void bpf_unwind_resume(void *ptr__ign)
+{
+	/*
+	 * Never reached: the verifier accepts this kfunc only as a frame
+	 * terminator and bpf_do_misc_fixups() lowers every one of them to
+	 * 'r0 = 0; exit', so no call to this body survives to run.
+	 */
+	WARN_ONCE(1, "exception cleanup resume was not lowered to a return\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