[PATCH bpf-next v6 03/21] bpf: Add the bpf_unwind() and bpf_unwind_resume() kfuncs

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

An exception cleanup needs two terminators, and the kernel provides both as
kfuncs so that a BPF program can name them.

bpf_unwind() begins an unwind: the frames between it and whatever catches
run their landing pads on the way out. It is separate from bpf_throw(),
which keeps its own meaning -- leaving for the exception boundary with the
frames in between discarded.

bpf_unwind_resume() ends a landing pad, carrying the unwind on once that
frame's cleanups have run. The compiler names it _Unwind_Resume, the base
unwind ABI's entry point for the same thing; a later libbpf patch resolves
that name to this one. Every unwind ABI hands _Unwind_Resume the exception
object, and LLVM emits that argument on BPF too. Nothing in the kernel
needs it now, but the kfunc takes it as ptr__ign so the prototype matches
the call the compiler makes.

Both are defined here and neither is registered with any program type yet.
A call to either is only valid in a particular place -- an unwind where a
record covers it, a resume 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 | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index a284f20c97d5..08aee86a155c 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 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