[PATCH RFC] bpf: Fix trampoline for functions with variable arguments

[Jiri Olsa <jolsa@xxxxxxxxxx>]

For functions with variable arguments like:

  void set_worker_desc(const char *fmt, ...)

the BTF data contains void argument at the end:

[4061] FUNC_PROTO '(anon)' ret_type_id=0 vlen=2
        'fmt' type_id=3
        '(anon)' type_id=0

When attaching function with this void argument the btf_distill_func_proto
will set last btf_func_model's argument with size 0 and that
will cause extra loop in save_regs/restore_regs functions and
generate trampoline code like:

  55             push   %rbp
  48 89 e5       mov    %rsp,%rbp
  48 83 ec 10    sub    $0x10,%rsp
  53             push   %rbx
  48 89 7d f0    mov    %rdi,-0x10(%rbp)
  75 f8          jne    0xffffffffa00cf007
                 ^^^ extra jump

It's causing soft lockups/crashes probably depends on what context
is the attached function called, like for set_worker_desc:

  watchdog: BUG: soft lockup - CPU#16 stuck for 22s! [kworker/u40:4:239]
  CPU: 16 PID: 239 Comm: kworker/u40:4 Not tainted 5.12.0-rc4qemu+ #178
  Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.14.0-1.fc33 04/01/2014
  Workqueue: writeback wb_workfn
  RIP: 0010:bpf_trampoline_6442464853_0+0xa/0x1000
  Code: Unable to access opcode bytes at RIP 0xffffffffa3597fe0.
  RSP: 0018:ffffc90000687da8 EFLAGS: 00000217
  Call Trace:
   set_worker_desc+0x5/0xb0
   wb_workfn+0x48/0x4d0
   ? psi_group_change+0x41/0x210
   ? __bpf_prog_exit+0x15/0x20
   ? bpf_trampoline_6442458903_0+0x3b/0x1000
   ? update_pasid+0x5/0x90
   ? __switch_to+0x187/0x450
   process_one_work+0x1e7/0x380
   worker_thread+0x50/0x3b0
   ? rescuer_thread+0x380/0x380
   kthread+0x11b/0x140
   ? __kthread_bind_mask+0x60/0x60
   ret_from_fork+0x22/0x30

This patch is removing the void argument from struct btf_func_model
in btf_distill_func_proto, but perhaps we should also check for this
in JIT's save_regs/restore_regs functions.

Signed-off-by: Jiri Olsa <jolsa@xxxxxxxxxx>
---
 kernel/bpf/btf.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
index b1a76fe046cb..017a80324139 100644
--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c
@@ -5133,6 +5133,11 @@ int btf_distill_func_proto(struct bpf_verifier_log *log,
 				tname, i, btf_kind_str[BTF_INFO_KIND(t->info)]);
 			return -EINVAL;
 		}
+		/* void at the end of args means '...' argument, skip it */
+		if (!ret && (i + 1 == nargs)) {
+			nargs--;
+			break;
+		}
 		m->arg_size[i] = ret;
 	}
 	m->nr_args = nargs;
-- 
2.30.2