kernel hangs when kprobe memcpy

[Tiezhu Yang <yangtiezhu@loongson.cn>]

Hi all,

(1) I have the following test environment, kernel hangs when kprobe memcpy:

system: x86_64 fedora 36
kernel version: Linux 5.7 (compile and update)
test case: modprobe kprobe_example symbol="memcpy" (CONFIG_SAMPLE_KPROBES=m)

In order to fix build errors, it needs to unset CONFIG_NFP and do the
following changes:
commit 52a9dab6d892 ("libsubcmd: Fix use-after-free for realloc(..., 0)")
commit de979c83574a ("x86/entry: Build thunk_$(BITS) only if 
CONFIG_PREEMPTION=y")

(2) Using the latest upstream mainline kernel, no hang problem due to the
commit e3a9e681adb7 ("x86/entry: Fixup bad_iret vs noinstr") to prohibit
probing memcpy which is put into the .noinstr.text section.

   # modprobe kprobe_example symbol="memcpy"
   modprobe: ERROR: could not insert 'kprobe_example': Invalid argument

In my opinion, according to the commit message, the above commit is not
intended to fix the memcpy hang problem, the problem was fixed by accident.

(3) If make handler_pre() and handler_post() as empty functions in the 5.7
kernel code, the above hang problem does not exist.

diff --git a/samples/kprobes/kprobe_example.c 
b/samples/kprobes/kprobe_example.c
index fd346f58ddba..c194171d8a46 100644
--- a/samples/kprobes/kprobe_example.c
+++ b/samples/kprobes/kprobe_example.c
@@ -28,8 +28,6 @@ static struct kprobe kp = {
  static int __kprobes handler_pre(struct kprobe *p, struct pt_regs *regs)
  {
  #ifdef CONFIG_X86
-	pr_info("<%s> p->addr = 0x%p, ip = %lx, flags = 0x%lx\n",
-		p->symbol_name, p->addr, regs->ip, regs->flags);
  #endif
  #ifdef CONFIG_PPC
  	pr_info("<%s> p->addr = 0x%p, nip = 0x%lx, msr = 0x%lx\n",
@@ -65,8 +63,6 @@ static void __kprobes handler_post(struct kprobe *p, 
struct pt_regs *regs,
  				unsigned long flags)
  {
  #ifdef CONFIG_X86
-	pr_info("<%s> p->addr = 0x%p, flags = 0x%lx\n",
-		p->symbol_name, p->addr, regs->flags);
  #endif
  #ifdef CONFIG_PPC
  	pr_info("<%s> p->addr = 0x%p, msr = 0x%lx\n",

I want to know what is the real reason of the hang problem when kprobe 
memcpy,
I guess it may be kprobe recursion, what do you think? Thank you.

By the way, kprobe memset has no problem whether or not handler_pre() and
handler_post() are empty functions.

Thanks,
Tiezhu