From mboxrd@z Thu Jan  1 00:00:00 1970
From: sironhide0null@gmail.com (zerons)
Date: Wed, 21 Dec 2016 19:35:57 +0800
Subject: A question about kprobe/kretprobe and kmalloc/kzalloc
Message-ID: <6e25ab27-c4e9-a452-1107-5cd9640e3a2d@gmail.com>
To: kernelnewbies@lists.kernelnewbies.org
List-Id: kernelnewbies.lists.kernelnewbies.org

Hi everyone.

I wrote a kernel module to test something. The module
uses kprobe and kretprobe, here is a bug I met today.

The pre_handler of kprobe, calls `do_something`. The probed
instructions are in the middle of a function.
The entry_handler of kretprobe, also calls `do_something`.
`do_something` calls `kmalloc`+`memset`.

Back to userspace, when I have all the functions probed,
then the test program cause a high CPU usage, and the
keyboard doesn't work. The system does not panic when
I set softlockup_panic=1.

If `do_something` is called by entry_handler of kretprobe,
the module works fine.
The bug happens when `do_something` called by the pre_handler
of kprobe.

So I use "#if 0" to locate the bug. It turns out to
be `kmalloc`+`memset`. When I change that to `kzalloc`,
problem solved.

Then I get confused.
`kzalloc` just calls `kmalloc` with a `__GFP_ZERO`.
Why the bug only happens when pre_handler of kprobe gets called?

Is it necessary to post the source code here? Thanks.