[PATCH v3 0/2] x86/kprobes: Fixes for CONFIG_RETHUNK

Masami Hiramatsu (Google)
2022-09-08T15:01:22Z

Hi Peter and Josh,

So here is 3rd version of the patches to fix kprobes and optprobe with
CONFIG_RETHUNK and CONFIG_SLS.
Previous version is here;

https://lore.kernel.org/all/166260087224.759381.4170102827490658262.stgit@devnote2/

In this version, I simplified all code and just checks the INT3 comes
from kgdb or not. Other INT3 are treated as one-byte instruction.

With CONFIG_RETHUNK/CONFIG_SLS, the kernel functions may includes INT3
for stopping speculative execution in the function code block (body) in
addition to the gaps between functions.

Since kprobes on x86 has to ensure the probe address is an instruction
bondary, it decodes the instructions in the function until the address.
If it finds an INT3 which is not embedded by kprobe, it stops decoding
because usually the INT3 is used for debugging as a software breakpoint
and such INT3 will replace the first byte of an original instruction.
Without recovering it, kprobes can not continue to decode it. Thus the
kprobes returns -EILSEQ as below.

# echo "p:probe/vfs_truncate_L19 vfs_truncate+98" >> kprobe_events
sh: write error: Invalid or incomplete multibyte or wide character

Actually, such INT3 can be ignored except the INT3 installed by kgdb.

To avoid this issue, just check whether the INT3 is owned by kgdb
or not and if so, it just stopped and return failure.

With thses fixes, kprobe and optprobe can probe the kernel again with
CONFIG_RETHUNK=y.

# echo "p:probe/vfs_truncate_L19 vfs_truncate+98" >> kprobe_events
# echo 1 > events/probe/vfs_truncate_L19/enable
# cat /sys/kernel/debug/kprobes/list
ffffffff81307b52 k vfs_truncate+0x62 [OPTIMIZED]

Thank you,

---

Masami Hiramatsu (Google) (2):
x86/kprobes: Fix kprobes instruction boudary check with CONFIG_RETHUNK
x86/kprobes: Fix optprobe optimization check with CONFIG_RETHUNK

arch/x86/kernel/kprobes/core.c | 10 +++++++---
arch/x86/kernel/kprobes/opt.c | 28 ++++++++--------------------
2 files changed, 15 insertions(+), 23 deletions(-)

--
Masami Hiramatsu (Google) <mhiramat@kernel.org>