Re: [PATCH v2 2/2] x86: Rewrite ret_from_fork() in C

Peter Zijlstra
2023-07-19T20:15:53Z

On Wed, Jul 19, 2023 at 10:02:22PM +0200, Peter Zijlstra wrote:
> On Wed, Jul 19, 2023 at 05:21:11PM +0200, Petr Mladek wrote:
>
> > This patch broke livepatching. Kthreads never have a reliable stack.
> > It works when I revert it.
>
> > > +SYM_CODE_START(ret_from_fork_asm)
> > > + UNWIND_HINT_REGS
>
> It works again when I change the above hint to UNWIND_HINT_END_OF_STACK,
> so yeah. Doing this makes objtool unhappy with something else though,
> so I'll go prod at things with something sharp...

The below cures things; Josh, did I miss anything?

---
arch/x86/entry/entry_64.S | 14 +++++++++++++-
1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 91f6818884fa..cfe7882ea9ae 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -285,7 +285,14 @@ SYM_FUNC_END(__switch_to_asm)
*/
.pushsection .text, "ax"
SYM_CODE_START(ret_from_fork_asm)
- UNWIND_HINT_REGS
+ /*
+ * This is the start of the kernel stack; even through there's a regs
+ * set at the top, there is no real exception frame and one cannot
+ * unwind further. This is the end.
+ *
+ * This ensures stack unwinds of kernel threads hit a known good state.
+ */
+ UNWIND_HINT_END_OF_STACK
ANNOTATE_NOENDBR // copy_thread
CALL_DEPTH_ACCOUNT

@@ -295,6 +302,11 @@ SYM_CODE_START(ret_from_fork_asm)
movq %r12, %rcx /* fn_arg */
call ret_from_fork

+ /*
+ * Set the stack state to what is expected for the target function
+ * -- also it is not wrong.
+ */
+ UNWIND_HINT_REGS
jmp swapgs_restore_regs_and_return_to_usermode
SYM_CODE_END(ret_from_fork_asm)
.popsection