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

[Peter Zijlstra <peterz@infradead.org>]

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