Commit 2e7e5bbb authored by Peter Zijlstra's avatar Peter Zijlstra
Browse files

x86: Fix kthread unwind 



The rewrite of ret_from_form() misplaced an unwind hint which caused
all kthread stack unwinds to be marked unreliable, breaking
livepatching.

Restore the annotation and add a comment to explain the how and why of
things.

Fixes: 3aec4ecb ("x86: Rewrite ret_from_fork() in C")
Reported-by: default avatarPetr Mladek <pmladek@suse.com>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: default avatarPetr Mladek <pmladek@suse.com>
Link: https://lkml.kernel.org/r/20230719201538.GA3553016@hirez.programming.kicks-ass.net
parent fdf0eaf1

arch/x86/entry/entry_64.S

+15 −1
Original line number Diff line number Diff line
@@ -285,7 +285,15 @@ 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

	 * register set at the top, the regset isn't necessarily coherent

	 * (consider kthreads) and one cannot unwind further.

	 *

	 * This ensures stack unwinds of kernel threads terminate in a known

	 * good state.

	 */

	UNWIND_HINT_END_OF_STACK

	ANNOTATE_NOENDBR // copy_thread

	CALL_DEPTH_ACCOUNT
@@ -295,6 +303,12 @@ 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

	 * -- at this point the register set should be a valid user set

	 * and unwind should work normally.

	 */

	UNWIND_HINT_REGS

	jmp	swapgs_restore_regs_and_return_to_usermode

SYM_CODE_END(ret_from_fork_asm)

.popsection