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
