On Sat, May 21, 2016 at 12:04:51PM -0400, Brian Gerst wrote: > --- a/arch/x86/entry/entry_64.S > +++ b/arch/x86/entry/entry_64.S > @@ -405,37 +405,29 @@ END(__switch_to_asm) > * A newly forked process directly context switches into this address. > * > * rax: prev task we switched from > + * rbx: kernel thread func > + * r12: kernel thread arg > */ > ENTRY(ret_from_fork) > movq %rax, %rdi > call schedule_tail /* rdi: 'prev' task parameter */ > > - testb $3, CS(%rsp) /* from kernel_thread? */ > + testq %rbx, %rbx /* from kernel_thread? */ > jnz 1f > > - /* > - * We came from kernel_thread. This code path is quite twisted, and > - * someone should clean it up. > - * > - * copy_thread_tls stashes the function pointer in RBX and the > - * parameter to be passed in RBP. The called function is permitted > - * to call do_execve and thereby jump to user mode. > - */ > - movq RBP(%rsp), %rdi > - call *RBX(%rsp) > - movq %rax, RAX(%rsp) > - > - /* > - * Fall through as though we're exiting a syscall. This makes a > - * twisted sort of sense if we just called do_execve. > - */ > - > -1: > +2: > movq %rsp, %rdi > call syscall_return_slowpath /* returns with IRQs disabled */ > TRACE_IRQS_ON /* user mode is traced as IRQS on */ > SWAPGS > jmp restore_regs_and_iret > + > +1: > + /* kernel thread */ > + movq %r12, %rdi > + call *%rbx > + movq %rax, RAX(%rsp) > + jmp 2b > END(ret_from_fork)
It seems really surprising that a kernel thread would be returning to user space. It would probably be a good idea to preserve the existing comments about that. -- Josh
