lkml.org 
[lkml]   [2016]   [May]   [23]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH 4/4] x86: Pass kernel thread parameters in fork_frame
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

\
 
 \ /
  Last update: 2016-05-23 17:41    [W:0.148 / U:0.056 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site