Re: [PATCH v2 5/6] x86: Pass kernel thread parameters in fork_frame

Borislav Petkov
2016-06-20T13:53:00Z

On Sat, Jun 18, 2016 at 04:56:17PM -0400, Brian Gerst wrote:
> Instead of setting up a fake pt_regs context, put the kernel thread
> function pointer and arg into the unused callee-restored registers
> of struct fork_frame.
>
> Signed-off-by: Brian Gerst <brgerst@gmail.com>

> ---
> arch/x86/entry/entry_32.S | 31 +++++++++++++++----------------
> arch/x86/entry/entry_64.S | 35 ++++++++++++++++-------------------
> arch/x86/kernel/process_32.c | 14 ++++----------
> arch/x86/kernel/process_64.c | 10 +++-------
> 4 files changed, 38 insertions(+), 52 deletions(-)

...

> diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
> index 7574528..23e764c 100644
> --- a/arch/x86/entry/entry_64.S
> +++ b/arch/x86/entry/entry_64.S
> @@ -404,37 +404,34 @@ 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)
> - movl $0, 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
> + /*
> + * A kernel thread is allowed to return here after successfully
> + * calling do_execve(). Exit to userspace to complete the execve()
> + * syscall.
> + */
> + movq $0, RAX(%rsp)
> + jmp 2b
> END(ret_from_fork)

Wouldn't it be simpler to reverse the JNZ after the TESTB above and thus
have a single label and simpler code flow (pasting the whole thing here
because a diff is unreadable):

ENTRY(ret_from_fork)
movq %rax, %rdi
call schedule_tail /* rdi: 'prev' task parameter */

testq %rbx, %rbx /* from kernel_thread? */
jz 1f

/* kernel thread */
movq %r12, %rdi
call *%rbx
/*
* A kernel thread is allowed to return here after successfully
* calling do_execve(). Exit to userspace to complete the execve()
* syscall.
*/
movq $0, RAX(%rsp)

1:
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
END(ret_from_fork)

It boots fine in my guest this way too.

Ditto for 32-bit.

--
Regards/Gruss,
Boris.

ECO tip #101: Trim your mails when you reply.

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