Messages in this thread | | | Date | Mon, 20 Jun 2016 15:51:58 +0200 | From | Borislav Petkov <> | Subject | Re: [PATCH v2 5/6] x86: Pass kernel thread parameters in fork_frame |
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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(-)
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> 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.
SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nürnberg) --
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