Re: 'khelper' (child) is stuck in endless loop: do_signal() and!user_mode(regs)

From: Dmitry ADAMUSHKA (EXT)
Date: Mon Mar 12 2012 - 12:35:46 EST



just wondering, whether this problem/scenario considered to be completely unrealistic or I just should resend the x86 patch alone to get more attention? :-)

--Dmitry

----- Original Message -----
> From: "Dmitry ADAMUSHKA (EXT)" <dmitry.adamushka_ext@xxxxxxxxxxxxxx>
> To: "Oleg Nesterov" <oleg@xxxxxxxxxx>
> Cc: "Ingo Molnar" <mingo@xxxxxxx>, "Ralf Baechle" <ralf@xxxxxxxxxxxxxx>, "wouter cloetens"
> <wouter.cloetens@xxxxxxxxxxxxxx>, linux-kernel@xxxxxxxxxxxxxxx, "Dmitry Adamushko" <dmitry.adamushko@xxxxxxxxx>, "H.
> Peter Anvin" <hpa@xxxxxxxxx>
> Sent: Thursday, March 8, 2012 5:58:29 PM
> Subject: Re: 'khelper' (child) is stuck in endless loop: do_signal() and !user_mode(regs)

> > On 03/08, Dmitry ADAMUSHKA (EXT) wrote:
> > >
> > > The following quick hack "fixes" it for x86.
> >
> > First of all let me repeat, I do not understand this asm ;)
> > Fortunately Ingo and Peter do.
> >
> > But,
> >
> > > --- arch/x86/kernel/entry_32.S.orig 2012-03-08 15:42:25.041296595
> > > +0100 +++ arch/x86/kernel/entry_32.S 2012-03-08 15:58:29.926081131
> > > +0100 @@ -98,12 +98,6 @@
> > > #endif .endm
> > >
> > > -#ifdef CONFIG_VM86
> > > -#define resume_userspace_sig check_userspace
> > > -#else -#define resume_userspace_sig resume_userspace
> > > -#endif -
> > > /*
> > > * User gs save/restore
> > > *
> > > @@ -327,10 +321,19 @@ ret_from_exception:
> > > preempt_stop(CLBR_ANY)
> > > ret_from_intr:
> > > GET_THREAD_INFO(%ebp)
> > > -check_userspace: +resume_userspace_sig: +#ifdef CONFIG_VM86
> > > movl PT_EFLAGS(%esp), %eax # mix EFLAGS and CS
> > > movb PT_CS(%esp), %al
> > > andl $(X86_EFLAGS_VM | SEGMENT_RPL_MASK), %eax
> > > +#else +/*
> > > + * We can be coming here from a syscall done in the kernel space,
> > > + * e.g. a failed kernel_execve().
> > > + */
> > > + movl PT_CS(%esp), %eax
> > > + andl $SEGMENT_RPL_MASK, %eax
> > > +#endif
> > > cmpl $USER_RPL, %eax
> > > jb resume_kernel # not returning to v8086 or userspace
> >
> > IIUC (I can be easily wrong) this breaks the endless loop, but
> > only after do_notify_resume() was already called.
>
> yeah, basically I'm simulating the approach of CONFIG_VM86 here, i.e.
> doing the check at the same place in the call chain. Well, and giving
> a possibility for that "if (!user_mode(regs))" code in do_signal() to
> execute :-))
>
> btw., what are the legitimate cases/code-paths for this part of
> do_signal()? I see that other archs just copy-past this approach. My
> initial thought was that it has something to do with handling (or
> rather preserving) some sort of signals that get delivered to
> not-yet-fully-created user-space tasks.. so that they get handled when
> these new tasks are up-and-running.
>
> >
> > _perhaps_ it would be better to avoid do_notify_resume() in this
> > case altogether. Say, fire_user_return_notifiers() doesn't look
> > right in this case, we are not going to return to the usermode.
> >
>
> yeah, there are some other corner cases I'm not sure about (like with
> syscall tracing). Also, there can be other scenarios of entering this
> loop... so one way or another, the loop should be broken.
>
> --Dmitry
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