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SubjectRe: git pull request for tip/tracing/urgent
On Tue, Feb 10, 2009 at 06:00:14PM -0500, Steven Rostedt wrote:
>
>
> On Tue, 10 Feb 2009, Frederic Weisbecker wrote:
> > >
> > > diff --git a/arch/x86/kernel/ftrace.c b/arch/x86/kernel/ftrace.c
> > > index 1b43086..9d549e4 100644
> > > --- a/arch/x86/kernel/ftrace.c
> > > +++ b/arch/x86/kernel/ftrace.c
> > > @@ -491,13 +491,15 @@ void prepare_ftrace_return(unsigned long *parent, unsigned long self_addr)
> > > "1: " _ASM_MOV " (%[parent_old]), %[old]\n"
> > > "2: " _ASM_MOV " %[return_hooker], (%[parent_replaced])\n"
> > > " movl $0, %[faulted]\n"
> > > + "3:\n"
> > >
> > > ".section .fixup, \"ax\"\n"
> > > - "3: movl $1, %[faulted]\n"
> > > + "4: movl $1, %[faulted]\n"
> > > + " jmp 3b\n"
> > > ".previous\n"
> >
> >
> > It thought after the fixup section, the code would continue to rest of the C code.
> > Where would it go without the jmp?
>
> To the next item the linker placed into the .fixup section. And that
> would jump back to the location for that fixup. Basically, what you have
> is this:
>
> (just picking random and factitious registers)
>
> .section .text
> [...]
> L1: mov %a, %b
> L2: cmp %x, $1
> <continue code>
>
>
> <Someplace else>
>
> .section .text
> [...]
> L3: mov %c, %d
> L4: cmp %x, $22
> [...]
>
> .section .fixup
> [...]
> L5: mov $1, %x
> jmp L2
> L6: mov $22, %x
> jmp L4
> [...]
>
>
> .section __ex_table
> [...]
> .long L1, L5
> .long L3, L6
> [...]
>
>
> So when we take an exception at label L1, the page fault code will look
> to see if it is OK, by doing a binary search of the exception table.
> When it finds the L1, L5 pair, it will then set up a return to the L5
> label.
>
> When the fault returns to L5, it loads that reg %x with $1 and jumps back
> to L2, where it can see that it took a fault.
>
> Now lets look at what happens when we do not have that jump back to L2.
> Instead of going back to the original code, it will load $22 into %x and
> jmp back to the wrong area. God knows what will happen then, since the
> stack pointer thinks it is from where the original fault occurred.


Heh, that's fairly logic. Don't ask me why, but I did not imagine each
part of .fixup unified in a separate contiguous section (but what else can it be?...).

Thanks for your explanations :-)


> -- Steve
>



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