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SubjectRe: ftrace introduces instability into kernel 2.6.27(-rc2,-rc3)
* Steven Rostedt (rostedt@goodmis.org) wrote:
>
>
> On Mon, 18 Aug 2008, Mathieu Desnoyers wrote:
>
> > * Steven Rostedt (rostedt@goodmis.org) wrote:
> > >
> > > On Tue, 19 Aug 2008, Benjamin Herrenschmidt wrote:
> > >
> > > >
> > > > > Hmm, this was originally copied from x86, where we did a cmpxchg, but that
> > > > > is probably not needed since all of this is done in kstop_machine. Also,
> > > > > only the "get" is needed. If we don't fault there, we wont fault on the
> > > > > put (unless we have permissions wrong, and that would be a bug).
> > > >
> > > > Would it ? How do we make sure the kernel text is mapped writeable ?
> > >
> > > We map it writeable if FTRACE is enabled.
> > >
> >
> > Argh. See text_poke(). It's there exactly for this purpose on x86.
> >
>
> OK, I just tried text_poke and it unfortunately fails. The problem is that
> it requires that the text you are changing is aligned and fits on one
> page. We have no control over that.
>
> -- Steve
>

Ok, there are two cases where it's ok :

1 - in stop_machine, considering we are not touching code executed in
NMI handlers.
2 - when using my replace_instruction_safe() which uses a temporary
breakpoint when doing the instruction replacement.

In those cases you could use text_poke_early().

See
http://git.kernel.org/?p=linux/kernel/git/compudj/linux-2.6-lttng.git;a=blob;f=arch/x86/kernel/immediate.c;h=7789e2c75bf03e645f15759d5dff0c1698493f92;hb=HEAD

For a use example. Basically it looks like :


360 pages[0] = virt_to_page((void *)bypass_eip);
361 vaddr = vmap(pages, 1, VM_MAP, PAGE_KERNEL);
362 BUG_ON(!vaddr);
363 text_poke_early(&vaddr[bypass_eip & ~PAGE_MASK],
364 (void *)addr, size);
365 /*
366 * Fill the rest with nops.
367 */
368 len = NR_NOPS - size;
369 add_nops((void *)
370 &vaddr[(bypass_eip & ~PAGE_MASK) + size],
371 len);
372 print_dbg_bytes("inserted nops",
373 &vaddr[(bypass_eip & ~PAGE_MASK) + size], len);
374 vunmap(vaddr);

Mathieu


--
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68


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