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SubjectRe: ftrace introduces instability into kernel 2.6.27(-rc2,-rc3)

On Tue, 19 Aug 2008, Benjamin Herrenschmidt wrote:

> On Mon, 2008-08-18 at 14:27 -0400, Steven Rostedt wrote:
> >
> > On Mon, 18 Aug 2008, Scott Wood wrote:
> >
> > > Mathieu Desnoyers wrote:
> > > > asm volatile (
> > > > "1: lwz %1, 0(%2)\n"
> > > > " cmpw %1, %5\n"
> > > > " bne 2f\n"
> > > > " stwu %3, 0(%2)\n"
> > > > "2:\n"
> > > > ".section .fixup, \"ax\"\n"
> > > > "3: li %0, 1\n"
> > > > " b 2b\n"
> > > > ".previous\n"
> > > > ".section __ex_table,\"a\"\n"
> > > > _ASM_ALIGN "\n"
> > > > _ASM_PTR "1b, 3b\n"
> > > > ".previous"
> > > > : "=r"(faulted), "=r"(replaced)
> > > > : "r"(ip), "r"(new),
> > > > "0"(faulted), "r"(old)
> > > > : "memory");
> > >
> > > Some (most likely unrelated) nits in the above inline asm:
>
>
> Why not use __get_user/__put_user ?

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).

So are you recommending something like

int cmd;

if (__get_user(cmd, ip))
goto fault;

if (cmd != old)
goto not_same;

WARN_ON_ONCE(__put_user(cmd, ip));

If we did this, we could probably put this into the generic code:


if (copy_from_user(cmd, ip, ARCH_CALL_SIZE))
goto fault;

if (memcmp(cmd, old, ARCH_CALL_SIZE) != 0)
goto not_same;

WARN_ON_ONCE(copy_to_user(cmd, ip, ARCH_CALL_SIZE));


-- Steve



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