Re: [PATCH] ptrace RSE bug

Shaohua Li
2007-10-18T01:54:01Z

On Wed, 2007-10-17 at 16:56 +0200, Petr Tesarik wrote:
> Shaohua Li wrote:
> > On Fri, 2007-09-07 at 09:11 -0600, David Mosberger-Tang wrote:
> >> Anything that avoids complicating the kernel exit path is worth doing!
> >> The exit path is complicated enough as it is.
> >>
> >> --david
> >>
> >> On 9/7/07, Petr Tesarik <ptesarik@suse.cz> wrote:
> >>> -----BEGIN PGP SIGNED MESSAGE-----
> >>> Hash: SHA1
> >>>
> >>> Shaohua Li wrote:
> >>>> On Thu, 2007-09-06 at 15:59 +0200, Petr Tesarik wrote:
> >>>>> [...]
> >>>>> So, what happens if upon syscall entry notification the debugger
> >>>>> modifies the part of the RBS (in user-space) which corresponds to the
> >>>>> arguments of that syscall? Currently, the syscall takes the modified
> >>>>> arguments, but with your change it would still take the stale data
> >>>>> from
> >>>>> the kernel RBS.
> >>>> The patch does sync from user RBS to kernel RBS just after syscall trace
> >>>> enter. this is an exception I said doing sync just before syscall
> >>>> return. I thought this covers your case, no?
> >>> Ah, I'm sorry, I missed that part of the patch. Well, if we have to do a
> >>> sync on every syscall_trace_enter() and syscall_trace_leave(), then the
> >>> only cases where introducing TIF_RESTORE_RSE saves us a duplicate sync
> >>> seems to be in the clone/fork and exit paths. In other words, it's
> >>> probably not worth the added complexity. But since you have written the
> >>> whole complex thing already, I have no objections against it.
> > Ok, this is a simplified patch. please review.
>
> Well, it's been quite some time, but here we go.
>
> I'm generally fine with this patch, but pleas note that it can't be
> included on its own:
>
> 1. There still is the race condition introduced by moving
> set_current_state(TASK_TRACED) after the spin_unlock_irq
I don't know the details, but Roland said if other parts are ok, he can help fix the issue.

> 2. You must couple it with the (planned) changes to the ptrace,
> because otherwise PTRACE_{PEEK,POKE}{TEXT,DATA} still access the kernel
> RBS, but it gets later overwritten back from userspace when it is synced.

> I have verified that failing to do so breaks "strace -f", because
> strace
> relies on intercepting the clone() system call and setting the
> CLONE_PTRACE bit in the flags argument. Of course, if the bit is only
> set in the kernel RBS, which is overwritten with the (old) value from
> the user RBS on a PTRACE_CONT, the new process is not traced.
The patch sync kernel RBS to user just before the task is suspended, so
I think we should be fine here. I did test 'strace -f', and test is ok.

Thanks,
Shaohua