Re: [PATCH] ptrace RSE bug

Shaohua Li
2007-09-07T01:02:44Z

On Thu, 2007-09-06 at 15:59 +0200, Petr Tesarik wrote:
> On Thu, 2007-09-06 at 11:16 +0800, Shaohua Li wrote:
> > On Wed, 2007-09-05 at 18:25 +0200, Petr Tesarik wrote:
> > > Shaohua Li wrote:
> > > > This is base kernel patch for ptrace RSE bug. It's basically a backport
> > > > from the utrace RSE patch I sent out several weeks ago. please review.
> > > >
> > > > when a thread is stopped (ptraced), debugger might change thread's user
> > > > stack (change memory directly), and we must avoid the RSE stored in
> > > > kernel to override user stack (user space's RSE is newer than kernel's
> > > > in the case). To workaround the issue, we copy kernel RSE to user RSE
> > > > before the task is stopped, so user RSE has updated data. we then copy
> > > > user RSE to kernel after the task is resummed from traced stop and
> > > > kernel will use the newer RSE to return to user.
> > >
> > > Hi Shaohua,
> > >
> > > somehow I fail to see the need for a dedicated bit in the task flags.
> > > The user RBS can only be more up-to-date than the kernel RBS if the
> > > values there have been modified by a debugger. So, it should be enough
> > > to copy the relevant part of the user RBS back into the kernel RBS when
> > > the ptraced process is resumed, which happens AFAIK only:
> > >
> > > 1. when the debugger lets it continue via ptrace(), or
> > > 2. when the debugger ceases to exist.
> > >
> > > Yes, we'd most likely have to add an arch_ptrace_resume() in addition to
> > > arch_ptrace_stop(), but there are some advantages, too:
> > >
> > > 1. It would save one bit in the task flags.
> > > 2. The usual path for task switches is not modified
> > > (OK, the few instructions needed to check for TIF_RESTORE_RSE are
> > > negligible)
> > >
> > > Or am I totally missing what you're trying to do?
> > No, you didn't. Ideaily the should do sync user RBS to kernel just after
> > ptraced process is resumed. In previous discussion, somebody thought
> > this might be too agressive, as ptraced task might do suspend/resume
> > several times in a single syscall (syscall trace, fork trace) and we
> > supposed debugger will only change user RBS just after syscall return.
> > so the flag is used to try to do less sync. If the assumption is not
> > true, we should always do sync just after ptraced task is resumed.
>
> Well, you're right, but are you sure it's really what you want? I did
> some testing and made sure that syscall arguments are stored just below
> ar.bsp as seen by ptrace().
>
> 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?

Thanks,
Shaohua