Re: [PATCH] ptrace RSE bug

Petr Tesarik
2007-10-18T10:59:02Z

Shaohua Li wrote:
> 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.

Maybe you're right. I was porting this to 2.6.16 for SUSE Linux
Enterprise Server 10, so my patch was a bit different. I'll retest with
latest git. Nevertheless, I still think that ia64_poke() can't do the
right thing here, because the changes made by PTRACE_PEEKDATA should
also be visible in /proc/<pid>/mem, for example.

Cheers,
Petr Tesarik