linux-kernel - Re: [PATCH] ptrace: kill BKL in ptrace syscall

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Message-ID: <520f0cf11003290506o629c9ab6n95c94c48ba24acf4@mail.gmail.com>
Date:	Mon, 29 Mar 2010 14:06:03 +0200
From:	John Kacur <jkacur@...il.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Roland McGrath <roland@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Arnd Bergmann <arnd@...db.de>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] ptrace: kill BKL in ptrace syscall

On Mon, Mar 29, 2010 at 1:40 PM, Oleg Nesterov <oleg@...hat.com> wrote:
> On 03/28, Frederic Weisbecker wrote:
>>
>> From: Arnd Bergmann <arnd@...db.de>
>>
>> The comment suggests that this usage is stale. There is no bkl in the
>> exec path so if there is a race lurking there, the bkl in ptrace is
>> not going to help in this regard.
>
> I never understood this comment too, and I do not see any potentional
> races bkl could prevent.
>
>> Overview of the possibility of "accidental" races this bkl might
>> protect:
>>
>> - ptrace_traceme() is protected against task removal and concurrent
>> read/write on current->ptrace as it locks write tasklist_lock.
>>
>> - arch_ptrace_attach() is serialized by ptrace_traceme() against
>> concurrent PTRACE_TRACEME or PTRACE_ATTACH
>>
>> - ptrace_attach() is protected the same way ptrace_traceme() and
>> in turn serializes arch_ptrace_attach()
>>
>> - ptrace_check_attach() does its own well described serializing too.
>>
>> There is no obvious race here.
>
> Yes, nothing inside sys_ptrace() pathes relies on bkl, and all recent
> changes were done assuming that lock_kernel() doesn't exist.
>
> I think the patch is correct.
>
> Acked-by: Oleg Nesterov <oleg@...hat.com>
>
>> Signed-off-by: Arnd Bergmann <arnd@...db.de>
>> Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>
>> Cc: Andrew Morton <akpm@...ux-foundation.org>
>> ---
>>  kernel/ptrace.c |   10 ----------
>>  1 files changed, 0 insertions(+), 10 deletions(-)
>>
>> diff --git a/kernel/ptrace.c b/kernel/ptrace.c
>> index 42ad8ae..5357502 100644
>> --- a/kernel/ptrace.c
>> +++ b/kernel/ptrace.c
>> @@ -666,10 +666,6 @@ SYSCALL_DEFINE4(ptrace, long, request, long, pid, long, addr, long, data)
>>       struct task_struct *child;
>>       long ret;
>>
>> -     /*
>> -      * This lock_kernel fixes a subtle race with suid exec
>> -      */
>> -     lock_kernel();
>>       if (request == PTRACE_TRACEME) {
>>               ret = ptrace_traceme();
>>               if (!ret)
>> @@ -703,7 +699,6 @@ SYSCALL_DEFINE4(ptrace, long, request, long, pid, long, addr, long, data)
>>   out_put_task_struct:
>>       put_task_struct(child);
>>   out:
>> -     unlock_kernel();
>>       return ret;
>>  }
>>
>> @@ -813,10 +808,6 @@ asmlinkage long compat_sys_ptrace(compat_long_t request, compat_long_t pid,
>>       struct task_struct *child;
>>       long ret;
>>
>> -     /*
>> -      * This lock_kernel fixes a subtle race with suid exec
>> -      */
>> -     lock_kernel();
>>       if (request == PTRACE_TRACEME) {
>>               ret = ptrace_traceme();
>>               goto out;
>> @@ -846,7 +837,6 @@ asmlinkage long compat_sys_ptrace(compat_long_t request, compat_long_t pid,
>>   out_put_task_struct:
>>       put_task_struct(child);
>>   out:
>> -     unlock_kernel();
>>       return ret;
>>  }
>>  #endif       /* CONFIG_COMPAT */
>> --
>> 1.6.2.3
>>

So, just to be clear about this particular patch, who is queuing it up
to send to Linus? Would that be you Oleg, as a ptrace maintainer?

Thanks
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