From: Nadav Amit <namit@vmware.com>
To: Sean Christopherson <sean.j.christopherson@intel.com>
Cc: Masami Hiramatsu <mhiramat@kernel.org>,
Thomas Gleixner <tglx@linutronix.de>,
LKML <linux-kernel@vger.kernel.org>,
Ingo Molnar <mingo@redhat.com>, X86 ML <x86@kernel.org>,
Arnd Bergmann <arnd@arndb.de>,
linux-arch <linux-arch@vger.kernel.org>,
Andy Lutomirski <luto@kernel.org>,
Kees Cook <keescook@chromium.org>
Subject: Re: [RFC PATCH 1/6] x86/alternative: assert text_mutex is taken
Date: Wed, 29 Aug 2018 22:56:23 +0000 [thread overview]
Message-ID: <DFC5DCBA-A572-4603-82C8-77A6C1303630@vmware.com> (raw)
In-Reply-To: <20180829210006.GA7166@linux.intel.com>
at 2:00 PM, Sean Christopherson <sean.j.christopherson@intel.com> wrote:
> On Wed, Aug 29, 2018 at 08:44:47PM +0000, Nadav Amit wrote:
>> at 1:13 PM, Sean Christopherson <sean.j.christopherson@intel.com> wrote:
>>
>>> On Wed, Aug 29, 2018 at 07:36:22PM +0000, Nadav Amit wrote:
>>>> at 10:11 AM, Nadav Amit <namit@vmware.com> wrote:
>>>>
>>>>> at 1:59 AM, Masami Hiramatsu <mhiramat@kernel.org> wrote:
>>>>>
>>>>>> On Wed, 29 Aug 2018 01:11:42 -0700
>>>>>> Nadav Amit <namit@vmware.com> wrote:
>>>>>>
>>>>>>> Use lockdep to ensure that text_mutex is taken when text_poke() is
>>>>>>> called.
>>>>>>>
>>>>>>> Actually it is not always taken, specifically when it is called by kgdb,
>>>>>>> so take the lock in these cases.
>>>>>>
>>>>>> Can we really take a mutex in kgdb context?
>>>>>>
>>>>>> kgdb_arch_remove_breakpoint
>>>>>> <- dbg_deactivate_sw_breakpoints
>>>>>> <- kgdb_reenter_check
>>>>>> <- kgdb_handle_exception
>>>>>> <- __kgdb_notify
>>>>>> <- kgdb_ll_trap
>>>>>> <- do_int3
>>>>>> <- kgdb_notify
>>>>>> <- die notifier
>>>>>>
>>>>>> kgdb_arch_set_breakpoint
>>>>>> <- dbg_activate_sw_breakpoints
>>>>>> <- kgdb_reenter_check
>>>>>> <- kgdb_handle_exception
>>>>>> ...
>>>>>>
>>>>>> Both seems called in exception context, so we can not take a mutex lock.
>>>>>> I think kgdb needs a special path.
>>>>>
>>>>> You are correct, but I don’t want a special path. Presumably text_mutex is
>>>>> guaranteed not to be taken according to the code.
>>>>>
>>>>> So I guess the only concern is lockdep. Do you see any problem if I change
>>>>> mutex_lock() into mutex_trylock()? It should always succeed, and I can add a
>>>>> warning and a failure path if it fails for some reason.
>>>>
>>>> Err.. This will not work. I think I will drop this patch, since I cannot
>>>> find a proper yet simple assertion. Creating special path just for the
>>>> assertion seems wrong.
>>>
>>> It's probably worth expanding the comment for text_poke() to call out
>>> the kgdb case and reference kgdb_arch_{set,remove}_breakpoint(), whose
>>> code and comments make it explicitly clear why its safe for them to
>>> call text_poke() without acquiring the lock. Might prevent someone
>>> from going down this path again in the future.
>>
>> I thought that the whole point of the patch was to avoid comments, and
>> instead enforce the right behavior. I don’t understand well enough kgdb
>> code, so I cannot attest it does the right thing. What happens if
>> kgdb_do_roundup==0?
>
> As is, the comment is wrong because there are obviously cases where
> text_poke() is called without text_mutex being held. I can't attest
> to the kgdb code either. My thought was to document the exception so
> that if someone does want to try and enforce the right behavior they
> can dive right into the problem instead of having to learn of the kgdb
> gotcha the hard way. Maybe a FIXME is the right approach?
Ok. I’ll add a FIXME comment as you propose, but this does not deserve a
separate patch. I’ll squash it into patch 5.
next prev parent reply other threads:[~2018-08-29 22:56 UTC|newest]
Thread overview: 34+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-08-29 8:11 [RFC PATCH 0/6] x86: text_poke() fixes Nadav Amit
2018-08-29 8:11 ` [RFC PATCH 1/6] x86/alternative: assert text_mutex is taken Nadav Amit
2018-08-29 8:59 ` Masami Hiramatsu
2018-08-29 17:11 ` Nadav Amit
2018-08-29 19:36 ` Nadav Amit
2018-08-29 20:13 ` Sean Christopherson
2018-08-29 20:44 ` Nadav Amit
2018-08-29 21:00 ` Sean Christopherson
2018-08-29 22:56 ` Nadav Amit [this message]
2018-08-30 2:26 ` Masami Hiramatsu
2018-08-30 5:23 ` Nadav Amit
2018-08-29 8:11 ` [RFC PATCH 2/6] x86/mm: temporary mm struct Nadav Amit
2018-08-29 9:49 ` Masami Hiramatsu
2018-08-29 15:41 ` Andy Lutomirski
2018-08-29 16:54 ` Nadav Amit
2018-08-29 21:38 ` Andy Lutomirski
2018-08-30 1:38 ` Masami Hiramatsu
2018-08-30 1:59 ` Andy Lutomirski
2018-08-31 4:42 ` Masami Hiramatsu
2018-08-29 15:46 ` Andy Lutomirski
2018-08-29 8:11 ` [RFC PATCH 3/6] fork: provide a function for copying init_mm Nadav Amit
2018-08-29 9:54 ` Masami Hiramatsu
2018-08-29 8:11 ` [RFC PATCH 4/6] x86/alternatives: initializing temporary mm for patching Nadav Amit
2018-08-29 13:21 ` Masami Hiramatsu
2018-08-29 17:45 ` Nadav Amit
2018-08-29 8:11 ` [RFC PATCH 5/6] x86/alternatives: use temporary mm for text poking Nadav Amit
2018-08-29 9:28 ` Peter Zijlstra
2018-08-29 15:46 ` Andy Lutomirski
2018-08-29 16:14 ` Peter Zijlstra
2018-08-29 16:32 ` Andy Lutomirski
2018-08-29 16:37 ` Dave Hansen
2018-08-29 8:11 ` [RFC PATCH 6/6] x86/alternatives: remove text_poke() return value Nadav Amit
2018-08-29 9:52 ` Masami Hiramatsu
2018-08-29 17:15 ` Nadav Amit
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