Re: [PATCH v2 5/5] exec: Add a exec_update_mutex to replace cred_guard_mutex

Eric W. Biederman
2020-03-09T18:26:54Z

ebiederm@xmission.com (Eric W. Biederman) writes:

> Bernd Edlinger <bernd.edlinger@hotmail.de> writes:
>
>> On 3/9/20 6:40 PM, Eric W. Biederman wrote:
>>> Bernd Edlinger <bernd.edlinger@hotmail.de> writes:
>>>
>>>> On 3/8/20 10:38 PM, Eric W. Biederman wrote:
>>>>>
>>>>> The cred_guard_mutex is problematic. The cred_guard_mutex is held
>>>>> over the userspace accesses as the arguments from userspace are read.
>>>>> The cred_guard_mutex is held of PTRACE_EVENT_EXIT as the the other
>>> ^ over
>>>>
>>>> ... is held while waiting for the trace parent to handle PTRACE_EVENT_EXIT
>>>> or something?
>>>
>>> Yes. Let me see if I can phrase that better.
>>>
>>>> I wonder if we also should mention that
>>>> it is held while waiting for the trace parent to
>>>> receive the exit code with "wait"?
>>>
>>> I don't think we have to spell out the details of how it all works,
>>> unless that makes things clearer. Kernel developers can be expected
>>> to figure out how the kernel works. The critical thing is that it is
>>> an indefinite wait for userspace to take action.
>>>
>>> But I will look.
>>>
>>>>> threads are killed. The cred_guard_mutex is held over
>>>>> "put_user(0, tsk->clear_child_tid)" in exit_mm().
>>>>>
>>>>> Any of those can result in deadlock, as the cred_guard_mutex is held
>>>>> over a possible indefinite userspace waits for userspace.
>>>>>
>>>>> Add exec_update_mutex that is only held over exec updating process
>>>>
>>>> Add ?
>>>
>>> Yes. That is what the change does: add exec_update_mutex.
>>>
>>
>> I just kind of missed the "subject" in this sentence,
>> like "This patch adds an exec_update_mutex that is ..."
>> but english is a foreign language for me, so may be okay as is.
>
> English has a lot of options. I think this is a stylistic difference.
>
> Instead of being an observer and describing what the change does:
> "This patch adds exec_update_mutex ..."
>
> I was being there in the moment and saying/commading what is happening:
> "Add exec_update_mutex ..."
>
> Using the more immdediate form ends up with more concise and clearer
> sentences.
>
> Every one of my writing teachers in school emphasized that point
> and I see the who it works when I write things. But writing is hard and
> I still tend toward long rambling sentences with many qualifiers that
> confuse and detract from the point rather than make it clear what is
> happening.

And reading through it all now I can see your confusion. That
description of my changes was not well done. Reworking it now.

Eric