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

Eric W. Biederman
2020-03-09T18:13:17Z

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.

Eric