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

Bernd Edlinger [off-list ref] writes:

On 3/9/20 6:40 PM, Eric W. Biederman wrote:

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Bernd Edlinger [off-list ref] writes:

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On 3/8/20 10:38 PM, Eric W. Biederman wrote:

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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

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... 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.

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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.

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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