linux-kernel - Re: [PATCH 2/2] coredump: exit_mm: clear ->mm first, then play with ->core

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Date:	Sat, 19 Jul 2008 19:32:58 -0700 (PDT)
From:	Roland McGrath <roland@...hat.com>
To:	Oleg Nesterov <oleg@...sign.ru>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	David Howells <dhowells@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] coredump: exit_mm: clear ->mm first, then play with ->core_state

> With the previous changes the sub-threads which participate in coredump do
> not need to have the valid ->mm when the coredump is in progress, now we
> can decouple exit_mm() from coredumping code.

I'm all for separating the code more cleanly.  But I don't think it can
work to change the order of the operations, i.e. it is not really true that
core dumps don't need each thread's ->mm link to be valid.  Is there a
benefit to unlinking the mm before waiting for the core dump to finish?

The issue is that the user_regset calls to get "thread state" might
actually read some user memory.  Those calls use a task_struct pointer and
you don't get to separately tell them the mm_struct describing the thread's
address space.  For example, the sparc64 "general registers" note for core
files includes the register window read from user memory.

So, it's not OK to clear the ->mm before everything examining the thread's
machine state is really done, i.e. core dump and anything else.

Thanks,
Roland
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