linux-kernel - Re: [SPARC32] NULL pointer derefference

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Message-Id: <20070730.234252.74747206.davem@davemloft.net>
Date:	Mon, 30 Jul 2007 23:42:52 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	mark@...hpc.demon.co.uk
Cc:	aaw@...gle.com, akpm@...ux-foundation.org,
	linux-arch@...r.kernel.org, sparclinux@...r.kernel.org,
	wli@...omorphy.com, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [SPARC32] NULL pointer derefference

From: Mark Fortescue <mark@...hpc.demon.co.uk>
Date: Tue, 31 Jul 2007 06:35:29 +0100 (BST)

> The original code did a job lot of pte stuf in install_arg_page. The
> new code seems to replace this using get_user_pages but I have not
> worked out how get_user_pages gets to the point at which it
> allocated pte's i.e.  maps the stack memory it is about to put the
> arguments into.

get_user_pages() essentially walks through the requested user address
space, faults in pages if necessary, and returns references to those
pages.

The logic of get_user_pages() you need to be concerned about is
this inner loop:

			while (!(page = follow_page(vma, start, foll_flags))) {
				int ret;
				ret = handle_mm_fault(mm, vma, start,
						foll_flags & FOLL_WRITE);
 ...
			}

handle_mm_fault() does all the dirty work of a page fault, and
is how we get to update_mmu_cache(), the sun4c implementation of
which is where you see the crash.
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