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unaligned exception handling

To: linux-mips@linux-mips.org
Subject: unaligned exception handling
From: "Kip Walker" <kwalker@broadcom.com>
Date: Mon, 30 Sep 2002 09:23:35 -0700
Organization: Broadcom Corp. BPBU
Original-recipient: rfc822;linux-mips@linux-mips.org
Sender: linux-mips-bounce@linux-mips.org
After inspecting a strange case in the mips64 kernel with address
errors, I'm starting to think there's a problem in the do_ade()
implementation.  I think the 32-bit kernel may have a similar problem,
but I haven't really inspected it.  The issue is where the kernel's
emulation of an address error causes another address error (NOT a page
fault).

Basically, I don't see how the exception table stuff in
emulate_load_store_insn is going to work.  Consider this scenario:

- user process does a 'sw' (for example) to an illegal address
  above xuseg but below xsseg
- do_ade calls emulate_load_store_insn, which tries swl/swr
- the swl again hits an illegal address, this time in the
  kernel's context
- do_ade does NOT check the exception table for the swl
- emulate_load_store_insn goes to the 'swl' part of the switch
- die_if_kernel DOES __die before the SIGBUS is delivered.

So I don't see how the ex_table stuff is useful at all.  Shouldn't
do_ade() do the exception table grovelling before calling
emulate_load_store_insn?

Kip


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