unaligned exception handling

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

You are right there is a potential problem in the 64-bit kernel, when emulating load/store instructions.
I have previously addressed this problem (and also send a patch).

The problem is the check_axs macro, that checks the access before actually doing the emulation (any load/stores).
It's a simple copy from the 32-bit kernel, where you do a check for user space by simple check the most significant bit.
That's fine in the 32-bit world, but it's not sufficient in the 64-bit case.

Locally I have a check_axs that look like this:

/*
* User code may only access USEG;
* Kernel code may access the entire address space, except the area
between
* USEG (XUSEG) and KSEG0.
*/

#define check_axs(pc,a,s)      \
        if (((pc < KUSIZE) && (((a) | ((a)+(s))) >= KUSIZE)) ||  \
     ((((a) | ((a)+(s))) < K0BASE) &&    \
      (((a) | ((a)+(s))) >= KUSIZE)))    \

goto sigbus;

Hope that helps you.
/Carsten

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