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Re: [Qemu-devel] [PATCH v2 18/19] linux-user: Avoid possible misalignmen

From: Peter Maydell
Subject: Re: [Qemu-devel] [PATCH v2 18/19] linux-user: Avoid possible misalignment in host_to_target_siginfo()
Date: Wed, 8 Jun 2016 12:29:51 +0100

On 8 June 2016 at 10:29, Laurent Vivier <address@hidden> wrote:
> Le 07/06/2016 à 23:08, Peter Maydell a écrit :
>> On 7 June 2016 at 20:36, Laurent Vivier <address@hidden> wrote:
>>> Le 27/05/2016 à 16:52, Peter Maydell a écrit :
>>>> host_to_target_siginfo() is implemented by a combination of
>>>> host_to_target_siginfo_noswap() followed by tswap_siginfo().
>>>> The first of these two functions assumes that the target_siginfo_t
>>>> it is writing to is correctly aligned, but the pointer passed
>>>> into host_to_target_siginfo() is directly from the guest and
>>>> might be misaligned. Use a local variable to avoid this problem.
>>>> (tswap_siginfo() does now correctly handle a misaligned destination.)
>>> You mean the pointer from the guest can not be correctly aligned for the
>>> guest?
>> Might not be correctly aligned for the host (for that matter
>> it might not be correctly aligned for the guest,
>> if the guest is being malicious or buggy, but it's the
>> host alignment we care about.)
> Because of the "abi_ulong _addr", I think this structure is always
> aligned for the guest.

No, because the address of the structure in guest memory comes from
a pointer passed to us by the guest. The guest could pass any value
at all that it likes for that pointer including one that is arbitrarily
misaligned. I think it's pretty much always a bug to do a direct
access to a structure field that's in guest memory, though we do
it a fair bit and get away with it because x86-64 doesn't have
alignment restrictions.

-- PMM

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