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Re: [Xen-devel] [PATCH 1/2] xen/ubsan: Don't perform alignment checking on supporting compilers
- To: Julien Grall <julien.grall@xxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Mon, 24 Jun 2019 12:04:34 +0100
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- Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Jan Beulich <JBeulich@xxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>
- Delivery-date: Mon, 24 Jun 2019 11:04:50 +0000
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- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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On 24/06/2019 11:33, Julien Grall wrote:
> Hi Andrew,
>
> On 6/24/19 11:17 AM, Andrew Cooper wrote:
>> GCC 5 introduced -fsanitize=alignment which is enabled by default by
>> CONFIG_UBSAN. This trips a load of wont-fix cases in the ACPI tables
>> and the
>> hypercall page and stubs writing logic.
>>
>> It also causes the native Xen boot to crash before the console is set
>> up, for
>> an as-yet unidentified reason (most likley a wont-fix case earlier on
>> boot).
>>
>> Disable alignment sanitisation on compilers which would try using it.
>>
>> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>> ---
>> CC: Jan Beulich <JBeulich@xxxxxxxx>
>> CC: Wei Liu <wl@xxxxxxx>
>> CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
>> CC: Stefano Stabellini <sstabellini@xxxxxxxxxx>
>> CC: Julien Grall <julien.grall@xxxxxxx>
>>
>> This isn't ideal, but we can't do better without a bit of an overhaul
>> which I
>> don't have time for now. Linux uses a whitelist of sanitisers but
>> I'm not
>> entirely sure we want to go that route. ARM currently isn't working
>> well with
>> UBSAN, but AFACIT, all ARM platforms that we support also disable
>> alignment
>> sanitisation in Linux.
>
> Linux has an option to disable/enable aligment sanitisation. However,
> IIRC, Linux allows unaligned access for both Arm32 and Arm64.
>
> For Xen:
> - On Arm32, alignment check is enabled, so any unaligned access will
> result to a crash.
> - On Arm64, alignment check is disabled, the only reason is because
> memcpy is using unaligned access (for performance reason). But we
> should still not rely on unaligned access as they are not atomic.
>
> The only limitation for using UBSAN on Xen on Arm today is the size of
> the binary (we only support up to 2MB). So my preference here would be
> to make the new flag x86 only.
Ok - that shouldn't be too difficult to arrange.
>
> Ideally longer plan would be to make a per-file decision on the
> sanitization to use.
ARM64's memcpy is written in assembly so not subject to UBSAN.
For GCC 8 and later, there is a new __attribute__((no_sanitize("foo",
"bar"))) to selectively disable specific checkers on a per-function basis.
~Andrew
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