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Re: [PATCH] device-tree/kernel: Use memcpy in output_length()
- To: "Orzel, Michal" <Michal.Orzel@xxxxxxx>
- From: Bertrand Marquis <Bertrand.Marquis@xxxxxxx>
- Date: Wed, 23 Jul 2025 06:44:05 +0000
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- Cc: Jason Andryuk <jason.andryuk@xxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Julien Grall <julien@xxxxxxx>
- Delivery-date: Wed, 23 Jul 2025 06:44:53 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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- Thread-index: AQHb+tzM3z6ZCe/riEG373GipwMM9LQ9kneAgAGvWoCAAALFAA==
- Thread-topic: [PATCH] device-tree/kernel: Use memcpy in output_length()
Hi,
> On 23 Jul 2025, at 08:33, Orzel, Michal <Michal.Orzel@xxxxxxx> wrote:
>
>
>
> On 22/07/2025 06:50, Jason Andryuk wrote:
>> On 2025-07-22 03:46, Michal Orzel wrote:
>>> Trying to boot a compressed kernel with UBSAN enabled, results in the
>>> following warning:
>>> (XEN) UBSAN: Undefined behaviour in common/device-tree/kernel.c:21:12
>>> (XEN) load of misaligned address 00000a0040f89867 for type 'uint32_t'
>>> (XEN) which requires 4 byte alignment
>>> ...
>>> (XEN) [<00000a0000529964>] kernel_decompress+0x2bc/0x5bc
>>> (XEN) [<00000a000052a354>] kernel_probe+0x6f0/0x734
>>> (XEN) [<00000a0000528714>] dom0less-build.c#construct_domU+0x188/0x9d8
>>>
>>> If &image[image_len - 4] is not aligned to 4B boundary it causes
>>> unaligned access which is undefined behavior on Arm. Use memcpy instead
>>> to be safe.
>>>
>>> Fixes: c1be0b102e0e ("xen/arm: support gzip compressed kernels")
>>> Signed-off-by: Michal Orzel <michal.orzel@xxxxxxx>
>>> ---
>>> xen/common/device-tree/kernel.c | 6 +++++-
>>> 1 file changed, 5 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/xen/common/device-tree/kernel.c
>>> b/xen/common/device-tree/kernel.c
>>> index ef393182b691..28096121a52d 100644
>>> --- a/xen/common/device-tree/kernel.c
>>> +++ b/xen/common/device-tree/kernel.c
>>> @@ -18,7 +18,11 @@
>>>
>>> static uint32_t __init output_length(char *image, unsigned long image_len)
>>> {
>>> - return *(uint32_t *)&image[image_len - 4];
>>
>> Maybe just:
>> return get_unaligned_le32(&image[image_len - 4]);
>>
>> You'll also need:
>> #include <xen/unaligned.h>
>>
>> The gzip size is little endian,
> I thought about this solution but decided to use memcpy because one day we may
> want to support other compression algorithms and forcing LE could cause
> issues.
That makes sense so:
Acked-by: Bertrand Marquis <bertrand.marquis@xxxxxxx>
Cheers
Bertrand
>
> I'm ok either way. Let other maintainers decide.
>
> ~Michal
>
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