Re: [PATCH] device-tree/kernel: Use memcpy in output_length()

[Jason Andryuk <jason.andryuk@amd.com>]

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@amd.com>
> ---
>   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, 
https://datatracker.ietf.org/doc/html/rfc1952:
       Within a computer, a number may occupy multiple bytes.  All
       multi-byte numbers in the format described here are stored with
       the least-significant byte first (at the lower memory address).

Regards,
Jason

> +    uint32_t val;
> +
> +    memcpy(&val, &image[image_len - 4], sizeof(val));
> +
> +    return val;
>   }
>   
>   int __init kernel_decompress(struct boot_module *mod, uint32_t offset)