Re: [PATCH v1] xz: fix arm fdt compile error for kmalloc replacement

[Kees Cook]

On Sun, Feb 22, 2026 at 08:11:00PM +0800, Haiyue Wang wrote:
> Align to the commit bf4afc53b77a ("Convert 'alloc_obj' family to use the new default GFP_KERNEL argument")
> update the 'kmalloc_obj' declaration for userspace to fix below comiple error:
> 
> In file included from arch/arm/boot/compressed/../../../../lib/decompress_unxz.c:241,
>                  from arch/arm/boot/compressed/decompress.c:56:
> arch/arm/boot/compressed/../../../../lib/xz/xz_dec_stream.c: In function 'xz_dec_init':
> arch/arm/boot/compressed/../../../../lib/xz/xz_dec_stream.c:787:28: error: implicit declaration of function 'kmalloc_obj'; did you mean 'kmalloc'? [-Wimplicit-function-declaration]
>    787 |         struct xz_dec *s = kmalloc_obj(*s);
>        |                            ^~~~~~~~~~~
>        |                            kmalloc
> [...]
> CC      arch/arm/boot/compressed/fdt.o

Which config & build target was this exposed by? I don't see it with
either "all" nor "zImage" for defconfig (which includes CONFIG_RD_XZ)
nor allmodconfig, and I see arch/arm/boot/compressed/fdt.o getting built.

Okay, I traced down the includes now, it looks like it's tripped by
CONFIG_KERNEL_XZ=y, which isn't in defconfig nor allmodconfig. :(

> Fixes: 69050f8d6d07 ("treewide: Replace kmalloc with kmalloc_obj for non-scalar types")

Also:

Fixes: bf4afc53b77a ("Convert 'alloc_obj' family to use the new default GFP_KERNEL argument")

> Signed-off-by: Haiyue Wang <haiyuewa@xxxxxxx>
> ---
>  lib/decompress_unxz.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/lib/decompress_unxz.c b/lib/decompress_unxz.c
> index 32138bb8ef77..05d5cb490a44 100644
> --- a/lib/decompress_unxz.c
> +++ b/lib/decompress_unxz.c
> @@ -157,11 +157,11 @@
>   * when XZ_DYNALLOC is used, but the pre-boot free() doesn't support it.
>   * Workaround it here because the other decompressors don't need it.
>   */
> -#undef kmalloc
> +#undef kmalloc_obj
>  #undef kfree
>  #undef vmalloc
>  #undef vfree
> -#define kmalloc(size, flags) malloc(size)
> +#define kmalloc_obj(type) malloc(sizeof(type))

It may be worth defining this as:

#define kmalloc_obj(type, ...) malloc(sizeof(type))

In case some non-GFP_KERNEL ever end up in here.

Either way:

Reviewed-by: Kees Cook <kees@xxxxxxxxxx>

-- 
Kees Cook