Re: [PATCH] bootconfig: Fix unaligned access when building footer

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

On Wed, 23 Jul 2025 12:49:25 +0200
Ben Hutchings <benh@debian.org> wrote:

> Currently we add padding between the bootconfig text and footer to
> ensure that the footer is aligned within the initramfs image.
> However, because only the bootconfig data is held in memory, not the
> full initramfs image, the footer may not be naturally aligned in
> memory.
> 
> This can result in an alignment fault (SIGBUS) when writing the footer
> on some architectures, such as sparc.

Aah, got it. 

> 
> Build the footer in a struct on the stack before adding it to the
> buffer.
> 
> References: https://buildd.debian.org/status/fetch.php?pkg=linux&arch=sparc64&ver=6.16%7Erc7-1%7Eexp1&stamp=1753209801&raw=0
> Signed-off-by: Ben Hutchings <benh@debian.org>
> ---
>  tools/bootconfig/main.c | 24 +++++++++++++-----------
>  1 file changed, 13 insertions(+), 11 deletions(-)
> 
> diff --git a/tools/bootconfig/main.c b/tools/bootconfig/main.c
> index 4988e23a1422..57c669d2aa90 100644
> --- a/tools/bootconfig/main.c
> +++ b/tools/bootconfig/main.c
> @@ -11,6 +11,7 @@
>  #include <string.h>
>  #include <errno.h>
>  #include <endian.h>
> +#include <assert.h>
>  
>  #include <linux/bootconfig.h>
>  
> @@ -363,7 +364,12 @@ static int delete_xbc(const char *path)
>  
>  static int apply_xbc(const char *path, const char *xbc_path)
>  {
> -	char *buf, *data, *p;
> +	struct {
> +		uint32_t size;
> +		uint32_t csum;
> +		char magic[BOOTCONFIG_MAGIC_LEN];
> +	} footer;

Don't we need __attribute__((__packed__)) for the footer?

Thank you,

> +	char *buf, *data;
>  	size_t total_size;
>  	struct stat stat;
>  	const char *msg;
> @@ -433,17 +439,13 @@ static int apply_xbc(const char *path, const char *xbc_path)
>  	size += pad;
>  
>  	/* Add a footer */
> -	p = data + size;
> -	*(uint32_t *)p = htole32(size);
> -	p += sizeof(uint32_t);
> +	footer.size = htole32(size);
> +	footer.csum = htole32(csum);
> +	memcpy(footer.magic, BOOTCONFIG_MAGIC, BOOTCONFIG_MAGIC_LEN);
> +	static_assert(sizeof(footer) == BOOTCONFIG_FOOTER_SIZE);
> +	memcpy(data + size, &footer, BOOTCONFIG_FOOTER_SIZE);
>  
> -	*(uint32_t *)p = htole32(csum);
> -	p += sizeof(uint32_t);
> -
> -	memcpy(p, BOOTCONFIG_MAGIC, BOOTCONFIG_MAGIC_LEN);
> -	p += BOOTCONFIG_MAGIC_LEN;
> -
> -	total_size = p - data;
> +	total_size = size + BOOTCONFIG_FOOTER_SIZE;
>  
>  	ret = write(fd, data, total_size);
>  	if (ret < total_size) {


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>