Re: [PATCH v4 3/4] tools/bootconfig: Align the bootconfig applied initrd image size to 4

[Steven Rostedt <rostedt@goodmis.org>]

On Thu, 19 Nov 2020 00:35:44 +0900
Masami Hiramatsu <mhiramat@kernel.org> wrote:

> +
> +	/* To align up the total size to BOOTCONFIG_ALIGN, get padding size */
> +	total_size = stat.st_size + size + sizeof(u32) * 2 + BOOTCONFIG_MAGIC_LEN;
> +	pad = BOOTCONFIG_ALIGN - total_size % BOOTCONFIG_ALIGN;
> +	if (pad == BOOTCONFIG_ALIGN)
> +		pad = 0;

If alignment is always a power of two, you could simply do:

	pad = (total_size + BOOTCONFIG_ALIGN - 1) &  ~(BOOTCONFIG_ALIGN - 1)) - total_size;

Which will give you the proper padding, without the if.

> +	size += pad;
> +
> +	/* Add a footer */
> +	*(u32 *)(data + size) = size;
> +	*(u32 *)(data + size + sizeof(u32)) = csum;
> +	memcpy(data + size + sizeof(u32) * 2, BOOTCONFIG_MAGIC, BOOTCONFIG_MAGIC_LEN);
> +	total_size = size + sizeof(u32) * 2 + BOOTCONFIG_MAGIC_LEN;

I wonder if it would be cleaner to just have a void pointer index for the above:

	void *p;

	p = data + size;
	*(u32 *)p = size;
	p += sizeof(u32);

	*(u32 *)p = csum;
	p += sizeof(u32);

	memcpy(p, BOOTCONFIG_MAGIC, BOOTCONFIG_MAGIC_LEN);
	p += BOOTCONFIG_MAGIC_LEN;

	total_size = p - (void *)data;

Also, how does this work if we run this on a little endian box for a big
endian crossbuild?

-- Steve


> +
> +	ret = write(fd, data, total_size);
> +	if (ret < total_size) {
>  		if (ret < 0)
>  			ret = -errno;
>  		pr_err("Failed to apply a boot config: %d\n", ret);
> -		if (ret < 0)
> -			goto out;
> -		goto out_rollback;
> -	}
> -	/* Write a magic word of the bootconfig */
> -	ret = write(fd, BOOTCONFIG_MAGIC, BOOTCONFIG_MAGIC_LEN);
> -	if (ret < BOOTCONFIG_MAGIC_LEN) {
> -		if (ret < 0)
> -			ret = -errno;
> -		pr_err("Failed to apply a boot config magic: %d\n", ret);
> -		goto out_rollback;
> -	}
> -	ret = 0;
> +		if (ret > 0)
> +			goto out_rollback;
> +	} else
> +		ret = 0;
> +
>  out:
>  	close(fd);
>  	free(data);