[LEDE-DEV] [PATCH] mount_root: implement overlay= boot option

[Josua Mayer]

Hi everybody,

Please take a look at this draft below. It attempts to solve the issue
of using an overlay device on regular block devices such as sata, emmc
or sdcards WITHOUT any unnecessary block2mtd layer in between.

I extended the find.method of ext4 code to include devname, because
partname is not easily available. It requires mtdparts or blockdevparts
kernel option, which requires specifying partition offsets and sizes.
Something that is totally redundant when there is a partition table, be
it MBR or GPT.

Ideally there would be 2 find-functions, one by partname and one by
devname. But that is currently hard to implement due to
a) lack of documentation for the intended purpose of each function
(init, find, read, write) implemented by drivers
b) lack of consistent behaviour of these functions across drivers (ext4,
mtd, ubi)
I feel like this driver api could use some rework to accomodate both mtd
and block devices, as well as potential future filesystems (f2fs?) in
addition to jffs, ubi and ext4.

br
Josua Mayer

Am 10.05.2016 um 14:25 schrieb josua.mayer97 at gmail.com:
> From: Josua Mayer <josua.mayer97 at gmail.com>
> 
> This change adds code to handle a new option on cmdline: overlay=
> This is used to find the rootfs_data partition / disk when it has a
> non-standard name or location.
> 
> It takes either the device node name, or the partition name:
> i.e. /dev/mmcblk0p3, mmcblk0p3, rootfs_data
> 
> This option has precedence over the default name "rootfs_data", and extroot.
> 
> Signed-off-by: Josua Mayer <josua.mayer97 at gmail.com>.
> ---
>  libfstools/ext4.c |  4 +--
>  mount_root.c      | 86 +++++++++++++++++++++++++++++++++++++++++++++++++++++--
>  2 files changed, 86 insertions(+), 4 deletions(-)
> 
> diff --git a/libfstools/ext4.c b/libfstools/ext4.c
> index f648aa8..2c6dd91 100644
> --- a/libfstools/ext4.c
> +++ b/libfstools/ext4.c
> @@ -77,8 +77,8 @@ ext4_part_match(char *dev, char *name, char *filename)
>  			strcpy(devname, buf + strlen("DEVNAME="));
>  			continue;
>  		}
> -		/* Match partition name */
> -		if (strstr(buf, name))  {
> +		/* Match either partition name or device node name */
> +		if (strstr(buf, name) || strstr(devname, name))  {
>  			ret = 0;
>  			break;
>  		}
> diff --git a/mount_root.c b/mount_root.c
> index 47a3409..64aef66 100644
> --- a/mount_root.c
> +++ b/mount_root.c
> @@ -28,11 +28,52 @@ static int
>  start(int argc, char *argv[1])
>  {
>  	struct volume *root;
> -	struct volume *data = volume_find("rootfs_data");
> +	struct volume *data = NULL;
>  
>  	if (!getenv("PREINIT"))
>  		return -1;
>  
> +	/*
> +	 * Check cmdline for a hint about overlay device
> +	 * e.g. /dev/mmcblk0p3
> +	 */
> +	{
> +		FILE *fp;
> +		char buffer[100] = {0};
> +
> +		fp = fopen("/proc/cmdline", "r");
> +		while (!feof(fp)) {
> +			if(fscanf(fp, "overlay=%s", buffer))
> +				break;
> +
> +			fseek(fp, 1, SEEK_CUR);
> +		}
> +		fclose(fp);
> +
> +		/*
> +		 * If an overlay= argument was found, look for a volume with that name
> +		 */
> +		if (buffer[0]) {
> +			/*
> +			 * strip /dev/ prefix if any
> +			 */
> +			int offset = 0;
> +			if (strstr(buffer, "/dev/"))
> +				offset = 5;
> +
> +			ULOG_NOTE("Looking for overlay device given on commandline\n");
> +			data = volume_find(buffer + offset);
> +		}
> +	}
> +
> +	/*
> +	 * Look for default rootfs_data partition name
> +	 */
> +	data = volume_find("rootfs_data");
> +
> +	/*
> +	 * In case rootfs_data doesn't exist, only mount /dev/root for now
> +	 */
>  	if (!data) {
>  		root = volume_find("rootfs");
>  		volume_init(root);
> @@ -95,8 +136,49 @@ stop(int argc, char *argv[1])
>  static int
>  done(int argc, char *argv[1])
>  {
> -	struct volume *v = volume_find("rootfs_data");
> +	struct volume *v = NULL;
> +	FILE *fp;
> +	struct stat fs = {0};
> +	char *buffer;
>  
> +	/*
> +	 * First check if there is an overlay device hint in cmdline
> +	 */
> +	fp = fopen("/proc/cmdline", "r");
> +	fstat(fileno(fp), &fs);
> +	buffer = calloc(1, fs.st_size+1);
> +	while (!feof(fp)) {
> +		if (fscanf(fp, "overlay=%s", buffer))
> +			break;
> +
> +		fseek(fp, 1, SEEK_CUR);
> +	}
> +	fclose(fp);
> +
> +	/*
> +	 * If an overlay= argument was found, look for a volume with that name
> +	 */
> +	if (buffer[0]) {
> +		/*
> +		 * strip /dev/ prefix if any
> +		 */
> +		int offset = 0;
> +		if (strstr(buffer, "/dev/"))
> +			offset = 5;
> +
> +		v = volume_find(buffer + offset);
> +	}
> +	free(buffer);
> +
> +	/*
> +	 * Now look for standard overlay partition name
> +	 */
> +	if (!v)
> +		v = volume_find("rootfs_data");
> +
> +	/*
> +	 * If no overlay device exists, then there is nothing to do here
> +	 */
>  	if (!v)
>  		return -1;
>  
>