Re: [PATCH v3 3/4] block: add support for partition table defined in OF

[Christian Marangi <ansuelsmth@gmail.com>]

On Mon, Sep 30, 2024 at 11:21:53AM +0200, Rasmus Villemoes wrote:
> Christian Marangi <ansuelsmth@gmail.com> writes:
> 
> > diff --git a/block/partitions/of.c b/block/partitions/of.c
> > new file mode 100644
> > index 000000000000..bc6200eb86b3
> > --- /dev/null
> > +++ b/block/partitions/of.c
> > @@ -0,0 +1,151 @@
> > +// SPDX-License-Identifier: GPL-2.0
> > +
> > +#include <linux/blkdev.h>
> > +#include <linux/major.h>
> > +#include <linux/of.h>
> > +#include "check.h"
> > +
> > +#define BOOT0_STR	"boot0"
> > +#define BOOT1_STR	"boot1"
> > +
> > +static struct device_node *get_partitions_node(struct device_node *disk_np,
> > +					       struct gendisk *disk)
> > +{
> > +	const char *node_name = "partitions";
> > +
> > +	/*
> > +	 * JEDEC specification 4.4 for eMMC introduced 3 additional partition
> > +	 * present on every eMMC. These additional partition are always hardcoded
> > +	 * from the eMMC driver as boot0, boot1 and rpmb. While rpmb is used to
> > +	 * store keys and exposed as a char device, the other 2 are exposed as
> > +	 * real separate disk with the boot0/1 appended to the disk name.
> > +	 *
> > +	 * Here we parse the disk_name in search for such suffix and select
> > +	 * the correct partition node.
> > +	 */
> > +	if (disk->major == MMC_BLOCK_MAJOR) {
> > +		const char *disk_name = disk->disk_name;
> > +
> > +		if (!memcmp(disk_name + strlen(disk_name) - strlen(BOOT0_STR),
> > +			    BOOT0_STR, sizeof(BOOT0_STR)))
> > +			node_name = "partitions-boot0";
> 
> If strlen(disk_name) is less than 5 (and I don't know if that's actually
> possible), this well end up doing out-of-bounds access.
> 
> We have a strstarts() helper, could you also add a strends() helper that
> handles this correctly? Something like
> 
> /**
>  * strends - does @str end with @suffix?
>  * @str: string to examine
>  * @suffix: suffix to look for.
>  */
> static inline bool strends(const char *str, const char *suffix)
> {
> 	size_t n = strlen(str);
>         size_t m = strlen(suffix);
>         return n >= m && !memcmp(str + n - m, suffix, m);
> }
> 
> [or name it str_has_suffix() or str_ends_with(), "strends" is not
> particularly readable, it's unfortunate that the existing strstarts is
> spelled like that].
>

Nice idea and thanks for checking the problem with the out-of-bounds
read.

Out of consistency with the unreadable strstarts I'm tempted to use
strends.

Since checking suffix of a string can't be something that unreal I
searched for the 3 function name and to my surprise all 3 suggested name
have a variant of the function statically defined hahaha.

To not pollute this series I will just introduce the helper but I will
add on my TODO list to convert the other function to make use of this
helper instead.

-- 
	Ansuel