Re: [PATCH] mtd: don't try to detect UBI in too small flash partitions

[Sascha Hauer <s.hauer@pengutronix.de>]

Hi Ahmad,

On Wed, Feb 28, 2024 at 05:03:04PM +0100, Ahmad Fatoum wrote:
> The detect callback will try to detect an UBI volume inside a partition.
> If that partition happens to be smaller than 512 bytes, detect will fail
> with -EINVAL.

This 512 bytes here is confusing. You are referring to the eraseblock
size which is usually much bigger than 512 bytes.

> 
> Avoid this confusing error by early exiting in this case.
> 
> Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
> ---
>  drivers/mtd/core.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/drivers/mtd/core.c b/drivers/mtd/core.c
> index 97a7996cf68e..34f919b96ea4 100644
> --- a/drivers/mtd/core.c
> +++ b/drivers/mtd/core.c
> @@ -599,6 +599,10 @@ static int mtd_detect(struct device *dev)
>  	enum filetype filetype;
>  	int npebs = mtd_div_by_eb(mtd->size, mtd);
>  
> +	/* No point in looking for UBI on a partition that's smaller than an erase block */
> +	if (!npebs)
> +		return 0;

When this triggers your partitioning is wrong. Partitions should always
align to eraseblock boundaries. While it is possible to have not
eraseblock aligned partitions, you'll be unable to write on these
partitions as you would always erase parts of the neighbour partition.

However, there's a minimal number of eraseblocks that a device must have
to be usable with UBI, see http://www.linux-mtd.infradead.org/doc/ubi.html#L_overhead
So instead of !npebs you can test for npebs < 5.

> +
>  	npebs = max(npebs, 64);

Not your patches fault, but as I just see it: This is wrong. It should
be min(), not max(). The goal of this is to check the first 64
eraseblocks, but not more pebs the device actually has.

Sascha

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