[PATCH] mtd: Make mtd_erase fail gracefully if CONFIG_MTD_WRITE is off

[Ahmad Fatoum]

Hi Christian,

Thanks for your patch!

On 9/3/25 10:49 AM, Christian Thießen via B4 Relay wrote:
> From: Christian Thießen <christian.thiessen at airbus.com>
> 
> When CONFIG_MTD_WRITE is unset, the mtd->_erase member of partition
> devices does not get initialized, so it remains NULL.
> However when running ubiattach on an UBI partition, mtd_erase() still
> gets called, tries to call mtd->_erase and crashes. Make it fail
> gracefully by returning ENOSYS ("Function not implemented") instead.

Reviewed-by: Ahmad Fatoum <a.fatoum at pengutronix.de>

with two minor comments below.

> 
> ---
> As discussed in https://github.com/barebox/barebox/issues/39 this patch
> enables ubiattach-ing a NAND partition even if CONFIG_MTD_WRITE is
> turned off, by making mtd_erase fail gracefully.
> 
> Signed-off-by: Christian Thießen <christian.thiessen at airbus.com>

Stuff between the --- is thrown away by git am, so you will want to move
the S-o-b above the ---. The rest is okay.

> ---
>  drivers/mtd/core.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/drivers/mtd/core.c b/drivers/mtd/core.c
> index 6162ec450fc1c3a0fa6339042c4d627e0f9567d0..7ca8fc900af4666070b03ef5f945748d60e6a6e7 100644
> --- a/drivers/mtd/core.c
> +++ b/drivers/mtd/core.c
> @@ -436,6 +436,7 @@ int mtd_erase(struct mtd_info *mtd, struct erase_info *instr)
>  {
>  	if (instr->addr >= mtd->size || instr->len > mtd->size - instr->addr)
>  		return -EINVAL;
> +#ifdef CONFIG_MTD_WRITE

Please replace with the following at the start of the function:

	if (!IS_ENABLED(CONFIG_MTD_WRITE))
		return -ENOSYS;

This results in the same code as IS_ENABLED() is evaluated at
compile-time. but the compiler can still catch compile errors in the
disabled parts, which makes maintenance easier.

Cheers,
Ahmad


>  	if (!(mtd->flags & MTD_WRITEABLE))
>  		return -EROFS;
>  	instr->fail_addr = MTD_FAIL_ADDR_UNKNOWN;
> @@ -443,6 +444,9 @@ int mtd_erase(struct mtd_info *mtd, struct erase_info *instr)
>  		return 0;
>  
>  	return mtd->_erase(mtd, instr);
> +#else
> +	return -ENOSYS;
> +#endif
>  }
>  
>  int mtd_read_oob(struct mtd_info *mtd, loff_t from, struct mtd_oob_ops *ops)
> 
> ---
> base-commit: b5561f3bdd4845b478e5b35aab04f1d8e71ea93b
> change-id: 20250902-mtd-erase-enosys-2d80e4bc77c8
> 
> Best regards,

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