[PATCH v2] mtd: block2mtd: Fix divide error when erase_size is zero

[Miquel Raynal]

On 12/08/2026 at 09:52:42 +08, Pei Xiao <xiaopei01 at kylinos.cn> wrote:

> The erase size is parsed from the "block2mtd" module parameter and can
> be set to zero. add_device() then evaluates
>
>         if (size % erase_size)
>
> with a zero divisor, which triggers a divide error:
>
>         divide error: 0000 [#1] PREEMPT SMP PTI
>         RIP: 0010:add_device drivers/mtd/devices/block2mtd.c:296 [inline]
>         RIP: 0010:block2mtd_setup2+0x592/0xda0 drivers/mtd/devices/block2mtd.c:459
>         Call Trace:
>          block2mtd_setup+0x27/0xe0 drivers/mtd/devices/block2mtd.c:476
>          param_attr_store+0x214/0x310 kernel/params.c:589
>          module_attr_store+0x65/0x90 kernel/params.c:904
>          kernfs_fop_write_iter+0x3a4/0x540 fs/kernfs/file.c:345
>          ...
>
> Reject a zero erase size before performing the modulo operation so the
> existing "erasesize must be a divisor of device size" error path
> reports the invalid argument and frees the device.
>
> While at it, drop the unnecessary (long) cast from the size operand of
> the modulo.
>
> Fixes: ea6d833a3fdd ("mtd: block2mtd: check device size")
> Reported-by: syzbot+b320a4d5f65a61dbbf89 at syzkaller.appspotmail.com
> Closes: https://lore.kernel.org/lkml/6a7b58ba.ac361c09.22ff0a.0045.GAE@google.com/
> Suggested-by: Jörn Engel <joern at barelysecure.org>
> Cc: stable at vger.kernel.org
> Signed-off-by: Pei Xiao <xiaopei01 at kylinos.cn>

Apparently, removing the cast leads to an unsupported operation with a
64-bit operand on 32-bit architectures (See Sashiko report).

I guess it is best to keep that cast in the end.

Sorry for the changes back-and-forth.

Miquèl