Re: [PATCH] block: sed-opal: reduce stack size of ioctl handler

On Wed, Feb 08, 2017 at 02:58:28PM -0700, Scott Bauer wrote:

On Wed, Feb 08, 2017 at 10:15:28PM +0100, Arnd Bergmann wrote:

quoted

When CONFIG_KASAN is in use, the sed_ioctl function uses unusually large stack,
as each possible ioctl argument gets its own stack area plus redzone:

block/sed-opal.c: In function 'sed_ioctl':
block/sed-opal.c:2447:1: error: the frame size of 2256 bytes is larger than 2048 bytes [-Werror=frame-larger-than=]

Moving the copy_from_user() calls into the individual functions has little
effect on readablility, but significantly reduces the stack size, with the
largest individual function (opal_enable_disable_shadow_mbr) now at
reasonable 456 bytes.

Fixes: 455a7b238cd6 ("block: Add Sed-opal library")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>

Hi Arnd,

Thank you for the report. We want to keep the function calls agnostic to userland.
In the future we will have in-kernel callers and I don't want to have to do any
get_fs(KERNEL_DS) wizardry.

Instead I think we can use a union to lessen the stack burden. I tested this patch just now
with config_ksasan and was able to build.

Nack on this patch, it only really masks the issue. Keith pointed out we have a call chain
up to this ioctl then deeper down into nvme then the block layer. If we use 25% of the stack
just for this function it's still too dangerous and we'll run into corruption later on and not
remember this fix. I'll come up with another solution.