Re: [PATCH v3] libfs: fix error cast of negative value in simple_attr_write()

[Luck, Tony]

On Sat, Nov 14, 2020 at 04:09:16PM +0800, Yicong Yang wrote:
> The attr->set() receive a value of u64, but simple_strtoll() is used
> for doing the conversion. It will lead to the error cast if user inputs
> a negative value.
> 
> Use kstrtoull() instead of simple_strtoll() to convert a string got
> from the user to an unsigned value. The former will return '-EINVAL' if
> it gets a negetive value, but the latter can't handle the situation
> correctly. Make 'val' unsigned long long as what kstrtoull() takes, this
> will eliminate the compile warning on no 64-bit architectures.
> 
> Fixes: f7b88631a897 ("fs/libfs.c: fix simple_attr_write() on 32bit machines")
> Signed-off-by: Yicong Yang <yangyicong@xxxxxxxxxxxxx>
> ---
> Change since v1:
> - address the compile warning for non-64 bit platform.
> Change since v2:
> Link: https://lore.kernel.org/linux-fsdevel/1605000324-7428-1-git-send-email-yangyicong@xxxxxxxxxxxxx/
> - make 'val' unsigned long long and mentioned in the commit
> Link: https://lore.kernel.org/linux-fsdevel/1605261369-551-1-git-send-email-yangyicong@xxxxxxxxxxxxx/

Belated error report on this. Some validation team just moved to
v5.10 and found their error injection scripts no longer work.

They have been using:

# echo $((-1 << 12)) > /sys/kernel/debug/apei/einj/param2

to write the mask value 0xfffffffffffff000 for many years ... but
now writing a negative value (-4096) to this file gives an EINVAL error.

Maybe they've been taking advantage of a bug all this time? The
comment for debugfs_create_x64() says it is for reading/writing
an unsigned value. But when a bug fix breaks user code ... then
we are supposed to ask whether that bug is actually a feature.

If there was a debugfs_create_s64() I might just fix einj.c to
use that ... but there isn't :-(

-Tony