Performing pointer arithmetic on a null pointer?

[Lukas Bulwahn]

On Thu, May 27, 2021 at 5:15 PM Hyeonggon Yoo <42.hyeyoo at gmail.com> wrote:
>
> Hello, I was compiling kernel with make CC=clang-10 W=1 -s
>
> there are some places that compiler complains about
> pointer arithmetic like below.it says it's undefined behavior.
>
> is it just OK to use UBs like this (I hope it's not),
> or am I missing something?
>
> fs/kernfs/file.c:128:15: warning: performing pointer arithmetic on
>         a null pointer has undefined behavior [-Wnull-pointer-arithmetic]
>                 return NULL + !*ppos;
>
>

Hyeonggon,

there is a patch series from Arnd Bergmann, a well-known kernel
developer and janitor, hanging around here:

https://git.kernel.org/pub/scm/linux/kernel/git/arnd/playground.git/log/?h=warnings-5.10-2

It includes a patch to address this issue:

https://git.kernel.org/pub/scm/linux/kernel/git/arnd/playground.git/commit/?h=warnings-5.10-2&id=cda293a8ee8a2bc763c33828cbc9ff21af987d45

The patch title and commit message indicates that this was submitted
in multiple versions to the mailing list.

You can find more on its progress on the mailing list here:
https://lore.kernel.org/lkml/?q=seq_file%3A+fix+clang+warning+for+NULL+pointer+arithmetic

It seems that it has not landed in linux-next yet, though:

https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/log/?qt=grep&q=seq_file%3A+fix+clang+warning+for+NULL+pointer+arithmetic

The commit and discussion around it will give you much more technical
details on the issue and its resolution.

I hope this helps.


Lukas