Performing pointer arithmetic on a null pointer?

Performing pointer arithmetic on a null pointer?

[Hyeonggon Yoo]

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;

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Re: Performing pointer arithmetic on a null pointer?

[Lukas Bulwahn]

On Thu, May 27, 2021 at 5:15 PM Hyeonggon Yoo <42.hyeyoo@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

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Re: Performing pointer arithmetic on a null pointer?

[Hyeonggon Yoo]

On Thu, May 27, 2021 at 05:27:06PM +0200, Lukas Bulwahn wrote:
> 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
> 

Oh, Arnd Bergmann was already working on it!

> 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
> 

Thank you for giving some informations,
it's always fun to read mailing list that I can understand :)

> 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
> 

Yes I was doing it on linux-next.. :D

> The commit and discussion around it will give you much more technical
> details on the issue and its resolution.
> 
> I hope this helps.
> 
> Lukas

Thank you for giving some links. it will be much helpful.
I'm going to read it now!

Hyeonggon

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Re: Performing pointer arithmetic on a null pointer?

[Greg KH]

On Fri, May 28, 2021 at 12:14:43AM +0900, Hyeonggon Yoo 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;

Should work just fine, what problems have you found with that code?

Also happens in a few other places in the vfs layer.  Tricky code, but
it works as pointer math is valid C code :)

thanks,

greg k-h

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Re: Performing pointer arithmetic on a null pointer?

[Hyeonggon Yoo]

On Thu, May 27, 2021 at 05:32:11PM +0200, Greg KH wrote:
> On Fri, May 28, 2021 at 12:14:43AM +0900, Hyeonggon Yoo 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;
> 
> Should work just fine, what problems have you found with that code?
> 

Hello Greg.

Yeah, that should work fine. I just wanted to understand
warning that clang says!

> Also happens in a few other places in the vfs layer.  Tricky code, but
> it works as pointer math is valid C code :)

I'm searching about pointer arithmetic.

Documents say that pointer math on void pointer (as NULL is void
pointer) is illegal by C standards. (I can be missing something!)

if above is true,then is it OK because
it's valid on gcc and clang?

Thanks,
Hyeonggon

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