Re: [PATCH] remove lots of IS_ERR_VALUE abuses

[Andrew Morton <akpm-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org>]

On Fri, 27 May 2016 23:23:25 +0200 Arnd Bergmann <arnd-r2nGTMty4D4@public.gmane.org> wrote:

> Most users of IS_ERR_VALUE() in the kernel are wrong, as they
> pass an 'int' into a function that takes an 'unsigned long'
> argument. This happens to work because the type is sign-extended
> on 64-bit architectures before it gets converted into an
> unsigned type.
> 
> However, anything that passes an 'unsigned short' or 'unsigned int'
> argument into IS_ERR_VALUE() is guaranteed to be broken, as are
> 8-bit integers and types that are wider than 'unsigned long'.
> 
> Andrzej Hajda has already fixed a lot of the worst abusers that
> were causing actual bugs, but it would be nice to prevent any
> users that are not passing 'unsigned long' arguments.
> 
> This patch changes all users of IS_ERR_VALUE() that I could find
> on 32-bit ARM randconfig builds and x86 allmodconfig. For the
> moment, this doesn't change the definition of IS_ERR_VALUE()
> because there are probably still architecture specific users
> elsewhere.

So you do plan to add some sort of typechecking into IS_ERR_VALUE()?

> Almost all the warnings I got are for files that are better off
> using 'if (err)' or 'if (err < 0)'.
> The only legitimate user I could find that we get a warning for
> is the (32-bit only) freescale fman driver, so I did not remove
> the IS_ERR_VALUE() there but changed the type to 'unsigned long'.
> For 9pfs, I just worked around one user whose calling conventions
> are so obscure that I did not dare change the behavior.
> 
> I was using this definition for testing:
> 
>  #define IS_ERR_VALUE(x) ((unsigned long*)NULL == (typeof (x)*)NULL && \
>        unlikely((unsigned long long)(x) >= (unsigned long long)(typeof(x))-MAX_ERRNO))
> 
> which ends up making all 16-bit or wider types work correctly with
> the most plausible interpretation of what IS_ERR_VALUE() was supposed
> to return according to its users, but also causes a compile-time
> warning for any users that do not pass an 'unsigned long' argument.
> 
> I suggested this approach earlier this year, but back then we ended
> up deciding to just fix the users that are obviously broken. After
> the initial warning that caused me to get involved in the discussion
> (fs/gfs2/dir.c) showed up again in the mainline kernel, Linus
> asked me to send the whole thing again.