Re: CONFIG_* symbols in UAPI headers?

[Arnd Bergmann]

On Mon, Apr 8, 2019 at 2:46 PM Christoph Hellwig <hch@xxxxxx> wrote:
>
> I just stumbled over the MAP_UNINITIALIZED defintion, initially
> added by:
>
> commit ea637639591def87a54cea811cbac796980cb30d
> Author: Jie Zhang <jie.zhang@xxxxxxxxxx>
> Date:   Mon Dec 14 18:00:02 2009 -0800
>
>     nommu: fix malloc performance by adding uninitialized flag
>
> The defintion depends on CONFIG_MMAP_ALLOW_UNINITIALIZED, which
> will never be set by userspace.  How is this supposed to work?
>
> Shoudn't we define the symbol unconditionally and just turn it
> into a no-op in the implementation?

Right, good catch. That should work. It can probably be done
by adding another check before the conditional, like:

       /* clear anonymous mappings that don't ask for uninitialized data */
       if (!vma->vm_file &&
           !(IS_ENABLED(CONFIG_MMAP_ALLOW_UNINITIALIZED) &&
             (flags & MAP_UNINITIALIZED))
               memset((void *)region->vm_start, 0,
                      region->vm_end - region->vm_start);

> There are a few similar issues, like struct elf_prstatus having
> a different layout depending on CONFIG_BINFMT_ELF_FDPIC, or
> MAX_SHARED_LIBS defending on CONFIG_BINFMT_SHARED_FLAT.

I thought we had cleaned them out a long time ago, but it looks
like those have been there since the uapi headers got split out,
and are still wrong.

Why doesn't scripts/headers_check.pl find those?

      Arnd