Re: [PATCH v1 0/7] tools: add linker table userspace sandbox

From: Luis R. Rodriguez
Date: Tue Aug 23 2016 - 12:05:54 EST


On Mon, Aug 22, 2016 at 11:59:46AM +0200, Vegard Nossum wrote:
> On 19 August 2016 at 23:41, <mcgrof@xxxxxxxxxx> wrote:
> > From: "Luis R. Rodriguez" <mcgrof@xxxxxxxxxx>
> >
> > The original v3 series for linker tables made reference only to
> > an external repository userspace sandbox application, however
> > Boris noted it'd be difficult ot keep this in sync with the
> > kernel so advised to consider integrate with the kernel. I've
> > taken steps in this direction.
> [...]
> > Please let me know if there are any issue or questions.
>
> +#define __VMLINUX_SYMBOL(x) x
>
> +#define VMLINUX_SYMBOL(x) __VMLINUX_SYMBOL(x)
>
> +#define LINUX_SECTION_START(name) VMLINUX_SYMBOL(name)
>
> +#define DECLARE_LINUX_SECTION(type, name) \
> + extern type VMLINUX_SYMBOL(name)[], \
> + VMLINUX_SYMBOL(name##__end)[]
>
> +#define DECLARE_LINKTABLE(type, name) \
> + DECLARE_LINUX_SECTION(type, name)
>
> +#define LINKTABLE_FOR_EACH(pointer, tbl) \
> + for (pointer = LINUX_SECTION_START(tbl); \
> + pointer < LINUX_SECTION_END(tbl); \
> + pointer++)
>
> I think this is subject to getting optimised out by newer gccs, since
> it sees the START(tbl) and END(tbl) symbols as two completely
> different arrays. See the short discussion here:
>
> https://lkml.org/lkml/2016/6/26/73 (the first attempt is wrong, so
> don't look at that)
>
> It is possible that < is different from != and always does the right
> thing, but I haven't checked.
>
> I have a WIP branch that converts most of the existing tables in the
> kernel to use the external_array() macro which makes gcc throw away
> any knowledge it had about a pointer being part of an array.

Was there no compiler option to disable the optimization ?

Please do Cc me on your patches, either way we can coordinate changes depending
on which series gets merged. If your external_array() change lands upstream
first I'll see it on linux-next shortly can rebase and adjust code then as
that is what I base my patches on.

Do we know if there exceptions to the optimization issue for this and if so what
patterns follow ? We can test with simple userspace code if this will be an
issue with gcc 7 by trying the demo in tools/linker-tables/ using this git tree
and branch 20160819-linker-table-v4 :

https://git.kernel.org/cgit/linux/kernel/git/mcgrof/linux-next.git/log/?h=20160819-linker-table-v4

Luis