Re: [PATCH v2] m68k: use conventional function parameters for do_sigreturn

Geert Uytterhoeven
2016-02-29T09:36:56Z

On Mon, Feb 15, 2016 at 7:36 AM, Greg Ungerer <gerg@uclinux.org> wrote:
> Create conventional stack parameters for the calls to do_sigreturn and
> do_rt_sigreturn. The current C code for do_sigreturn and do_rt_sigreturn
> dig into the stack to create local pointers to the saved switch stack
> and the pt_regs structs.
>
> The motivation for this change is a problem with non-MMU targets that
> have broken signal return paths on newer versions of gcc. It appears as
> though gcc has determined that the pointers into the saved stack structs,
> and the saved structs themselves, are function parameters and updates to
> them will be lost on function return, so they are optimized away. This
> results in large parts of restore_sigcontext() and mangle_kernel_stack()
> functions being removed. Of course this results in non-functional code
> causing kernel oops. This problem has been observed with gcc version
> 5.2 and 5.3, and probably exists in earlier versions as well.
>
> Using conventional stack parameter pointers passed to these functions has
> the advantage of the code here not needing to know the exact details of
> how the underlying entry handler layed these structs out on the stack.
> So the rather ugly pointer setup casting and arg referencing can be
> removed.
>
> The resulting code after this change is a few bytes larger (due to the
> overhead of creating the stack args and their tear down). Not being hot
> paths I don't think this is too much of a problem here.
>
> An alternative solution is to put a barrier() in the do_sigreturn() code,
> but this doesn't feel quite as clean as this solution.
>
> This change has been compile tested on all defconfigs, and run tested on
> Atari (through aranym), ColdFire with MMU (M5407EVB) and ColdFire with
> no-MMU (QEMU and M5208EVB).
>
> Signed-off-by: Greg Ungerer <gerg@uclinux.org>

Thanks,.applied and queued for v4.6 with Andreas' Ack.

Gr{oetje,eeting}s,

Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
-- Linus Torvalds