linux-kernel - Re: [PATCH 1/2] sh: align .bss section padding to 8-byte boundary

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Message-ID: <CAMuHMdXaw117mimN_7UgwbfbG9grVg09WfjKa2Lz4eGqM24EvA@mail.gmail.com>
Date: Wed, 12 Mar 2025 10:47:00 +0100
From: Geert Uytterhoeven <geert@...ux-m68k.org>
To: Uros Bizjak <ubizjak@...il.com>
Cc: John Paul Adrian Glaubitz <glaubitz@...sik.fu-berlin.de>, Artur Rojek <contact@...ur-rojek.eu>, 
	Yoshinori Sato <ysato@...rs.sourceforge.jp>, Rich Felker <dalias@...c.org>, 
	Daniel Lezcano <daniel.lezcano@...aro.org>, Thomas Gleixner <tglx@...utronix.de>, 
	Geert Uytterhoeven <geert+renesas@...der.be>, "D . Jeff Dionne" <jeff@...esemi.io>, 
	Rob Landley <rob@...dley.net>, linux-sh@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] sh: align .bss section padding to 8-byte boundary

Hi Uros,

On Wed, 12 Mar 2025 at 09:32, Uros Bizjak <ubizjak@...il.com> wrote:
> On Wed, Mar 12, 2025 at 9:22 AM John Paul Adrian Glaubitz
> <glaubitz@...sik.fu-berlin.de> wrote:
> > > In the original BSS_SECTION(0, PAGE_SIZE, 4), the last argument inserts
> > > a 4 byte padding after the closing brace of .bss section definition,
> > > causing the __bss_stop symbol offset to grow, but not the .bss section
> > > itself:
> > >
> > > #define BSS_SECTION(sbss_align, bss_align, stop_align)                        \
> > >       . = ALIGN(sbss_align);                                          \
> > >       __bss_start = .;                                                \
> > >       SBSS(sbss_align)                                                \
> > >       BSS(bss_align)                                                  \
> > >       . = ALIGN(stop_align);                                          \
> > >       __bss_stop = .;
> >
> > OK, that's really odd. So, the __bss_stop would be moved to the desired
> > position but the section itself still remains small? What exactly does the
> > linker fill the region with? Sounds very strange.
> >
> > > TurtleBoard loader is only concerned with the .bss section size - it
> > > doesn't care about any symbol offsets - and hence this seemingly cryptic
> > > change (you can display the section size information with
> > > readelf -t kernel_image).
> >
> > Looking at the actual kernel image with readelf is a very good suggestion!
> >
> > > The rest of the changes are simply to "inline" the BSS() macro (as I
> > > needed to access that closing brace), and the former sbss_align,
> > > bss_align (that's your PAGE_SIZE) and stop_align arguments are passed
> > > accordingly, the same way they used to be passed before. The only
> > > visible effect should be the move of ALIGN(stop_align) inside of .bss
> > > section definition, and the change of stop_align value from 4 to 8.
> >
> > OK. FWIW, do you understand what SBSS is for? I couldn't find any explanation
> > for it.
>
> Small BSS section. The compiler can put data objects under a certain
> size threshold to the .sbss section. Looking at GCC sh config, sh does
> not use this section.

Hence the moment gcc (or clang) starts using that section, the
TurtleBoard loader is broken again...

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-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