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

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Message-ID: <CAFULd4b8+HsmJC2XkW50pxtw=fHNrL9gH1_WM90jh+rfLCbSHw@mail.gmail.com>
Date: Wed, 12 Mar 2025 09:32:33 +0100
From: Uros Bizjak <ubizjak@...il.com>
To: John Paul Adrian Glaubitz <glaubitz@...sik.fu-berlin.de>
Cc: 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

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.

Uros.