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SubjectRe: pointer to .subsection and .previous usage in kernel?
On Thu, 27 Feb 2003, Steve Kenton wrote:

> Anyone have a pointer to what we are really trying to accomplish
> with .subsection and .previous in the in-line and asm stuff?
> Like maybe a readme or coding style doc for in-line and asm code?
> That was the main thing I tripped over while trying to build
> a 2.5 kernel using the cygwin toolchain with an eye towards
> uml under windows. I found very little documentation about
> the directives themselves even in the gnu info stuff and nada,
> except one thread about abstracting them in spinlocks,
> bout how/why they are used in the kernel. They appear to be
> elf specific which is why the i386pe stuff puked on them. I
> assume that most of this is to maximize cache hits, but that's
> just a guess. There are a largish handful of them scattered
> around the kernel outside of locks. Any pointers would be appreciated.
> Steve
> -

The data storage exists in ".sections". The common ones are:

.text Where the code is
.bss Data initialized upon startup to zeros.
.data Where non-local data are kept.
.rodata Where non-writable strings are put.
.othernames Defined by the implementor so that it can
remain isolated.

The "othernames" are things like __ex_table and _text_lock.
A subsection is a finer-granularity section. They can't be
named, only numbered. Since each of the ".sections" are
separate, one can set up a specific trap on a section. For
instance, one can grab an out-of-bounds access in a ".section"
without seg-faulting the whole kernel. This helps provide
for a recovery mechanism should the kernel try to write to
an invalid user-provided buffer.

The keyword ".previous" is shorthand for just that. It tells
the assembler to emit data and/or code according to the previous

If you are writing code under cygwin, you should not be encountering
spin-locks and kernel-specific things. Perhaps you are including
the wrong header files? You should never do:

#include <linux/xxx.h>
#include <asm/xxx.h>

... unless you are writing modules. And you don't do that in cygwin.

Dick Johnson
Penguin : Linux version 2.4.18 on an i686 machine (797.90 BogoMips).
Why is the government concerned about the lunatic fringe? Think about it.

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