extable, core_kernel_data(): Make sure all archs define _sdata

A new utility function (core_kernel_data()) is used to determine if a
passed in address is part of core kernel data or not. It may or may not
return true for RO data, but this utility must work for RW data.

Thus both _sdata and _edata must be defined and continuous,
without .init sections that may later be freed and replaced by
volatile memory (memory that can be freed).

This utility function is used to determine if data is safe from
ever being freed. Thus it should return true for all RW global
data that is not in a module or has been allocated, or false
otherwise.

Also change core_kernel_data() back to the more precise _sdata condition
and document the function.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Acked-by: Ralf Baechle <ralf@linux-mips.org>
Acked-by: Hirokazu Takata <takata@linux-m32r.org>
Cc: Richard Henderson <rth@twiddle.net>
Cc: Ivan Kokshaysky <ink@jurassic.park.msu.ru>
Cc: Matt Turner <mattst88@gmail.com>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Roman Zippel <zippel@linux-m68k.org>
Cc: linux-m68k@lists.linux-m68k.org
Cc: Kyle McMartin <kyle@mcmartin.ca>
Cc: Helge Deller <deller@gmx.de>
Cc: JamesE.J.Bottomley <jejb@parisc-linux.org>
Link: http://lkml.kernel.org/r/1305855298.1465.19.camel@gandalf.stny.rr.com
Signed-off-by: Ingo Molnar <mingo@elte.hu>
----
 arch/alpha/kernel/vmlinux.lds.S   |    1 +
 arch/m32r/kernel/vmlinux.lds.S    |    1 +
 arch/m68k/kernel/vmlinux-std.lds  |    2 ++
 arch/m68k/kernel/vmlinux-sun3.lds |    1 +
 arch/mips/kernel/vmlinux.lds.S    |    1 +
 arch/parisc/kernel/vmlinux.lds.S  |    3 +++
 kernel/extable.c                  |   12 +++++++++++-
 7 files changed, 20 insertions(+), 1 deletion(-)

arch/alpha/kernel/vmlinux.lds.S

@@ -46,6 +46,7 @@ SECTIONS
 	__init_end = .;
 	/* Freed after init ends here */
 
+	_sdata = .;	/* Start of rw data section */
 	_data = .;
 	RW_DATA_SECTION(L1_CACHE_BYTES, PAGE_SIZE, THREAD_SIZE)
 

arch/m32r/kernel/vmlinux.lds.S

@@ -44,6 +44,7 @@ SECTIONS
   EXCEPTION_TABLE(16)
   NOTES
 
+  _sdata = .;			/* Start of data section */
   RODATA
   RW_DATA_SECTION(32, PAGE_SIZE, THREAD_SIZE)
   _edata = .;			/* End of data section */

arch/m68k/kernel/vmlinux-std.lds

@@ -25,6 +25,8 @@ SECTIONS
 
   EXCEPTION_TABLE(16)
 
+  _sdata = .;			/* Start of data section */
+
   RODATA
 
   RW_DATA_SECTION(16, PAGE_SIZE, THREAD_SIZE)

arch/m68k/kernel/vmlinux-sun3.lds

@@ -25,6 +25,7 @@ SECTIONS
   _etext = .;			/* End of text section */
 
   EXCEPTION_TABLE(16) :data
+  _sdata = .;			/* Start of rw data section */
   RW_DATA_SECTION(16, PAGE_SIZE, THREAD_SIZE) :data
   /* End of data goes *here* so that freeing init code works properly. */
   _edata = .;

arch/mips/kernel/vmlinux.lds.S

@@ -65,6 +65,7 @@ SECTIONS
 	NOTES :text :note
 	.dummy : { *(.dummy) } :text
 
+	_sdata = .;			/* Start of data section */
 	RODATA
 
 	/* writeable */

arch/parisc/kernel/vmlinux.lds.S

@@ -69,6 +69,9 @@ SECTIONS
 	/* End of text section */
 	_etext = .;
 
+	/* Start of data section */
+	_sdata = .;
+
 	RODATA
 
 	/* writeable */

kernel/extable.c

@@ -72,9 +72,19 @@ int core_kernel_text(unsigned long addr)
 	return 0;
 }
 
+/**
+ * core_kernel_data - tell if addr points to kernel data
+ * @addr: address to test
+ *
+ * Returns true if @addr passed in is from the core kernel data
+ * section.
+ *
+ * Note: On some archs it may return true for core RODATA, and false
+ *  for others. But will always be true for core RW data.
+ */
 int core_kernel_data(unsigned long addr)
 {
-	if (addr >= (unsigned long)_stext &&
+	if (addr >= (unsigned long)_sdata &&
 	    addr < (unsigned long)_edata)
 		return 1;
 	return 0;