[RFC] Proposal of marker implementation

[Masami Hiramatsu <masami dot hiramatsu dot pt at hitachi dot com>]

Hi,

I'd like to suggest my marker idea which I spoke in OLS.
My idea is based on the "section" of elf binary and the djprobe.

Here is the concept code on i386 architecture.
---
#define __MARKER_NOP(name) \
        asm volatile ("771:\n\t" ASM_NOP6 "\n772:\n"            \
                      ".section .markers,\"a\"\n"               \
                      "  .align 4\n"                            \
                      "  .long 771b\n"            /* label */   \
                      "  .byte 772b-771b\n"       /* length */  \
                      "  .string \"" #name "\"\n" /* name */    \
                      ".previous\n"                             \
                      ::: "memory")

#define MARKER(n) __MARKER_NOP(n)
---

This code comes from the "alternative" macros in the asm/alternative.h.
We can extract this marker's information by using the readelf command as below;

---
$ readelf -x ".markers" marker.ko
Hex dump of section '.markers':
  0x00000000 00000010 00000074 696e6906 00000003 .....init.......
  0x00000010                       0074 69786506 .exit.
---

This section contains two markers;
mark address = 00000003
mark length  = 06
marker name  = 696e697400 = "init"

mark address = 00000010
mark length  = 06
marker name  = 6578697400 = "exit"

So, we can check it by using the "objdump" command.
---
$ objdump -d marker.ko
marker.ko:     file format elf32-i386

Disassembly of section .text:

00000000 <init_module>:
   0:	55                   	push   %ebp
   1:	89 e5                	mov    %esp,%ebp
   3:	8d b6 00 00 00 00    	lea    0x0(%esi),%esi <--- here is "init" marker
   9:	31 c0                	xor    %eax,%eax
   b:	5d                   	pop    %ebp
   c:	c3                   	ret

0000000d <cleanup_module>:
   d:	55                   	push   %ebp
   e:	89 e5                	mov    %esp,%ebp
  10:	8d b6 00 00 00 00    	lea    0x0(%esi),%esi <--- here is "exit" marker
  16:	5d                   	pop    %ebp
  17:	c3                   	ret
---
Each marker has 6bytes NOP. I think we can replace it with a jump code
safely by using djprobe. Note: we can not replace it with a "call" code
because it will break some caller-save registers.

I think this marker has many advantages.
- Minimal overhead (don't touch any memory if it is deactivated)
- Multiple markers can have the same name (we can activate those at once)
- There are no additional marking symbols in the kernel.
- Easily extensible format (we can add some additional information in the section)
Also has some disadvantages.
- Architecture dependency (but the "section" itself doesn't depend on the arch)
- Need djprobe for safety (we can use kprobes until it goes fine)
Any comments are welcome.

By the way, flight recorder patches and STPtracer (re-implementation
of LKST on SystemTap) package are now under prior examination by
PR members.
So, I'll send it in the week after next.

Best regards,
-- 
Masami HIRAMATSU
2nd Research Dept.
Hitachi, Ltd., Systems Development Laboratory
E-mail: masami.hiramatsu.pt@hitachi.com