why kernel do eight times nop ?

[Valdis.Kletnieks at vt.edu]

On Thu, 15 May 2014 14:19:25 +0800, Chuck Huang said:
> hi All,
>
> is anyone can explain why kernel do eight times nop in
> arch/arm/boot/compressed/head.S ?
>
> 128 start:
> 129                 .type   start,#function
> 130                 .rept   8
> 131                 mov     r0, r0
> 132                 .endr
>
> what's the purpose of that ?

With 'git', you could figure that out yourself....

git blame arch/arm/boot/compressed/head.S     says:

^1da177e4 (Linus Torvalds                 2005-04-16 15:20:36 -0700  118) /*
^1da177e4 (Linus Torvalds                 2005-04-16 15:20:36 -0700  119)  * sort out different calling conventions
^1da177e4 (Linus Torvalds                 2005-04-16 15:20:36 -0700  120)  */
^1da177e4 (Linus Torvalds                 2005-04-16 15:20:36 -0700  121)               .align
26e5ca93d (Dave Martin                    2010-11-29 19:43:27 +0100  122)               .arm                            @ Always enter in ARM state
^1da177e4 (Linus Torvalds                 2005-04-16 15:20:36 -0700  123) start:
^1da177e4 (Linus Torvalds                 2005-04-16 15:20:36 -0700  124)               .type   start,#function
b11fe3888 (Nicolas Pitre                  2011-02-12 22:25:27 +0100  125)               .rept   7
^1da177e4 (Linus Torvalds                 2005-04-16 15:20:36 -0700  126)               mov     r0, r0
^1da177e4 (Linus Torvalds                 2005-04-16 15:20:36 -0700  127)               .endr
b11fe3888 (Nicolas Pitre                  2011-02-12 22:25:27 +0100  128)    ARM(               mov     r0, r0          )
b11fe3888 (Nicolas Pitre                  2011-02-12 22:25:27 +0100  129)    ARM(               b       1f              )
b11fe3888 (Nicolas Pitre                  2011-02-12 22:25:27 +0100  130)  THUMB(               adr     r12, BSYM(1f)   )
b11fe3888 (Nicolas Pitre                  2011-02-12 22:25:27 +0100  131)  THUMB(               bx      r12             )
^1da177e4 (Linus Torvalds                 2005-04-16 15:20:36 -0700  132)

So the .rept 7 and related were added in commit b11fe38888 in Feb 2011.

git log b11fe3888

commit b11fe38883d1de76f2f847943e085a808f83f189
Author: Nicolas Pitre <nico at fluxnic.net>
Date:   Sat Feb 12 22:25:27 2011 +0100

    ARM: 6663/1: make Thumb2 kernel entry point more similar to the ARM one

    Some installers would binary patch the kernel zImage to replace the
    first few nops with custom instructions.  This breaks the Thumb2 kernel
    as the mode switch is right at the beginning.  Let's move it towards the
    end of the nop sequence instead.

    Signed-off-by: Nicolas Pitre <nicolas.pitre at linaro.org>
    Signed-off-by: Russell King <rmk+kernel at arm.linux.org.uk>

So there you have it - it's a nop sled for installers that would otherwise
trample the start of executable code.

If you're still confused, you'll have to ask Nicolas or Russell or an actual
ARM expert. :)
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