.align may cause data to be interpreted as instructions

[Jon Medhurst (Tixy)]

On Wed, 2013-10-16 at 01:38 +0300, Taras Kondratiuk wrote:
> Hi
> 
> I was debugging kprobes-test for BE8 and noticed that some data fields
> are stored in LE instead of BE. It happens because these data fields
> get interpreted as instructions.
> 
> Is it a known issue?
> 
> For example:
> test_align_fail_data:
> 	bx	lr
> 	.byte 0xaa
> 	.align
> 	.word 0x12345678
> 
> I would expect to see something like this:
> 00000000 <test_align_fail_data>:
>    0:	e12fff1e 	bx	lr
>    4:	aa          	.byte	0xaa
>    5:	00          	.byte	0x00
>    6:	0000      	.short	0x0000
>    8:	12345678 	.word	0x12345678
> 
> But instead I have:
> 00000000 <test_align_fail_data>:
>    0:	e12fff1e 	bx	lr
>    4:	aa          	.byte	0xaa
>    5:	00          	.byte	0x00
>    6:	0000      	.short	0x0000
>    8:	12345678 	eorsne	r5, r4, #120, 12	; 0x7800000
> 
> As a result the word 0x12345678 will be stored in LE.

Hmm looks like a GCC bug, or we're doing something it considers
undefined behaviour?

> I've run several tests and here are my observations:
> - Double ".align" fixes the issue :)

Temping as a simple fix, but rather risky to rely on strange compiler
behaviour to fix another strange compiler error. :-)

> - Behavior is the same for LE/BE, ARM/Thumb, GCC 4.4.1/4.6.x/4.8.2

At least the compiler is being consistent and it's obviously an issue we
need to work around in the kernel.

> - Size of alignment doesn't matter.
> - Issue happens only if previous data is not instruction-aligned and
>     0's are added before NOPs.
> - Explicit filling with 0's (.align , 0) fixes the issue, but as a side
>     effect data @0x4 is interpreted as a single ".word 0xaa000000"
>     instead of ".byte .byte .short". I'm not sure if there can be any
>     functional difference because of this.
> - Issue doesn't happen if there is no instructions before data
>   (no "bx lr" in the example).
> - Issue doesn't happen if data after .align is defined as
>     ".type <symbol>,%object".

Seems like something which should be reported to the GCC people, and you
would think with the detailed diagnosis you've provided someone familiar
with the code would be able to quickly get to the bottom of it.

Meanwhile, I like the solution Den Dooks came up with, I'll comment
about that in my reply to his mail...

-- 
Tixy