On Fri, 2015-01-09 at 17:57 +0000, Russell King - ARM Linux wrote:
[...]
> For me:
>
> $ echo 'asm ("bx r2\n");' | arm-linux-gcc -x c -c -marm -march=armv4 -v - -o
> o.o
>
> calls the assembler thusly:
>
> /usr/local/lib/gcc/arm-linux-gnueabi/4.7.4/../../../../arm-linux-gnueabi/bin/as
> \
> -v -march=armv4 -meabi=5 --fix-v4bx -o o.o /tmp/ccB0cZgO.s
>
> Sure enough, the object file contains:
>
> 00000000 <.text>:
> 0: e12fff12 bx r2
> 0: R_ARM_V4BX *ABS*
>
> so it looks like it's been told... Then if you do:
>
> $ arm-linux-ld --fix-v4bx -o o1.o o.o
> $ arm-linux-objdump -dr o1.o
>
> you get:
>
> 8074: e1a0f002 mov pc, r2
I get results consistent with what you get above. A bit of googling
seems to indicate the generate-BX-and-fix-it-in-the-linker came in many
years ago with AEABI support.
> Hmm, I wonder if this means we should have the kernel linker deal with
> V4BX relocations on ARMv4, converting them to their mov pc, X variant.
>
> Also, do we need --fix-v4bx for the link of vmlinux?
I guess the answer is yes if we want to catch uses of BX in inline
assembly. A quick and not very thorough grep of arch/arm for 'bx'
doesn't seem to turn up any existing dodgy uses, except in the kprobes
test code I wrote :-(
--
Tixy
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