Fix asm for LTO on ARM

[arter97]

So, I've managed to merge LTO changes from this github repository to a ARM Linux 3.4 kernel.

With plugin enabled gcc-ar, I've managed to get almost every errors passed until this happened;

LDFINAL .tmp_vmlinux1
/tmp/ccxBqOkb.ltrans31.ltrans.o: In function kernel_thread': ccxBqOkb.ltrans31.o:(.text+0x6a74): undefined reference tokernel_thread_exit'
ccxBqOkb.ltrans31.o:(.text+0x6a78): undefined reference to kernel_thread_helper' ccxBqOkb.ltrans31.o:(.text+0x6a8c): undefined reference tokernel_thread_exit'
ccxBqOkb.ltrans31.o:(.text+0x6a90): undefined reference to kernel_thread_helper' kprobe_handler' referenced in section .kprobes.text' of /tmp/ccxBqOkb.ltrans31.ltrans.o: defined in discarded section.text' of arch/arm/kernel/built-in.o (symbol from plugin)
collect2: error: ld returned 1 exit status
make: *** [.tmp_vmlinux1] Error 1

The 'kernel_thread', 'kernel_thread_exit', 'kernel_thread_helper' is declared in arch/arm/kernel/process.c, but it's "asm" function seems to be not working with LTO.

How can I fix those errors??

Thanks!

PS1. My kernel has CONFIG_ARM_UNWIND enabled;
PS2. The latest Linux kernels apparently doesn't have those kernel_thread functions on arch/arm/kernel/process.c.

[arter97]

Anyone? :(

[peterbowey]

@arter97

It is possible you are using 'ld.gold' rather than the absolutely required 'ld.bfd' binutils linker...

I have proven that the this LTO kernel will not compile using the 'ld.gold', with similar errors to yours.

For me, under Fedora 20 X86_64, it is required that I do a "/usr/sbin/alternatives --set ld /usr/bin/ld.bfd" to change this 'state' for GCC 4.10

[arter97]

@peterbowey I'm already using bfd ld.
GNU ld (Linux/GNU Binutils) 2.24.51.0.3.20140127

ps. are you building for ARM?

[rmbq]

Fixed with this commit: https://github.com/rmbq/android_kernel_lge_hammerhead/commit/b174c8a001da10283c792414a51e5114627a4495

patch:

arch/arm/kernel/process.c | 2 ++
1 file changed, 2 insertions(+)

diff --git a/arch/arm/kernel/process.c b/arch/arm/kernel/process.c
index 4d24305..2160c96 100644
--- a/arch/arm/kernel/process.c
+++ b/arch/arm/kernel/process.c
@@ -582,6 +582,7 @@ extern void kernel_thread_helper(void);
asm( ".pushsection .text\n"
" .align\n"
" .type kernel_thread_helper, #function\n"
+" .global kernel_thread_helper\n"
"kernel_thread_helper:\n"
#ifdef CONFIG_TRACE_IRQFLAGS
" bl trace_hardirqs_on\n"
@@ -598,6 +599,7 @@ extern void kernel_thread_exit(long code);
asm( ".pushsection .text\n"
" .align\n"
" .type kernel_thread_exit, #function\n"
+" .global kernel_thread_exit\n"
"kernel_thread_exit:\n"
" .fnstart\n"
" .cantunwind\n"

[andikleen]

Thanks.

I checked upstream and it looks like the function has been removed completely there. So I assume it's resolved in the latest trees.