[PATCH 0/7] gcov kernel support

[PATCH 0/7] gcov kernel support

[Peter Oberparleiter]

This is version #2 of the gcov kernel support patch set (see further
below for an in-depth explanation). Patch base is 2.6.26-rc3.

Changes are mostly based on mailing list feedback:

 * added more documentation
 * moved gcov config menu to general setup
 * renamed some structures and variables (removed _t, lowercase ctors)
 * split mod versioning change into separate patch (3)
 * split GCC_VERSION_LOWER into separate patch (2)
 * split gcov core patch into two patches (5+6) for better readability
 * absolute paths only used when CONFIG_GCOV_PROFILE is set
 * fixed compile problem with CONFIG_MODULES=n
 * fixed memory leak in gcov fs code
 * more style changes

Patch overview:

[PATCH 1/7] kernel: call constructors
[PATCH 2/7] kernel: introduce GCC_VERSION_LOWER macro
[PATCH 3/7] kbuild: delay object file renaming during module versioning
[PATCH 4/7] seq_file: add function to write binary data
[PATCH 5/7] gcov: add gcov profiling infrastructure
[PATCH 6/7] kbuild: make source and include paths absolute
[PATCH 7/7] gcov: architecture specific compile flag adjustments

===

This set of patches enables the use of GCC's coverage testing tool
gcov [1] with kernel 2.6.25. Coverage data for the running kernel is
exported via debugfs in a gcov-compatible format. To get coverage
data for a specific file, simply use gcov with the -o option:

# gcov -o /sys/kernel/debug/gcov/tmp/linux-2.6.25/kernel spinlock.c

This will create source code files annotated with execution counts
in the current directory. In addition, graphical gcov front-ends such
as lcov [2] can be used to automate the process of collecting data
for the entire kernel and provide coverage overviews in HTML format.

Possible uses:

* debugging (has this line been executed at all?)
* test improvement (how do I change my test to cover these lines?)
* minimizing kernel configurations (do I need this option if the
  associated code is never executed?)


Known issues:

* some architecture specific problems: the patch has been tested
  successfully on s390 and i386. Known problems exist on x86_64 and
  arm (to be investigated)
* GCC's profiling mechanism together with optimization sometimes
  produces skewed data (see [1])
* GCC's profiling code assumes single-threaded execution
* gcov assumes that a program has finished when coverage data is
  analyzed

Despite these issues, the data which can be obtained has been proven
to be sufficiently accurate for most practical uses.


History:

Hubertus Franke <frankeh@us.ibm.com> wrote the first version of this
patch around 2002. Since then it has been adapted to new versions of
the kernel and GCC with contributions by several people (see file
kernel/gcov/fs.c, write me if I missed anyone). Due to regular
requests, I rewrote the gcov-kernel patch from scratch so that it
would (hopefully) be fit for inclusion into the upstream kernel.

--
[1] http://gcc.gnu.org/onlinedocs/gcc/Gcov.html
[2] http://ltp.sourceforge.net/coverage/lcov.php

Re: [PATCH 0/7] gcov kernel support

[Andi Kleen]

Peter Oberparleiter <peter.oberparleiter@de.ibm.com> writes:

> This is version #2 of the gcov kernel support patch set (see further
> below for an in-depth explanation). Patch base is 2.6.26-rc3.

I reviewed the patches and they look good to me (except for
some minor nits, probably not worth fixing)

-Andi

Re: [PATCH 0/7] gcov kernel support

[Andrew Morton]

On Mon, 19 May 2008 10:42:54 +0200 Peter Oberparleiter <peter.oberparleiter@de.ibm.com> wrote:

> This is version #2 of the gcov kernel support patch set

powerpc blew up:

/opt/crosstool/gcc-4.1.0-glibc-2.3.6/powerpc64-unknown-linux-gnu/lib/gcc/powerpc64-unknown-linux-gnu/4.1.0/../../../../powerpc64-unknown-linux-gnu/bin/ld: skipping incompatible /opt/crosstool/gcc-4.1.0-glibc-2.3.6/powerpc64-unknown-linux-gnu/lib/gcc/powerpc64-unknown-linux-gnu/4.1.0/./libgcov.a when searching for -lgcov
/opt/crosstool/gcc-4.1.0-glibc-2.3.6/powerpc64-unknown-linux-gnu/lib/gcc/powerpc64-unknown-linux-gnu/4.1.0/../../../../powerpc64-unknown-linux-gnu/bin/ld: cannot find -lgcov
collect2: ld returned 1 exit status
make[2]: *** [arch/powerpc/kernel/vdso32/vdso32.so.dbg] Error 1
make[1]: *** [arch/powerpc/kernel/vdso32] Error 2

Re: [PATCH 0/7] gcov kernel support

[Segher Boessenkool]

>> This is version #2 of the gcov kernel support patch set
>
> powerpc blew up:
>
> /opt/crosstool/gcc-4.1.0-glibc-2.3.6/powerpc64-unknown-linux-gnu/lib/ 
> gcc/powerpc64-unknown-linux-gnu/4.1.0/../../../../powerpc64-unknown- 
> linux-gnu/bin/ld: skipping incompatible  
> /opt/crosstool/gcc-4.1.0-glibc-2.3.6/powerpc64-unknown-linux-gnu/lib/ 
> gcc/powerpc64-unknown-linux-gnu/4.1.0/./libgcov.a when searching for  
> -lgcov
> /opt/crosstool/gcc-4.1.0-glibc-2.3.6/powerpc64-unknown-linux-gnu/lib/ 
> gcc/powerpc64-unknown-linux-gnu/4.1.0/../../../../powerpc64-unknown- 
> linux-gnu/bin/ld: cannot find -lgcov
> collect2: ld returned 1 exit status
> make[2]: *** [arch/powerpc/kernel/vdso32/vdso32.so.dbg] Error 1
> make[1]: *** [arch/powerpc/kernel/vdso32] Error 2

Sounds like your toolchain was built without biarch support, but you're
using it for the 32-bit parts of the kernel anyway.  Which works as long
as you're not using any libraries, but now you need libgcov.a .

What does

/opt/crosstool/gcc-4.1.0-glibc-2.3.6/powerpc64-unknown-linux-gnu/bin/ 
powerpc64-unknown-linux-gnu-gcc -v

say?  (Or whatever the correct path to that compiler is, the directory
structure is a bit unusual here it seems).  There shouldn't be a
--disable-biarch there.

Or, what does

find  
/opt/crosstool/gcc-4.1.0-glibc-2.3.6/powerpc64-unknown-linux-gnu/4.1.0/  
-name libgcov.a

say?  (It should usually find three versions: 64-bit, 32-bit, 32-bit no  
float).


You *can* build with a non-biarch compiler, but then you need to point
the kernel build system at a 32-bit toolchain to use for the 32-bit
parts (via CROSS32_COMPILE).


Segher

Re: [PATCH 0/7] gcov kernel support

[Andrew Morton]

On Fri, 23 May 2008 13:06:00 +0200 Segher Boessenkool <segher@kernel.crashing.org> wrote:

> >> This is version #2 of the gcov kernel support patch set
> >
> > powerpc blew up:
> >
> > /opt/crosstool/gcc-4.1.0-glibc-2.3.6/powerpc64-unknown-linux-gnu/lib/ 
> > gcc/powerpc64-unknown-linux-gnu/4.1.0/../../../../powerpc64-unknown- 
> > linux-gnu/bin/ld: skipping incompatible  
> > /opt/crosstool/gcc-4.1.0-glibc-2.3.6/powerpc64-unknown-linux-gnu/lib/ 
> > gcc/powerpc64-unknown-linux-gnu/4.1.0/./libgcov.a when searching for  
> > -lgcov
> > /opt/crosstool/gcc-4.1.0-glibc-2.3.6/powerpc64-unknown-linux-gnu/lib/ 
> > gcc/powerpc64-unknown-linux-gnu/4.1.0/../../../../powerpc64-unknown- 
> > linux-gnu/bin/ld: cannot find -lgcov
> > collect2: ld returned 1 exit status
> > make[2]: *** [arch/powerpc/kernel/vdso32/vdso32.so.dbg] Error 1
> > make[1]: *** [arch/powerpc/kernel/vdso32] Error 2
> 
> Sounds like your toolchain was built without biarch support, but you're
> using it for the 32-bit parts of the kernel anyway.  Which works as long
> as you're not using any libraries, but now you need libgcov.a .

ok..

> What does
> 
> /opt/crosstool/gcc-4.1.0-glibc-2.3.6/powerpc64-unknown-linux-gnu/bin/ 
> powerpc64-unknown-linux-gnu-gcc -v
> 
> say?

y:/home/akpm> /opt/crosstool/gcc-4.1.0-glibc-2.3.6/powerpc64-unknown-linux-gnu/bin/powerpc64-unknown-linux-gnu-gcc -v
Using built-in specs.
Target: powerpc64-unknown-linux-gnu
Configured with: /home/axboe/crosstool-0.43/build/powerpc64-unknown-linux-gnu/gcc-4.1.0-glibc-2.3.6/gcc-4.1.0/configure --target=powerpc64-unknown-linux-gnu --host=x86_64-host_unknown-linux-gnu --prefix=/opt/crosstool/gcc-4.1.0-glibc-2.3.6/powerpc64-unknown-linux-gnu --disable-multilib --with-sysroot=/opt/crosstool/gcc-4.1.0-glibc-2.3.6/powerpc64-unknown-linux-gnu/powerpc64-unknown-linux-gnu/sys-root --with-local-prefix=/opt/crosstool/gcc-4.1.0-glibc-2.3.6/powerpc64-unknown-linux-gnu/powerpc64-unknown-linux-gnu/sys-root --disable-nls --enable-threads=posix --enable-symvers=gnu --enable-__cxa_atexit --enable-languages=c --enable-shared --enable-c99 --enable-long-long
Thread model: posix
gcc version 4.1.0

>  (Or whatever the correct path to that compiler is, the directory
> structure is a bit unusual here it seems).  There shouldn't be a
> --disable-biarch there.
> 
> Or, what does
> 
> find  
> /opt/crosstool/gcc-4.1.0-glibc-2.3.6/powerpc64-unknown-linux-gnu/4.1.0/  
> -name libgcov.a
> 
> say?  (It should usually find three versions: 64-bit, 32-bit, 32-bit no  
> float).

Only a single hit:

/opt/crosstool/gcc-4.1.0-glibc-2.3.6/powerpc64-unknown-linux-gnu/lib/gcc/powerpc64-unknown-linux-gnu/4.1.0/libgcov.a

and it contains

y:/home/akpm/ppc> ar x libgcov.a 
y:/home/akpm/ppc> l
total 316
-rw-r--r-- 1 akpm akpm  44192 May 23 09:53 _gcov.o
-rw-r--r-- 1 akpm akpm   8448 May 23 09:53 _gcov_execl.o
-rw-r--r-- 1 akpm akpm   8584 May 23 09:53 _gcov_execle.o
-rw-r--r-- 1 akpm akpm   8456 May 23 09:53 _gcov_execlp.o
-rw-r--r-- 1 akpm akpm   7384 May 23 09:53 _gcov_execv.o
-rw-r--r-- 1 akpm akpm   7544 May 23 09:53 _gcov_execve.o
-rw-r--r-- 1 akpm akpm   7392 May 23 09:53 _gcov_execvp.o
-rw-r--r-- 1 akpm akpm   7264 May 23 09:53 _gcov_fork.o
-rw-r--r-- 1 akpm akpm   7240 May 23 09:53 _gcov_interval_profiler.o
-rw-r--r-- 1 akpm akpm   7464 May 23 09:53 _gcov_merge_add.o
-rw-r--r-- 1 akpm akpm   8544 May 23 09:53 _gcov_merge_delta.o
-rw-r--r-- 1 akpm akpm   8488 May 23 09:53 _gcov_merge_single.o
-rw-r--r-- 1 akpm akpm   6976 May 23 09:53 _gcov_one_value_profiler.o
-rw-r--r-- 1 akpm akpm   6856 May 23 09:53 _gcov_pow2_profiler.o
-rw-r--r-- 1 akpm akpm 146492 May 23 09:52 libgcov.a
y:/home/akpm/ppc> file _gcov_merge_delta.o
_gcov_merge_delta.o: ELF 64-bit MSB relocatable, 64-bit PowerPC or cisco 7500, version 1 (SYSV), not stripped

> 
> You *can* build with a non-biarch compiler, but then you need to point
> the kernel build system at a 32-bit toolchain to use for the 32-bit
> parts (via CROSS32_COMPILE).

Jens built this one, using crosstool.  It is
http://userweb.kernel.org/~akpm/cross-compilers/power64-cross.tar.bz2

It was all working happily until gcov came along :(

Re: [PATCH 0/7] gcov kernel support

[Segher Boessenkool]

>> Sounds like your toolchain was built without biarch support, but  
>> you're
>> using it for the 32-bit parts of the kernel anyway.  Which works as  
>> long
>> as you're not using any libraries, but now you need libgcov.a .
>
> ok..
>
>> What does
>>
>> /opt/crosstool/gcc-4.1.0-glibc-2.3.6/powerpc64-unknown-linux-gnu/bin/
>> powerpc64-unknown-linux-gnu-gcc -v
>>
>> say?
>
> y:/home/akpm>  
> /opt/crosstool/gcc-4.1.0-glibc-2.3.6/powerpc64-unknown-linux-gnu/bin/ 
> powerpc64-unknown-linux-gnu-gcc -v
> Using built-in specs.
> Target: powerpc64-unknown-linux-gnu
> Configured with:  
> /home/axboe/crosstool-0.43/build/powerpc64-unknown-linux-gnu/gcc 
> -4.1.0-glibc-2.3.6/gcc-4.1.0/configure  
> --target=powerpc64-unknown-linux-gnu  
> --host=x86_64-host_unknown-linux-gnu  
> --prefix=/opt/crosstool/gcc-4.1.0-glibc-2.3.6/powerpc64-unknown-linux- 
> gnu

> --disable-multilib

That's the culprit.  I believe 4.1.0 enables multilib by default on  
PowerPC
compilers that target 64-bit by default (the patch for that is dated  
almost
a full year before the 4.1.0 release).  I have no idea why crosstool
disables this support; perhaps there were some problems with some older
GCC version (or this one, even; it's ancient as well).

>> Or, what does
>>
>> find
>> /opt/crosstool/gcc-4.1.0-glibc-2.3.6/powerpc64-unknown-linux-gnu/ 
>> 4.1.0/
>> -name libgcov.a
>>
>> say?  (It should usually find three versions: 64-bit, 32-bit, 32-bit  
>> no
>> float).
>
> Only a single hit:

Right, as expected.

>> You *can* build with a non-biarch compiler, but then you need to point
>> the kernel build system at a 32-bit toolchain to use for the 32-bit
>> parts (via CROSS32_COMPILE).
>
> Jens built this one, using crosstool.  It is
> http://userweb.kernel.org/~akpm/cross-compilers/power64-cross.tar.bz2
>
> It was all working happily until gcov came along :(

You were lucky, yes.  If your compiler would have been just a little
bit older it wouldn't have been able to build 32-bit at all.

Do you have a separate 32-bit PowerPC compiler?  If so, just set
CROSS32_COMPILE to it; if not, you'll need a new toolchain.  In
that case, it's probably worthwhile to upgrade to 4.1.2 at least...


Segher

Re: [PATCH 0/7] gcov kernel support

[Andrew Morton]

On Fri, 23 May 2008 23:52:43 +0200
Segher Boessenkool <segher@kernel.crashing.org> wrote:

> >> You *can* build with a non-biarch compiler, but then you need to point
> >> the kernel build system at a 32-bit toolchain to use for the 32-bit
> >> parts (via CROSS32_COMPILE).
> >
> > Jens built this one, using crosstool.  It is
> > http://userweb.kernel.org/~akpm/cross-compilers/power64-cross.tar.bz2
> >
> > It was all working happily until gcov came along :(
> 
> You were lucky, yes.  If your compiler would have been just a little
> bit older it wouldn't have been able to build 32-bit at all.
> 
> Do you have a separate 32-bit PowerPC compiler?

I dont' cross-compile for 32-bit power.  This is all 64-bit stuff.

>  If so, just set
> CROSS32_COMPILE to it; if not, you'll need a new toolchain.  In
> that case, it's probably worthwhile to upgrade to 4.1.2 at least...

Sigh.  Life's too short.  I'll go back to testing powerpc builds on the
powerpc box.

Re: [PATCH 0/7] gcov kernel support

[Peter Oberparleiter]

Andrew Morton wrote:
> On Mon, 19 May 2008 10:42:54 +0200 Peter Oberparleiter <peter.oberparleiter@de.ibm.com> wrote:
> 
>> This is version #2 of the gcov kernel support patch set
> 
> powerpc blew up:
> 
> /opt/crosstool/gcc-4.1.0-glibc-2.3.6/powerpc64-unknown-linux-gnu/lib/gcc/powerpc64-unknown-linux-gnu/4.1.0/../../../../powerpc64-unknown-linux-gnu/bin/ld: skipping incompatible /opt/crosstool/gcc-4.1.0-glibc-2.3.6/powerpc64-unknown-linux-gnu/lib/gcc/powerpc64-unknown-linux-gnu/4.1.0/./libgcov.a when searching for -lgcov
> /opt/crosstool/gcc-4.1.0-glibc-2.3.6/powerpc64-unknown-linux-gnu/lib/gcc/powerpc64-unknown-linux-gnu/4.1.0/../../../../powerpc64-unknown-linux-gnu/bin/ld: cannot find -lgcov
> collect2: ld returned 1 exit status
> make[2]: *** [arch/powerpc/kernel/vdso32/vdso32.so.dbg] Error 1
> make[1]: *** [arch/powerpc/kernel/vdso32] Error 2

This looks like a secondary linking process. It's best to exclude the
whole directory from GCOV profiling (which should ony work for C files
anyway).

Below is an update for patch #7 of the gcov series that includes the
corresponding changes.

---

[PATCH 7/7] gcov: architecture specific compile flag adjustments

From: Peter Oberparleiter <peter.oberparleiter@de.ibm.com>

Exclude code from profiling which is not linked to the main kernel or
which is running in a context not compatible with the assumptions made
by gcc's profiling code.

Signed-off-by: Peter Oberparleiter <peter.oberparleiter@de.ibm.com>
---
 arch/arm/boot/compressed/Makefile      |    1 +
 arch/powerpc/kernel/vdso32/Makefile    |    1 +
 arch/powerpc/kernel/vdso64/Makefile    |    1 +
 arch/ppc/boot/Makefile                 |    1 +
 arch/ppc/boot/common/Makefile          |    1 +
 arch/ppc/boot/simple/Makefile          |    1 +
 arch/x86/boot/Makefile                 |    1 +
 arch/x86/boot/compressed/Makefile      |    1 +
 arch/x86/kernel/acpi/realmode/Makefile |    1 +
 9 files changed, 9 insertions(+)

Index: linux-2.6.26-rc3/arch/arm/boot/compressed/Makefile
===================================================================
--- linux-2.6.26-rc3.orig/arch/arm/boot/compressed/Makefile
+++ linux-2.6.26-rc3/arch/arm/boot/compressed/Makefile
@@ -7,6 +7,7 @@
 HEAD	= head.o
 OBJS	= misc.o
 FONTC	= $(srctree)/drivers/video/console/font_acorn_8x8.c
+GCOV	:= n
 
 #
 # Architecture dependencies
Index: linux-2.6.26-rc3/arch/x86/boot/compressed/Makefile
===================================================================
--- linux-2.6.26-rc3.orig/arch/x86/boot/compressed/Makefile
+++ linux-2.6.26-rc3/arch/x86/boot/compressed/Makefile
@@ -12,6 +12,7 @@ cflags-$(CONFIG_X86_64) := -mcmodel=smal
 KBUILD_CFLAGS += $(cflags-y)
 KBUILD_CFLAGS += $(call cc-option,-ffreestanding)
 KBUILD_CFLAGS += $(call cc-option,-fno-stack-protector)
+GCOV := n
 
 KBUILD_AFLAGS  := $(KBUILD_CFLAGS) -D__ASSEMBLY__
 
Index: linux-2.6.26-rc3/arch/ppc/boot/Makefile
===================================================================
--- linux-2.6.26-rc3.orig/arch/ppc/boot/Makefile
+++ linux-2.6.26-rc3/arch/ppc/boot/Makefile
@@ -16,6 +16,7 @@
 # KBUILD_CFLAGS used when building rest of boot (takes effect recursively)
 KBUILD_CFLAGS 	+= -fno-builtin -D__BOOTER__ -Iarch/$(ARCH)/boot/include
 HOSTCFLAGS	+= -Iarch/$(ARCH)/boot/include
+GCOV		:= n
 
 BOOT_TARGETS	= zImage zImage.initrd znetboot znetboot.initrd
 
Index: linux-2.6.26-rc3/arch/ppc/boot/common/Makefile
===================================================================
--- linux-2.6.26-rc3.orig/arch/ppc/boot/common/Makefile
+++ linux-2.6.26-rc3/arch/ppc/boot/common/Makefile
@@ -5,6 +5,7 @@
 # Tom Rini	January 2001
 #
 
+GCOV := n
 lib-y					:= string.o util.o misc-common.o \
 						serial_stub.o bootinfo.o
 lib-$(CONFIG_SERIAL_8250_CONSOLE)	+= ns16550.o
Index: linux-2.6.26-rc3/arch/ppc/boot/simple/Makefile
===================================================================
--- linux-2.6.26-rc3.orig/arch/ppc/boot/simple/Makefile
+++ linux-2.6.26-rc3/arch/ppc/boot/simple/Makefile
@@ -21,6 +21,7 @@
 # XXX_memory.o file for this to work, as well as editing the
 # misc-$(CONFIG_MACHINE) variable.
 
+GCOV				:= n
 boot				:= arch/ppc/boot
 common				:= $(boot)/common
 utils				:= $(boot)/utils
Index: linux-2.6.26-rc3/arch/x86/boot/Makefile
===================================================================
--- linux-2.6.26-rc3.orig/arch/x86/boot/Makefile
+++ linux-2.6.26-rc3/arch/x86/boot/Makefile
@@ -71,6 +71,7 @@ KBUILD_CFLAGS	:= $(LINUXINCLUDE) -g -Os 
 		   $(call cc-option, -mpreferred-stack-boundary=2)
 KBUILD_CFLAGS +=   $(call cc-option,-m32)
 KBUILD_AFLAGS	:= $(KBUILD_CFLAGS) -D__ASSEMBLY__
+GCOV		:= n
 
 $(obj)/zImage:  IMAGE_OFFSET := 0x1000
 $(obj)/zImage:  asflags-y := $(SVGA_MODE) $(RAMDISK)
Index: linux-2.6.26-rc3/arch/x86/kernel/acpi/realmode/Makefile
===================================================================
--- linux-2.6.26-rc3.orig/arch/x86/kernel/acpi/realmode/Makefile
+++ linux-2.6.26-rc3/arch/x86/kernel/acpi/realmode/Makefile
@@ -42,6 +42,7 @@ KBUILD_CFLAGS	:= $(LINUXINCLUDE) -g -Os 
 		   $(call cc-option, -mpreferred-stack-boundary=2)
 KBUILD_CFLAGS	+= $(call cc-option, -m32)
 KBUILD_AFLAGS	:= $(KBUILD_CFLAGS) -D__ASSEMBLY__
+GCOV		:= n
 
 WAKEUP_OBJS = $(addprefix $(obj)/,$(wakeup-y))
 
Index: linux-2.6.26-rc3/arch/powerpc/kernel/vdso32/Makefile
===================================================================
--- linux-2.6.26-rc3.orig/arch/powerpc/kernel/vdso32/Makefile
+++ linux-2.6.26-rc3/arch/powerpc/kernel/vdso32/Makefile
@@ -17,6 +17,7 @@ EXTRA_CFLAGS := -shared -fno-common -fno
 EXTRA_CFLAGS += -nostdlib -Wl,-soname=linux-vdso32.so.1 \
 		$(call ld-option, -Wl$(comma)--hash-style=sysv)
 EXTRA_AFLAGS := -D__VDSO32__ -s
+GCOV := n
 
 obj-y += vdso32_wrapper.o
 extra-y += vdso32.lds
Index: linux-2.6.26-rc3/arch/powerpc/kernel/vdso64/Makefile
===================================================================
--- linux-2.6.26-rc3.orig/arch/powerpc/kernel/vdso64/Makefile
+++ linux-2.6.26-rc3/arch/powerpc/kernel/vdso64/Makefile
@@ -11,6 +11,7 @@ EXTRA_CFLAGS := -shared -fno-common -fno
 EXTRA_CFLAGS += -nostdlib -Wl,-soname=linux-vdso64.so.1 \
 		$(call ld-option, -Wl$(comma)--hash-style=sysv)
 EXTRA_AFLAGS := -D__VDSO64__ -s
+GCOV := n
 
 obj-y += vdso64_wrapper.o
 extra-y += vdso64.lds