Re: [PATCH] vDSO, kbuild: Provide vDSO debug variants at runtime

[H. Peter Anvin]

On July 9, 2026 2:57:09 AM PDT, Vincenzo Frascino <vincenzo.frascino@xxxxxxx> wrote:
>
>
>On 08/07/2026 14:56, Thomas Weißschuh wrote:
>> Finding the debug version of the vDSO is not trivial as there is no common
>> scheme where it is placed. That's especially problematic for CI testing.
>> 
>> The vDSO futex unlock mechanism requires for testing to have access to the
>> inner labels of the unlock assembly, which are only accessible via the
>> debug so.
>> 
>> Also for general debugging purposes it's convenient to have access to the
>> debug vDSO at a well defined place.
>> 
>> The files are placed in /sys/kernel/vdso_debug.tar.xz. They use the
>> regular 'make vdso_install' layout, including build-id symlinks to find
>> the correct file for each process.
>> 
>> The design is kept close to the ones of the similar IKCONFIG and IKHEADERS.
>> 
>> On x86 the x32 vDSO is derived from the x86_64 one, necessitating an
>> explicit dependency to avoid errors due to concurrent builds.
>> 
>> Suggested-by: Thomas Gleixner <tglx@xxxxxxxxxx>
>> Link: https://lore.kernel.org/lkml/20260602090536.045586688@xxxxxxxxxx/
>> Signed-off-by: Thomas Weißschuh <thomas.weissschuh@xxxxxxxxxxxxx>
>
>This is a really good idea. Thanks!
>
>Reviewed-by: Vincenzo Frascino <vincenzo.frascino@xxxxxxx>
>
>> ---
>>  Makefile            | 33 ++++++++++++++++++++++++++++++++-
>>  arch/x86/Makefile   |  3 +++
>>  init/Kconfig        | 10 ++++++++++
>>  kernel/Makefile     |  1 +
>>  kernel/vdso_debug.c | 43 +++++++++++++++++++++++++++++++++++++++++++
>>  5 files changed, 89 insertions(+), 1 deletion(-)
>> 
>> diff --git a/Makefile b/Makefile
>> index b9c5792c79e0..8049f10c27b2 100644
>> --- a/Makefile
>> +++ b/Makefile
>> @@ -298,7 +298,7 @@ no-dot-config-targets := $(clean-targets) \
>>  			 %asm-generic kernelversion %src-pkg dt_binding_check \
>>  			 dt_style_selftest \
>>  			 outputmakefile rustavailable rustfmt rustfmtcheck \
>> -			 run-command
>> +			 run-command have-vdso-debug
>>  no-sync-config-targets := $(no-dot-config-targets) %install modules_sign kernelrelease \
>>  			  image_name
>>  single-targets := %.a %.i %.ko %.lds %.ll %.lst %.mod %.o %.rsi %.s %/
>> @@ -1539,6 +1539,37 @@ vdso_install: export INSTALL_FILES = $(vdso-install-y)
>>  vdso_install:
>>  	$(Q)$(MAKE) -f $(srctree)/scripts/Makefile.vdsoinst
>>  
>> +# ---------------------------------------------------------------------------
>> +# vDSO embedded debug symbols
>> +
>> +PHONY += vdso_prepare
>> +
>> +# Build the targets in $(vdso-install-y)
>> +# Some architectures may do this already through vdso_prepare,
>> +# so add a dependency to avoid race conditions
>> +$(vdso-install-y): vdso_prepare prepare0 FORCE
>> +	@$(MAKE) $(build)=$(patsubst %/,%,$(dir $@)) $@
>> +
>> +tar-opts := --mtime='1970-01-01 00:00:00' --owner=0 --group=0 --sort=name --numeric-owner --mode=u=rw,go=r,a+X
>> +
>> +quiet_cmd_vdso_debug_tar_xz = VDSODBG $@
>> +      cmd_vdso_debug_tar_xz = \
>> +		rm -rf $(tmp-target); mkdir -p $(tmp-target)/vdso/; \
>> +		$(MAKE) quiet=@ MODLIB=$(tmp-target) vdso_install; \
>> +		$(TAR) $(tar-opts) -a -c -f $@ -C $(tmp-target)/vdso/ .
>> +
>> +targets += vdso_debug.tar.xz
>> +vdso_debug.tar.xz: $(vdso-install-y) FORCE
>> +	$(call if_changed,vdso_debug_tar_xz)
>> +
>> +ifdef CONFIG_IVDSODEBUG
>> +prepare: vdso_debug.tar.xz
>> +endif
>> +
>> +PHONY += have-vdso-debug
>> +have-vdso-debug:
>> +	@echo $(if $(vdso-install-y)$(vdso-install-),y,n)
>> +
>>  # ---------------------------------------------------------------------------
>>  # Tools
>>  
>> diff --git a/arch/x86/Makefile b/arch/x86/Makefile
>> index 598f178102ee..b762fddb620e 100644
>> --- a/arch/x86/Makefile
>> +++ b/arch/x86/Makefile
>> @@ -316,6 +316,9 @@ vdso-install-$(CONFIG_X86_64)	   += arch/x86/entry/vdso/vdso64/vdso64.so.dbg
>>  vdso-install-$(CONFIG_X86_X32_ABI) += arch/x86/entry/vdso/vdso64/vdsox32.so.dbg
>>  vdso-install-$(CONFIG_COMPAT_32)   += arch/x86/entry/vdso/vdso32/vdso32.so.dbg
>>  
>> +# vdsox32.so.dbg is derived from vdso64.so.dbg. Avoid races with CONFIG_IVDSODEBUG
>> +arch/x86/entry/vdso/vdso64/vdsox32.so.dbg: arch/x86/entry/vdso/vdso64/vdso64.so.dbg
>> +
>>  archprepare: checkbin
>>  checkbin:
>>  ifdef CONFIG_MITIGATION_RETPOLINE
>> diff --git a/init/Kconfig b/init/Kconfig
>> index 5230d4879b1c..f09132a44264 100644
>> --- a/init/Kconfig
>> +++ b/init/Kconfig
>> @@ -801,6 +801,16 @@ config IKHEADERS
>>  	  or similar programs.  If you build the headers as a module, a module called
>>  	  kheaders.ko is built which can be loaded on-demand to get access to headers.
>>  
>> +config IVDSODEBUG
>> +	tristate "Enable vDSO debug information through /sys/kernel/vdso_debug.tar.xz"
>> +	depends on SYSFS
>> +	depends on "$(shell,make have-vdso-debug)"
>> +	help
>> +	  This option enables access to the vDSO debug information. That can be
>> +	  used to debug applications and test the vDSO implementation.
>> +
>> +	  If configured as M the module will be called vdso_debug.ko.
>> +
>>  config LOG_BUF_SHIFT
>>  	int "Kernel log buffer size (16 => 64KB, 17 => 128KB)"
>>  	range 12 25
>> diff --git a/kernel/Makefile b/kernel/Makefile
>> index 1e1a31673577..8adcc26ef859 100644
>> --- a/kernel/Makefile
>> +++ b/kernel/Makefile
>> @@ -93,6 +93,7 @@ obj-$(CONFIG_USER_NS) += user_namespace.o
>>  obj-$(CONFIG_PID_NS) += pid_namespace.o
>>  obj-$(CONFIG_IKCONFIG) += configs.o
>>  obj-$(CONFIG_IKHEADERS) += kheaders.o
>> +obj-$(CONFIG_IVDSODEBUG) += vdso_debug.o
>>  obj-$(CONFIG_SMP) += stop_machine.o
>>  obj-$(CONFIG_AUDIT) += audit.o auditfilter.o
>>  obj-$(CONFIG_AUDITSYSCALL) += auditsc.o audit_watch.o audit_fsnotify.o audit_tree.o
>> diff --git a/kernel/vdso_debug.c b/kernel/vdso_debug.c
>> new file mode 100644
>> index 000000000000..72d4f11327fc
>> --- /dev/null
>> +++ b/kernel/vdso_debug.c
>> @@ -0,0 +1,43 @@
>> +// SPDX-License-Identifier: GPL-2.0
>> +
>> +#include <linux/cache.h>
>> +#include <linux/init.h>
>> +#include <linux/kobject.h>
>> +#include <linux/module.h>
>> +#include <linux/sysfs.h>
>> +
>> +asm (
>> +"	.pushsection .rodata, \"a\"	\n"
>> +"	.global vdso_debug_data		\n"
>> +"vdso_debug_data:			\n"
>> +"	.incbin \"vdso_debug.tar.xz\"	\n"
>> +"	.global vdso_debug_data_end	\n"
>> +"vdso_debug_data_end:			\n"
>> +"	.popsection			\n"
>> +);
>> +
>> +extern char vdso_debug_data[];
>> +extern char vdso_debug_data_end[];
>> +
>> +static struct bin_attribute vdso_debug_attr __ro_after_init =
>> +	__BIN_ATTR_SIMPLE_RO(vdso_debug.tar.xz, 0444);
>> +
>> +static int __init vdso_debug_init(void)
>> +{
>> +	vdso_debug_attr.private = vdso_debug_data;
>> +	vdso_debug_attr.size = vdso_debug_data_end - vdso_debug_data;
>> +
>> +	return sysfs_create_bin_file(kernel_kobj, &vdso_debug_attr);
>> +}
>> +
>> +static void __exit vdso_debug_exit(void)
>> +{
>> +	sysfs_remove_bin_file(kernel_kobj, &vdso_debug_attr);
>> +}
>> +
>> +module_init(vdso_debug_init);
>> +module_exit(vdso_debug_exit);
>> +
>> +MODULE_LICENSE("GPL");
>> +MODULE_AUTHOR("Thomas Weißschuh <thomas.weissschuh@xxxxxxxxxxxxx>");
>> +MODULE_DESCRIPTION("Provide vDSO debug information at runtime");
>> 
>> ---
>> base-commit: dc59e4fea9d83f03bad6bddf3fa2e52491777482
>> change-id: 20260624-vdso-sysfs-abfd14fde5a7
>> 
>> Best regards,
>> --  
>> Thomas Weißschuh <thomas.weissschuh@xxxxxxxxxxxxx>
>> 
>

Why stuff them into a tarball?