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

[Vincenzo Frascino]

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>
> 

-- 
Regards,
Vincenzo