Re: [PATCH v2] module: Add stubs.c for CONFIG_MODULES=n module_put_and_kthread_exit()

[Aaron Tomlin]

On Fri, Oct 02, 2026 at 10:40:38AM +0200, Petr Pavlu wrote:
> Module-related stubs for CONFIG_MODULES=n can normally live in
> linux/module.h. This works well when the stub does nothing or simply
> returns a constant. However, some non-module implementations may be more
> expensive, either because they have a real implementation or because they
> require additional dependencies to be pulled into linux/module.h. This
> header appears in roughly 15k #include directives across the kernel, so it
> should avoid bringing in unnecessary definitions.
> 
> Specifically, linux/module.h defines module_put_and_kthread_exit() as an
> alias for kthread_exit() when CONFIG_MODULES=n. That in turn requires
> linux/kthread.h, even though linux/module.h otherwise has no need for it.
> Note that linux/module.h currently doesn't include linux/kthread.h and
> requires its users to pick it up through other paths, which is itself not
> quite right.
> 
> Introduce kernel/module/stubs.c to hold such stubs, built only for
> CONFIG_MODULES=n. This mirrors kernel/time/posix-stubs.c, which is used for
> CONFIG_POSIX_TIMERS=n.
> 
> Define module_put_and_kthread_exit() in both CONFIG_MODULES configurations
> to call __module_put_and_kthread_exit() and add a CONFIG_MODULES=n
> implementation of that function to kernel/module/stubs.c. Since
> __module_put_and_kthread_exit() is now implemented even when
> CONFIG_MODULES=n, remove the corresponding '#ifdef CONFIG_MODULES' from the
> noreturn_deny BPF list.
> 
> Signed-off-by: Petr Pavlu <petr.pavlu@xxxxxxxx>
> ---
> kernel/module/stubs.c should also be used for add_taint_module() [1] in the
> future.
> 
> [1] https://lore.kernel.org/linux-modules/a79a6a06-cc48-4036-882d-95e9b4cc01cf@xxxxxxxx/
> ---
> Changes in v2:
> - Remove the unnecessary include of internal.h in stubs.c.
> - Remove the unnecessary EXPORT_SYMBOL(__module_put_and_kthread_exit) in
>   stubs.c.
> - Link to v1: https://patch.msgid.link/20260922-module-stubs-v1-1-abe465a24879@xxxxxxxx
> ---
>  include/linux/module.h | 9 +++------
>  kernel/Makefile        | 2 +-
>  kernel/bpf/verifier.c  | 2 --
>  kernel/module/Makefile | 4 ++++
>  kernel/module/main.c   | 1 +
>  kernel/module/stubs.c  | 9 +++++++++
>  6 files changed, 18 insertions(+), 9 deletions(-)
> 
> diff --git a/include/linux/module.h b/include/linux/module.h
> index 96cc98568eea..4c7bd7f9f889 100644
> --- a/include/linux/module.h
> +++ b/include/linux/module.h
> @@ -658,10 +658,6 @@ static inline bool within_module(unsigned long addr, const struct module *mod)
>  /* Search for module by name: must be in a RCU critical section. */
>  struct module *find_module(const char *name);
>  
> -extern void __noreturn __module_put_and_kthread_exit(struct module *mod,
> -			long code);
> -#define module_put_and_kthread_exit(code) __module_put_and_kthread_exit(THIS_MODULE, code)
> -
>  #ifdef CONFIG_MODULE_UNLOAD
>  int module_refcount(struct module *mod);
>  void __symbol_put(const char *symbol);
> @@ -850,8 +846,6 @@ static inline int unregister_module_notifier(struct notifier_block *nb)
>  	return 0;
>  }
>  
> -#define module_put_and_kthread_exit(code) kthread_exit(code)
> -
>  static inline void print_modules(void)
>  {
>  }
> @@ -879,6 +873,9 @@ static inline void module_for_each_mod(int(*func)(struct module *mod, void *data
>  }
>  #endif /* CONFIG_MODULES */
>  
> +void __noreturn __module_put_and_kthread_exit(struct module *mod, long code);
> +#define module_put_and_kthread_exit(code) __module_put_and_kthread_exit(THIS_MODULE, code)
> +
>  #ifdef CONFIG_SYSFS
>  extern struct kset *module_kset;
>  extern const struct kobj_type module_ktype;
> diff --git a/kernel/Makefile b/kernel/Makefile
> index 1e1a31673577..08a94594975d 100644
> --- a/kernel/Makefile
> +++ b/kernel/Makefile
> @@ -58,7 +58,7 @@ obj-y += liveupdate/
>  obj-y += dma/
>  obj-y += entry/
>  obj-y += unwind/
> -obj-$(CONFIG_MODULES) += module/
> +obj-y += module/
>  
>  obj-$(CONFIG_KCMP) += kcmp.o
>  obj-$(CONFIG_FREEZER) += freezer.o
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 72a3f5998dd2..be70f10e4968 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -20417,9 +20417,7 @@ BTF_ID(func, __ia32_sys_exit_group)
>  BTF_ID(func, __kunit_abort)
>  BTF_ID(func, kunit_try_catch_throw)
>  #endif
> -#ifdef CONFIG_MODULES
>  BTF_ID(func, __module_put_and_kthread_exit)
> -#endif
>  #ifdef CONFIG_X86_64
>  BTF_ID(func, __x64_sys_exit)
>  BTF_ID(func, __x64_sys_exit_group)
> diff --git a/kernel/module/Makefile b/kernel/module/Makefile
> index 50ffcc413b54..72fd2972eacf 100644
> --- a/kernel/module/Makefile
> +++ b/kernel/module/Makefile
> @@ -7,6 +7,7 @@
>  # and produce insane amounts of uninteresting coverage.
>  KCOV_INSTRUMENT_main.o := n
>  
> +ifeq ($(CONFIG_MODULES),y)
>  obj-y += main.o
>  obj-y += strict_rwx.o
>  obj-y += kmod.o
> @@ -23,3 +24,6 @@ obj-$(CONFIG_KGDB_KDB) += kdb.o
>  obj-$(CONFIG_MODVERSIONS) += version.o
>  obj-$(CONFIG_MODULE_UNLOAD_TAINT_TRACKING) += tracking.o
>  obj-$(CONFIG_MODULE_STATS) += stats.o
> +else
> +obj-y += stubs.o
> +endif
> diff --git a/kernel/module/main.c b/kernel/module/main.c
> index d0e1e0bd2ad0..a8bf803afa81 100644
> --- a/kernel/module/main.c
> +++ b/kernel/module/main.c
> @@ -60,6 +60,7 @@
>  #include <linux/codetag.h>
>  #include <linux/debugfs.h>
>  #include <linux/execmem.h>
> +#include <linux/kthread.h>
>  #include <uapi/linux/module.h>
>  #include "internal.h"
>  
> diff --git a/kernel/module/stubs.c b/kernel/module/stubs.c
> new file mode 100644
> index 000000000000..1385c87e8169
> --- /dev/null
> +++ b/kernel/module/stubs.c
> @@ -0,0 +1,9 @@
> +// SPDX-License-Identifier: GPL-2.0-or-later
> +
> +#include <linux/kthread.h>
> +#include <linux/module.h>
> +
> +void __noreturn __module_put_and_kthread_exit(struct module *mod, long code)
> +{
> +	kthread_exit(code);
> +}
> 
> ---
> base-commit: 93f51579e7df248780214094418f205253383cc5
> change-id: 20260918-module-stubs-dccefe41dace

Hi Petr,

I noticed your response to Sashiko [1].

I agree. Marking the stub function with "__used __retain" in
kernel/module/stubs.c (e.g. as per "#define __bpf_kfunc __used __retain
__noclone noinline") should be sufficient. Thank you.

Reviewed-by: Aaron Tomlin <atomlin@xxxxxxxxxxx>

[1]: https://lore.kernel.org/sashiko-reviews/c166a29c-14f4-44ee-81ac-d79c82175fc8@xxxxxxxx/

Kind regards,
-- 
Aaron Tomlin