crash_dump, nvme: select CONFIGFS_FS as built-in

Configfs can be configured as a loadable module, which causes a link-time
failure for dm-crypt crash dump support:

crash_dump_dm_crypt.c:(.text+0x3a4): undefined reference to `config_item_init_type_name'
aarch64-linux-ld: kernel/crash_dump_dm_crypt.o: in function `configfs_dmcrypt_keys_init':
crash_dump_dm_crypt.c:(.init.text+0x90): undefined reference to `config_group_init'
aarch64-linux-ld: crash_dump_dm_crypt.c:(.init.text+0xb4): undefined reference to `configfs_register_subsystem'
aarch64-linux-ld: crash_dump_dm_crypt.c:(.init.text+0xd8): undefined reference to `configfs_unregister_subsystem'

This could be avoided with a dependency on CONFIGFS_FS=y, but the
dependency has an additional problem of causing Kconfig dependency loops
since most other uses select the symbol.

Using a simple 'select CONFIGFS_FS' here in turn fails with
CONFIG_DM_CRYPT=m, because that still only causes configfs to be a
loadable module.

The only version I found that fixes this reliably uses an additional
Kconfig symbol to ensure the 'select' actually turns on configfs as
builtin, with two additional changes to avoid dependency loops with nvme
and sysfs.

There is no compile-time dependency between configfs and sysfs, so
selecting configfs from a driver with sysfs disabled does not cause link
failures, only the default /sys/kernel/config mount point will not be
created.

Link: https://lkml.kernel.org/r/20250521160359.2132363-1-arnd@kernel.org
Fixes: 6b23858fd63b ("crash_dump: make dm crypt keys persist for the kdump kernel")
Fixes: 1fb4704084 ("nvme-loop: add configfs dependency")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Andreas Hindborg <a.hindborg@kernel.org>
Cc: Breno Leitao <leitao@debian.org>
Cc: Chaitanya Kulkarni <kch@nvidia.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Coiby Xu <coxu@redhat.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

drivers/nvme/target/Kconfig

@@ -3,7 +3,7 @@
 config NVME_TARGET
 	tristate "NVMe Target support"
 	depends on BLOCK
-	depends on CONFIGFS_FS
+	select CONFIGFS_FS
 	select NVME_KEYRING if NVME_TARGET_TCP_TLS
 	select KEYS if NVME_TARGET_TCP_TLS
 	select SGL_ALLOC

fs/configfs/Kconfig

@@ -1,7 +1,6 @@
 # SPDX-License-Identifier: GPL-2.0-only
 config CONFIGFS_FS
 	tristate "Userspace-driven configuration filesystem"
-	select SYSFS
 	help
 	  configfs is a RAM-based filesystem that provides the converse
 	  of sysfs's functionality. Where sysfs is a filesystem-based

kernel/Kconfig.kexec

@@ -120,12 +120,18 @@ config CRASH_DM_CRYPT
 	depends on KEXEC_FILE
 	depends on CRASH_DUMP
 	depends on DM_CRYPT
-	depends on CONFIGFS_FS
 	help
 	  With this option enabled, user space can intereact with
 	  /sys/kernel/config/crash_dm_crypt_keys to make the dm crypt keys
 	  persistent for the dump-capture kernel.
 
+config CRASH_DM_CRYPT_CONFIGS
+	def_tristate CRASH_DM_CRYPT
+	select CONFIGFS_FS
+	help
+	  CRASH_DM_CRYPT cannot directly select CONFIGFS_FS, because that
+	  is required to be built-in.
+
 config CRASH_HOTPLUG
 	bool "Update the crash elfcorehdr on system configuration changes"
 	default y