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linux/kernel/module/Kconfig
Luis Chamberlain 8660484ed1 module: add debugging auto-load duplicate module support 
The finit_module() system call can in the worst case use up to more than
twice of a module's size in virtual memory. Duplicate finit_module()
system calls are non fatal, however they unnecessarily strain virtual
memory during bootup and in the worst case can cause a system to fail
to boot. This is only known to currently be an issue on systems with
larger number of CPUs.

To help debug this situation we need to consider the different sources for
finit_module(). Requests from the kernel that rely on module auto-loading,
ie, the kernel's *request_module() API, are one source of calls. Although
modprobe checks to see if a module is already loaded prior to calling
finit_module() there is a small race possible allowing userspace to
trigger multiple modprobe calls racing against modprobe and this not
seeing the module yet loaded.

This adds debugging support to the kernel module auto-loader (*request_module()
calls) to easily detect duplicate module requests. To aid with possible bootup
failure issues incurred by this, it will converge duplicates requests to a
single request. This avoids any possible strain on virtual memory during
bootup which could be incurred by duplicate module autoloading requests.

Folks debugging virtual memory abuse on bootup can and should enable
this to see what pr_warn()s come on, to see if module auto-loading is to
blame for their wores. If they see duplicates they can further debug this
by enabling the module.enable_dups_trace kernel parameter or by enabling
CONFIG_MODULE_DEBUG_AUTOLOAD_DUPS_TRACE.

Current evidence seems to point to only a few duplicates for module
auto-loading. And so the source for other duplicates creating heavy
virtual memory pressure due to larger number of CPUs should becoming
from another place (likely udev).

Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
2023-04-19 17:26:01 -07:00

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# SPDX-License-Identifier: GPL-2.0-only
menuconfig MODULES
bool "Enable loadable module support"
modules
help
Kernel modules are small pieces of compiled code which can
be inserted in the running kernel, rather than being
permanently built into the kernel. You use the "modprobe"
tool to add (and sometimes remove) them. If you say Y here,
many parts of the kernel can be built as modules (by
answering M instead of Y where indicated): this is most
useful for infrequently used options which are not required
for booting. For more information, see the man pages for
modprobe, lsmod, modinfo, insmod and rmmod.
If you say Y here, you will need to run "make
modules_install" to put the modules under /lib/modules/
where modprobe can find them (you may need to be root to do
this).
If unsure, say Y.
if MODULES
config MODULE_DEBUGFS
bool
config MODULE_DEBUG
bool "Module debugging"
depends on DEBUG_FS
help
Allows you to enable / disable features which can help you debug
modules. You don't need these options on production systems.
if MODULE_DEBUG
config MODULE_STATS
bool "Module statistics"
depends on DEBUG_FS
select MODULE_DEBUGFS
help
This option allows you to maintain a record of module statistics.
For example, size of all modules, average size, text size, a list
of failed modules and the size for each of those. For failed
modules we keep track of modules which failed due to either the
existing module taking too long to load or that module was already
loaded.
You should enable this if you are debugging production loads
and want to see if userspace or the kernel is doing stupid things
with loading modules when it shouldn't or if you want to help
optimize userspace / kernel space module autoloading schemes.
You might want to do this because failed modules tend to use
up significant amount of memory, and so you'd be doing everyone a
favor in avoiding these failures proactively.
This functionality is also useful for those experimenting with
module .text ELF section optimization.
If unsure, say N.
config MODULE_DEBUG_AUTOLOAD_DUPS
bool "Debug duplicate modules with auto-loading"
help
Module autoloading allows in-kernel code to request modules through
the *request_module*() API calls. This in turn just calls userspace
modprobe. Although modprobe checks to see if a module is already
loaded before trying to load a module there is a small time window in
which multiple duplicate requests can end up in userspace and multiple
modprobe calls race calling finit_module() around the same time for
duplicate modules. The finit_module() system call can consume in the
worst case more than twice the respective module size in virtual
memory for each duplicate module requests. Although duplicate module
requests are non-fatal virtual memory is a limited resource and each
duplicate module request ends up just unnecessarily straining virtual
memory.
This debugging facility will create pr_warn() splats for duplicate
module requests to help identify if module auto-loading may be the
culprit to your early boot virtual memory pressure. Since virtual
memory abuse caused by duplicate module requests could render a
system unusable this functionality will also converge races in
requests for the same module to a single request. You can boot with
the module.enable_dups_trace=1 kernel parameter to use WARN_ON()
instead of the pr_warn().
If the first module request used request_module_nowait() we cannot
use that as the anchor to wait for duplicate module requests, since
users of request_module() do want a proper return value. If a call
for the same module happened earlier with request_module() though,
then a duplicate request_module_nowait() would be detected. The
non-wait request_module() call is synchronous and waits until modprobe
completes. Subsequent auto-loading requests for the same module do
not trigger a new finit_module() calls and do not strain virtual
memory, and so as soon as modprobe successfully completes we remove
tracking for duplicates for that module.
Enable this functionality to try to debug virtual memory abuse during
boot on systems which are failing to boot or if you suspect you may be
straining virtual memory during boot, and you want to identify if the
abuse was due to module auto-loading. These issues are currently only
known to occur on systems with many CPUs (over 400) and is likely the
result of udev issuing duplicate module requests for each CPU, and so
module auto-loading is not the culprit. There may very well still be
many duplicate module auto-loading requests which could be optimized
for and this debugging facility can be used to help identify them.
Only enable this for debugging system functionality, never have it
enabled on real systems.
config MODULE_DEBUG_AUTOLOAD_DUPS_TRACE
bool "Force full stack trace when duplicates are found"
depends on MODULE_DEBUG_AUTOLOAD_DUPS
help
Enabling this will force a full stack trace for duplicate module
auto-loading requests using WARN_ON() instead of pr_warn(). You
should keep this disabled at all times unless you are a developer
and are doing a manual inspection and want to debug exactly why
these duplicates occur.
endif # MODULE_DEBUG
config MODULE_FORCE_LOAD
bool "Forced module loading"
default n
help
Allow loading of modules without version information (ie. modprobe
--force). Forced module loading sets the 'F' (forced) taint flag and
is usually a really bad idea.
config MODULE_UNLOAD
bool "Module unloading"
help
Without this option you will not be able to unload any
modules (note that some modules may not be unloadable
anyway), which makes your kernel smaller, faster
and simpler. If unsure, say Y.
config MODULE_FORCE_UNLOAD
bool "Forced module unloading"
depends on MODULE_UNLOAD
help
This option allows you to force a module to unload, even if the
kernel believes it is unsafe: the kernel will remove the module
without waiting for anyone to stop using it (using the -f option to
rmmod). This is mainly for kernel developers and desperate users.
If unsure, say N.
config MODULE_UNLOAD_TAINT_TRACKING
bool "Tainted module unload tracking"
depends on MODULE_UNLOAD
select MODULE_DEBUGFS
help
This option allows you to maintain a record of each unloaded
module that tainted the kernel. In addition to displaying a
list of linked (or loaded) modules e.g. on detection of a bad
page (see bad_page()), the aforementioned details are also
shown. If unsure, say N.
config MODVERSIONS
bool "Module versioning support"
help
Usually, you have to use modules compiled with your kernel.
Saying Y here makes it sometimes possible to use modules
compiled for different kernels, by adding enough information
to the modules to (hopefully) spot any changes which would
make them incompatible with the kernel you are running. If
unsure, say N.
config ASM_MODVERSIONS
bool
default HAVE_ASM_MODVERSIONS && MODVERSIONS
help
This enables module versioning for exported symbols also from
assembly. This can be enabled only when the target architecture
supports it.
config MODULE_SRCVERSION_ALL
bool "Source checksum for all modules"
help
Modules which contain a MODULE_VERSION get an extra "srcversion"
field inserted into their modinfo section, which contains a
sum of the source files which made it. This helps maintainers
see exactly which source was used to build a module (since
others sometimes change the module source without updating
the version). With this option, such a "srcversion" field
will be created for all modules. If unsure, say N.
config MODULE_SIG
bool "Module signature verification"
select MODULE_SIG_FORMAT
help
Check modules for valid signatures upon load: the signature
is simply appended to the module. For more information see
<file:Documentation/admin-guide/module-signing.rst>.
Note that this option adds the OpenSSL development packages as a
kernel build dependency so that the signing tool can use its crypto
library.
You should enable this option if you wish to use either
CONFIG_SECURITY_LOCKDOWN_LSM or lockdown functionality imposed via
another LSM - otherwise unsigned modules will be loadable regardless
of the lockdown policy.
!!!WARNING!!! If you enable this option, you MUST make sure that the
module DOES NOT get stripped after being signed. This includes the
debuginfo strip done by some packagers (such as rpmbuild) and
inclusion into an initramfs that wants the module size reduced.
config MODULE_SIG_FORCE
bool "Require modules to be validly signed"
depends on MODULE_SIG
help
Reject unsigned modules or signed modules for which we don't have a
key. Without this, such modules will simply taint the kernel.
config MODULE_SIG_ALL
bool "Automatically sign all modules"
default y
depends on MODULE_SIG || IMA_APPRAISE_MODSIG
help
Sign all modules during make modules_install. Without this option,
modules must be signed manually, using the scripts/sign-file tool.
comment "Do not forget to sign required modules with scripts/sign-file"
depends on MODULE_SIG_FORCE && !MODULE_SIG_ALL
choice
prompt "Which hash algorithm should modules be signed with?"
depends on MODULE_SIG || IMA_APPRAISE_MODSIG
help
This determines which sort of hashing algorithm will be used during
signature generation. This algorithm _must_ be built into the kernel
directly so that signature verification can take place. It is not
possible to load a signed module containing the algorithm to check
the signature on that module.
config MODULE_SIG_SHA1
bool "Sign modules with SHA-1"
select CRYPTO_SHA1
config MODULE_SIG_SHA224
bool "Sign modules with SHA-224"
select CRYPTO_SHA256
config MODULE_SIG_SHA256
bool "Sign modules with SHA-256"
select CRYPTO_SHA256
config MODULE_SIG_SHA384
bool "Sign modules with SHA-384"
select CRYPTO_SHA512
config MODULE_SIG_SHA512
bool "Sign modules with SHA-512"
select CRYPTO_SHA512
endchoice
config MODULE_SIG_HASH
string
depends on MODULE_SIG || IMA_APPRAISE_MODSIG
default "sha1" if MODULE_SIG_SHA1
default "sha224" if MODULE_SIG_SHA224
default "sha256" if MODULE_SIG_SHA256
default "sha384" if MODULE_SIG_SHA384
default "sha512" if MODULE_SIG_SHA512
choice
prompt "Module compression mode"
help
This option allows you to choose the algorithm which will be used to
compress modules when 'make modules_install' is run. (or, you can
choose to not compress modules at all.)
External modules will also be compressed in the same way during the
installation.
For modules inside an initrd or initramfs, it's more efficient to
compress the whole initrd or initramfs instead.
This is fully compatible with signed modules.
Please note that the tool used to load modules needs to support the
corresponding algorithm. module-init-tools MAY support gzip, and kmod
MAY support gzip, xz and zstd.
Your build system needs to provide the appropriate compression tool
to compress the modules.
If in doubt, select 'None'.
config MODULE_COMPRESS_NONE
bool "None"
help
Do not compress modules. The installed modules are suffixed
with .ko.
config MODULE_COMPRESS_GZIP
bool "GZIP"
help
Compress modules with GZIP. The installed modules are suffixed
with .ko.gz.
config MODULE_COMPRESS_XZ
bool "XZ"
help
Compress modules with XZ. The installed modules are suffixed
with .ko.xz.
config MODULE_COMPRESS_ZSTD
bool "ZSTD"
help
Compress modules with ZSTD. The installed modules are suffixed
with .ko.zst.
endchoice
config MODULE_DECOMPRESS
bool "Support in-kernel module decompression"
depends on MODULE_COMPRESS_GZIP || MODULE_COMPRESS_XZ || MODULE_COMPRESS_ZSTD
select ZLIB_INFLATE if MODULE_COMPRESS_GZIP
select XZ_DEC if MODULE_COMPRESS_XZ
select ZSTD_DECOMPRESS if MODULE_COMPRESS_ZSTD
help
Support for decompressing kernel modules by the kernel itself
instead of relying on userspace to perform this task. Useful when
load pinning security policy is enabled.
If unsure, say N.
config MODULE_ALLOW_MISSING_NAMESPACE_IMPORTS
bool "Allow loading of modules with missing namespace imports"
help
Symbols exported with EXPORT_SYMBOL_NS*() are considered exported in
a namespace. A module that makes use of a symbol exported with such a
namespace is required to import the namespace via MODULE_IMPORT_NS().
There is no technical reason to enforce correct namespace imports,
but it creates consistency between symbols defining namespaces and
users importing namespaces they make use of. This option relaxes this
requirement and lifts the enforcement when loading a module.
If unsure, say N.
config MODPROBE_PATH
string "Path to modprobe binary"
default "/sbin/modprobe"
help
When kernel code requests a module, it does so by calling
the "modprobe" userspace utility. This option allows you to
set the path where that binary is found. This can be changed
at runtime via the sysctl file
/proc/sys/kernel/modprobe. Setting this to the empty string
removes the kernel's ability to request modules (but
userspace can still load modules explicitly).
config TRIM_UNUSED_KSYMS
bool "Trim unused exported kernel symbols" if EXPERT
depends on !COMPILE_TEST
help
The kernel and some modules make many symbols available for
other modules to use via EXPORT_SYMBOL() and variants. Depending
on the set of modules being selected in your kernel configuration,
many of those exported symbols might never be used.
This option allows for unused exported symbols to be dropped from
the build. In turn, this provides the compiler more opportunities
(especially when using LTO) for optimizing the code and reducing
binary size. This might have some security advantages as well.
If unsure, or if you need to build out-of-tree modules, say N.
config UNUSED_KSYMS_WHITELIST
string "Whitelist of symbols to keep in ksymtab"
depends on TRIM_UNUSED_KSYMS
help
By default, all unused exported symbols will be un-exported from the
build when TRIM_UNUSED_KSYMS is selected.
UNUSED_KSYMS_WHITELIST allows to whitelist symbols that must be kept
exported at all times, even in absence of in-tree users. The value to
set here is the path to a text file containing the list of symbols,
one per line. The path can be absolute, or relative to the kernel
source tree.
config MODULES_TREE_LOOKUP
def_bool y
depends on PERF_EVENTS || TRACING || CFI_CLANG
endif # MODULES
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