The lib/cmdline.c is basically a set of some small string parsers
which are wide used in the kernel. Their prototypes belong to the
string.h rather then kernel.h.
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20231003130142.2936503-1-andriy.shevchenko@linux.intel.comSigned-off-by: Kees Cook <keescook@chromium.org>

Introduce --kallsyms argument for scanning binary files for known symbol
addresses. This would have found the exposure in /sys/kernel/notes:

$ scripts/leaking_addresses.pl --kallsyms=<(sudo cat /proc/kallsyms)
/sys/kernel/notes: hypercall_page @ 156
/sys/kernel/notes: xen_hypercall_set_trap_table @ 156
/sys/kernel/notes: startup_xen @ 132
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Tycho Andersen <tandersen@netflix.com>
Link: https://lore.kernel.org/r/20240222220053.1475824-4-keescook@chromium.orgSigned-off-by: Kees Cook <keescook@chromium.org>

These are false positives from the input subsystem:

/proc/bus/input/devices: B: KEY=402000000 3803078f800d001 feffffdfffefffff fffffffffffffffe
/sys/devices/platform/i8042/serio0/input/input1/uevent: KEY=402000000 3803078f800d001 feffffdfffefffff fffffffffffffffe
/sys/devices/platform/i8042/serio0/input/input1/capabilities/key: 402000000 3803078f800d001 feffffdf

Pass in the filename for more context and expand the "ignored pattern"
matcher to notice these.
Reviewed-by: Tycho Andersen <tandersen@netflix.com>
Link: https://lore.kernel.org/r/20240222220053.1475824-3-keescook@chromium.orgSigned-off-by: Kees Cook <keescook@chromium.org>

Instead of using a statically named path in /tmp, use File::Temp to create
(and remove) the temporary file used for parsing /proc/config.gz.
Reviewed-by: Tycho Andersen <tandersen@netflix.com>
Link: https://lore.kernel.org/r/20240222220053.1475824-2-keescook@chromium.orgSigned-off-by: Kees Cook <keescook@chromium.org>

Tobin hasn't been involved lately, and I can step up to be a reviewer
with Tycho. I'll carry changes via the hardening tree.
Reviewed-by: Tycho Andersen <tandersen@netflix.com>
Link: https://lore.kernel.org/r/20240222220053.1475824-1-keescook@chromium.orgSigned-off-by: Kees Cook <keescook@chromium.org>

Improve the reporting of buffer overflows under CONFIG_FORTIFY_SOURCE to
help accelerate debugging efforts. The calculations are all just sitting
in registers anyway, so pass them along to the function to be reported.

For example, before:

  detected buffer overflow in memcpy

and after:

  memcpy: detected buffer overflow: 4096 byte read of buffer size 1

Link: https://lore.kernel.org/r/20230407192717.636137-10-keescook@chromium.orgSigned-off-by: Kees Cook <keescook@chromium.org>

With fortify overflows able to be redirected, we can use KUnit to
exercise the overflow conditions. Add tests for every API covered by
CONFIG_FORTIFY_SOURCE, except for memset() and memcpy(), which are
special-cased for now.

Disable warnings in the Makefile since we're explicitly testing
known-bad string handling code patterns.

Note that this makes the LKDTM FORTIFY_STR* tests obsolete, but those
can be removed separately.
Signed-off-by: Kees Cook <keescook@chromium.org>

The standard C string APIs were not designed to have a failure mode;
they were expected to always succeed without memory safety issues.
Normally, CONFIG_FORTIFY_SOURCE will use fortify_panic() to stop
processing, as truncating a read or write may provide an even worse
system state. However, this creates a problem for testing under things
like KUnit, which needs a way to survive failures.

When building with CONFIG_KUNIT, provide a failure path for all users
of fortify_panic, and track whether the failure was a read overflow or
a write overflow, for KUnit tests to examine. Inspired by similar logic
in the slab tests.
Signed-off-by: Kees Cook <keescook@chromium.org>

In order for CI systems to notice all the skipped tests related to
CONFIG_FORTIFY_SOURCE, allow the FORTIFY_SOURCE KUnit tests to build
with or without CONFIG_FORTIFY_SOURCE.
Signed-off-by: Kees Cook <keescook@chromium.org>

In preparation for KUnit testing and further improvements in fortify
failure reporting, split out the report and encode the function and access
failure (read or write overflow) into a single u8 argument. This mainly
ends up saving a tiny bit of space in the data segment. For a defconfig
with FORTIFY_SOURCE enabled:

$ size gcc/vmlinux.before gcc/vmlinux.after
   text  	  data     bss     dec    	    hex filename
26132309        9760658 2195460 38088427        2452eeb gcc/vmlinux.before
26132386        9748382 2195460 38076228        244ff44 gcc/vmlinux.after
Reviewed-by: Alexander Lobakin <aleksander.lobakin@intel.com>
Signed-off-by: Kees Cook <keescook@chromium.org>

Mark the various refcount_t functions with __signed_wrap, as we depend
on the wrapping behavior to detect the overflow and perform saturation.
Silences warnings seen with the LKDTM REFCOUNT_* tests:

  UBSAN: signed-integer-overflow in ../include/linux/refcount.h:189:11
  2147483647 + 1 cannot be represented in type 'int'
Reviewed-by: Miguel Ojeda <ojeda@kernel.org>
Link: https://lore.kernel.org/r/20240221051634.work.287-kees@kernel.orgSigned-off-by: Kees Cook <keescook@chromium.org>

include/linux/overflow.h includes helper macros intended for calculating
sizes of allocations. These macros prevent accidental overflow by
saturating at SIZE_MAX.

In general when calculating such sizes use of the macros is preferred. Add
a semantic patch which can detect code patterns which can be replaced by
struct_size.

Note that I set the confidence to medium because this patch doesn't make an
attempt to ensure that the relevant array is actually a flexible array. The
struct_size macro does specifically require a flexible array. In many cases
the detected code could be refactored to a flexible array, but this is not
always possible (such as if there are multiple over-allocations).
Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Link: https://lore.kernel.org/r/20230227202428.3657443-1-jacob.e.keller@intel.comSigned-off-by: Kees Cook <keescook@chromium.org>

Add rules for finding places where str_plural() can be used. This
currently finds:
 54 files changed, 62 insertions(+), 61 deletions(-)
Co-developed-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Link: https://lore.kernel.org/all/fc1b25a8-6381-47c2-831c-ab6b8201a82b@intel.com/Signed-off-by: Kees Cook <keescook@chromium.org>

Add str_plural() helper to replace existing open implementations
used by many drivers and help improve future user facing messages.
Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Link: https://lore.kernel.org/r/20240214165015.1656-1-michal.wajdeczko@intel.comSigned-off-by: Kees Cook <keescook@chromium.org>

This allows replacements of the idioms "var += offset" and "var -=
offset" with the wrapping_assign_add() and wrapping_assign_sub() helpers
respectively. They will avoid wrap-around sanitizer instrumentation.

Add to the selftests to validate behavior and lack of side-effects.
Reviewed-by: Marco Elver <elver@google.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Kees Cook <keescook@chromium.org>

Provide helpers that will perform wrapping addition, subtraction, or
multiplication without tripping the arithmetic wrap-around sanitizers. The
first argument is the type under which the wrap-around should happen
with. In other words, these two calls will get very different results:

	wrapping_mul(int, 50, 50) == 2500
	wrapping_mul(u8,  50, 50) ==  196

Add to the selftests to validate behavior and lack of side-effects.
Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Marco Elver <elver@google.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Kees Cook <keescook@chromium.org>

The check_*_overflow() helpers will return results with potentially
wrapped-around values. These values have always been checked by the
selftests, so avoid the confusing language in the kern-doc. The idea of
"safe for use" was relative to the expectation of whether or not the
caller wants a wrapped value -- the calculation itself will always follow
arithmetic wrapping rules.
Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Kees Cook <keescook@chromium.org>

The wordpart.h header is collecting APIs related to the handling
parts of the word (usually in byte granularity). The upper_*_bits()
and lower_*_bits() are good candidates to be moved to there.

This helps to clean up header dependency hell with regard to kernel.h
as the latter gathers completely unrelated stuff together and slows
down compilation (especially when it's included into other header).
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20240214172752.3605073-1-andriy.shevchenko@linux.intel.comReviewed-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Kees Cook <keescook@chromium.org>

KFENCE is not a security mitigation mechanism (due to sampling), but has
the performance characteristics of unintrusive hardening techniques.
When used at scale, however, it improves overall security by allowing
kernel developers to detect heap memory-safety bugs cheaply.

Link: https://lkml.kernel.org/r/79B9A832-B3DE-4229-9D87-748B2CFB7D12@kernel.org
Cc: Matthieu Baerts <matttbe@kernel.org>
Cc: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Marco Elver <elver@google.com>
Link: https://lore.kernel.org/r/20240212130116.997627-1-elver@google.comSigned-off-by: Kees Cook <keescook@chromium.org>

Commit 94f8f319 ("drm: Remove Kconfig option for legacy support
(CONFIG_DRM_LEGACY)") removes the config DRM_LEGACY, but one reference to
that config is left in the hardening.config fragment.

As there is no drm legacy driver left, we do not need to recommend this
attack surface reduction anymore.

Drop this reference in hardening.config fragment.
Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Link: https://lore.kernel.org/r/20240208091045.9219-3-lukas.bulwahn@gmail.comSigned-off-by: Kees Cook <keescook@chromium.org>

Commit 7a628f818499 ("ubsan: Remove CONFIG_UBSAN_SANITIZE_ALL") removes the
config UBSAN_SANITIZE_ALL, but one reference to that config is left in the
hardening.config fragment.

Drop this reference in hardening.config fragment.

Note that CONFIG_UBSAN is still enabled in the hardening.config fragment,
so the functionality when using this fragment remains the same.
Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Link: https://lore.kernel.org/r/20240208091045.9219-2-lukas.bulwahn@gmail.comSigned-off-by: Kees Cook <keescook@chromium.org>

The ARCH=um build has its own idea about strscpy()'s definition. Adjust
the callers to remove the redundant sizeof() arguments ahead of treewide
changes, since it needs a manual adjustment for the newly named
sized_strscpy() export.

Cc: Richard Weinberger <richard@nod.at>
Cc: linux-um@lists.infradead.org
Signed-off-by: Kees Cook <keescook@chromium.org>

Similar to strscpy(), update strscpy_pad()'s 3rd argument to be
optional when the destination is a compile-time known size array.

Cc: Andy Shevchenko <andy@kernel.org>
Cc:  <linux-hardening@vger.kernel.org>
Reviewed-by: Justin Stitt <justinstitt@google.com>
Signed-off-by: Kees Cook <keescook@chromium.org>

Using sizeof(dst) for the "size" argument in strscpy() is the
overwhelmingly common case. Instead of requiring this everywhere, allow a
2-argument version to be used that will use the sizeof() internally. There
are other functions in the kernel with optional arguments[1], so this
isn't unprecedented, and improves readability. Update and relocate the
kern-doc for strscpy() too, and drop __HAVE_ARCH_STRSCPY as it is unused.

Adjust ARCH=um build to notice the changed export name, as it doesn't
do full header includes for the string helpers.

This could additionally let us save a few hundred lines of code:
 1177 files changed, 2455 insertions(+), 3026 deletions(-)
with a treewide cleanup using Coccinelle:

@needless_arg@
expression DST, SRC;
@@

        strscpy(DST, SRC
-, sizeof(DST)
        )

Link: https://elixir.bootlin.com/linux/v6.7/source/include/linux/pci.h#L1517 [1]
Reviewed-by: Justin Stitt <justinstitt@google.com>
Cc: Andy Shevchenko <andy@kernel.org>
Cc: linux-hardening@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>

In preparation for making strscpy_pad()'s 3rd argument optional, redefine
it as a macro. This also has the benefit of allowing greater FORITFY
introspection, as it couldn't see into the strscpy() nor the memset()
within strscpy_pad().

Cc: Andy Shevchenko <andy@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc:  <linux-hardening@vger.kernel.org>
Reviewed-by: Justin Stitt <justinstitt@google.com>
Signed-off-by: Kees Cook <keescook@chromium.org>

In order to mitigate unexpected signed wrap-around[1], bring back the
signed integer overflow sanitizer. It was removed in commit 6aaa31ae
("ubsan: remove overflow checks") because it was effectively a no-op
when combined with -fno-strict-overflow (which correctly changes signed
overflow from being "undefined" to being explicitly "wrap around").

Compilers are adjusting their sanitizers to trap wrap-around and to
detecting common code patterns that should not be instrumented
(e.g. "var + offset < var"). Prepare for this and explicitly rename
the option from "OVERFLOW" to "WRAP" to more accurately describe the
behavior.

To annotate intentional wrap-around arithmetic, the helpers
wrapping_add/sub/mul_wrap() can be used for individual statements. At
the function level, the __signed_wrap attribute can be used to mark an
entire function as expecting its signed arithmetic to wrap around. For a
single object file the Makefile can use "UBSAN_SIGNED_WRAP_target.o := n"
to mark it as wrapping, and for an entire directory, "UBSAN_SIGNED_WRAP :=
n" can be used.

Additionally keep these disabled under CONFIG_COMPILE_TEST for now.

Link: https://github.com/KSPP/linux/issues/26 [1]
Cc: Miguel Ojeda <ojeda@kernel.org>
Cc: Nathan Chancellor <nathan@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Hao Luo <haoluo@google.com>
Reviewed-by: Marco Elver <elver@google.com>
Reviewed-by: Justin Stitt <justinstitt@google.com>
Signed-off-by: Kees Cook <keescook@chromium.org>

For simplicity in splitting out UBSan options into separate rules,
remove CONFIG_UBSAN_SANITIZE_ALL, effectively defaulting to "y", which
is how it is generally used anyway. (There are no ":= y" cases beyond
where a specific file is enabled when a top-level ":= n" is in effect.)

Cc: Andrey Konovalov <andreyknvl@gmail.com>
Cc: Marco Elver <elver@google.com>
Cc: linux-doc@vger.kernel.org
Cc: linux-kbuild@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>

Silence a handful of W=1 warnings in the UBSan selftest, which set
variables without using them. For example:

   lib/test_ubsan.c:101:6: warning: variable 'val1' set but not used [-Wunused-but-set-variable]
     101 |         int val1 = 10;
         |             ^
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202401310423.XpCIk6KO-lkp@intel.com/Reviewed-by: Marco Elver <elver@google.com>
Signed-off-by: Kees Cook <keescook@chromium.org>

Clang changed the way it enables UBSan trapping mode. Update the Makefile
logic to discover it.
Suggested-by: Fangrui Song <maskray@google.com>
Link: https://lore.kernel.org/lkml/CAFP8O3JivZh+AAV7N90Nk7U2BHRNST6MRP0zHtfQ-Vj0m4+pDA@mail.gmail.com/Reviewed-by: Fangrui Song <maskray@google.com>
Reviewed-by: Justin Stitt <justinstitt@google.com>
Cc: Nathan Chancellor <nathan@kernel.org>
Cc: Masahiro Yamada <masahiroy@kernel.org>
Cc: Nicolas Schier <nicolas@fjasle.eu>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: Bill Wendling <morbo@google.com>
Cc: linux-kbuild@vger.kernel.org
Cc: llvm@lists.linux.dev
Signed-off-by: Kees Cook <keescook@chromium.org>

The kernel hardening efforts have continued to depend more and more
heavily on UBSAN, so make an actual MAINTAINERS entry for it.

Cc: Andrey Ryabinin <ryabinin.a.a@gmail.com>
Acked-by: Andrey Konovalov <andreyknvl@gmail.com>
Acked-by: Marco Elver <elver@google.com>
Signed-off-by: Kees Cook <keescook@chromium.org>

Syzkaller hit 'WARNING in dg_dispatch_as_host' bug.

memcpy: detected field-spanning write (size 56) of single field "&dg_info->msg"
at drivers/misc/vmw_vmci/vmci_datagram.c:237 (size 24)

WARNING: CPU: 0 PID: 1555 at drivers/misc/vmw_vmci/vmci_datagram.c:237
dg_dispatch_as_host+0x88e/0xa60 drivers/misc/vmw_vmci/vmci_datagram.c:237

Some code commentry, based on my understanding:

544 #define VMCI_DG_SIZE(_dg) (VMCI_DG_HEADERSIZE + (size_t)(_dg)->payload_size)
/// This is 24 + payload_size

memcpy(&dg_info->msg, dg, dg_size);
	Destination = dg_info->msg ---> this is a 24 byte
					structure(struct vmci_datagram)
	Source = dg --> this is a 24 byte structure (struct vmci_datagram)
	Size = dg_size = 24 + payload_size

{payload_size = 56-24 =32} -- Syzkaller managed to set payload_size to 32.

 35 struct delayed_datagram_info {
 36         struct datagram_entry *entry;
 37         struct work_struct work;
 38         bool in_dg_host_queue;
 39         /* msg and msg_payload must be together. */
 40         struct vmci_datagram msg;
 41         u8 msg_payload[];
 42 };

So those extra bytes of payload are copied into msg_payload[], a run time
warning is seen while fuzzing with Syzkaller.

One possible way to fix the warning is to split the memcpy() into
two parts -- one -- direct assignment of msg and second taking care of payload.

Gustavo quoted:
"Under FORTIFY_SOURCE we should not copy data across multiple members
in a structure."
Reported-by: syzkaller <syzkaller@googlegroups.com>
Suggested-by: Vegard Nossum <vegard.nossum@oracle.com>
Suggested-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Harshit Mogalapalli <harshit.m.mogalapalli@oracle.com>
Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Dan Carpenter <dan.carpenter@linaro.org>
Link: https://lore.kernel.org/r/20240105164001.2129796-2-harshit.m.mogalapalli@oracle.comSigned-off-by: Kees Cook <keescook@chromium.org>

Use struct_size() instead of open coding.
Suggested-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Harshit Mogalapalli <harshit.m.mogalapalli@oracle.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/20240105164001.2129796-1-harshit.m.mogalapalli@oracle.comSigned-off-by: Kees Cook <keescook@chromium.org>

CAP_SYSLOG was separated from CAP_SYS_ADMIN and introduced in Linux
2.6.37 (2010-11). For a long time, certain syslog actions required
CAP_SYS_ADMIN or CAP_SYSLOG. Maybe it’s time to officially remove
CAP_SYS_ADMIN for more fine-grained control.

CAP_SYS_ADMIN was once removed but added back for backwards
compatibility reasons. In commit 38ef4c2e ("syslog: check cap_syslog
when dmesg_restrict") (2010-12), CAP_SYS_ADMIN was no longer needed. And
in commit ee24aebf ("cap_syslog: accept CAP_SYS_ADMIN for now")
(2011-02), it was accepted again. Since then, CAP_SYS_ADMIN has been
preserved.

Now that almost 13 years have passed, the legacy application may have
had enough time to be updated.
Signed-off-by: Jingzi Meng <mengjingzi@iie.ac.cn>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20240105062007.26965-1-mengjingzi@iie.ac.cnSigned-off-by: Kees Cook <keescook@chromium.org>

This diff uses an open source tool include-what-you-use (IWYU) to modify
the include list, changing indirect includes to direct includes. IWYU is
implemented using the IWYUScripts github repository which is a tool that
is currently undergoing development. These changes seek to improve build
times.

This change to lib/string.c resulted in a preprocessed size of
lib/string.i from 26371 lines to 5321 lines (-80%) for the x86
defconfig.

Link: https://github.com/ClangBuiltLinux/IWYUScriptsReviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Tanzir Hasan <tanzirh@google.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20231226-libstringheader-v6-2-80aa08c7652c@google.comSigned-off-by: Kees Cook <keescook@chromium.org>

This patch creates wordpart.h and includes it in asm/word-at-a-time.h
for all architectures. WORD_AT_A_TIME_CONSTANTS depends on kernel.h
because of REPEAT_BYTE. Moving this to another header and including it
where necessary allows us to not include the bloated kernel.h. Making
this implicit dependency on REPEAT_BYTE explicit allows for later
improvements in the lib/string.c inclusion list.
Suggested-by: Al Viro <viro@zeniv.linux.org.uk>
Suggested-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Tanzir Hasan <tanzirh@google.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20231226-libstringheader-v6-1-80aa08c7652c@google.comSigned-off-by: Kees Cook <keescook@chromium.org>

In commit edb6538da3df ("lkdtm/bugs: Adjust lkdtm_HUNG_TASK() to avoid
tail call optimization") we marked lkdtm_HUNG_TASK() as
__noreturn. The compiler gets unhappy if it thinks a __noreturn
function might return, so there's a BUG_ON(1) at the end. Any human
can see that the function won't return and the compiler can figure
that out too. Except when it can't.

The MIPS architecture defines HAVE_ARCH_BUG_ON and defines its own
version of BUG_ON(). The MIPS version of BUG_ON() is not a macro but
is instead an inline function. Apparently this prevents the compiler
from realizing that the condition to BUG_ON() is constant and that the
function will never return.

Let's change the BUG_ON(1) to just BUG(), which it should have been to
begin with. The only reason I used BUG_ON(1) to begin with was because
I was used to using WARN_ON(1) when writing test code and WARN() and
BUG() are oddly inconsistent in this manner. :-/

Fixes: edb6538da3df ("lkdtm/bugs: Adjust lkdtm_HUNG_TASK() to avoid tail call optimization")
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202401262204.wUFKRYZF-lkp@intel.com/Acked-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20240126072852.1.Ib065e528a8620474a72f15baa2feead1f3d89865@changeidSigned-off-by: Kees Cook <keescook@chromium.org>

When testing with lkdtm_HUNG_TASK() and looking at the output, I
expected to see lkdtm_HUNG_TASK() in the stack crawl but it wasn't
there. Instead, the top function on at least some devices was
schedule() due to tail call optimization.

Let's do two things to help here:
1. We'll mark this as "__noreturn". On GCC at least this is documented
   to prevent tail call optimization. The docs [1] say "In order to
   preserve backtraces, GCC will never turn calls to noreturn
   functions into tail calls."
2. We'll add a BUG_ON(1) at the end which means that schedule() is no
   longer a tail call. Note that this is potentially important because
   if we _did_ end up returning from schedule() due to some weird
   issue then we'd potentially be violating the "noreturn" that we
   told the compiler about. BUG is the right thing to do here.

[1] https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.htmlSigned-off-by: Douglas Anderson <dianders@chromium.org>
Link: https://lore.kernel.org/r/20240122164935.2.I26e8f68c312824fcc80c19d4e91de2d2bef958f0@changeidSigned-off-by: Kees Cook <keescook@chromium.org>

The comments for lkdtm_do_action() explicitly call out that it
shouldn't be inlined because we want it to show up in stack
crawls. However, at least with some compilers / options it's still
vanishing due to tail call optimization. Let's add a return value to
the function to make it harder for the compiler to do tail call
optimization here.

Now that we have a return value, we can actually use it in the
callers, which is a minor improvement in the code.
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Link: https://lore.kernel.org/r/20240122164935.1.I345e485f36babad76370c59659a706723750d950@changeidSigned-off-by: Kees Cook <keescook@chromium.org>

Pull cxl fixes from Dan Williams:
 "A build regression fix, a device compatibility fix, and an original
  bug preventing creation of large (16 device) interleave sets:

   - Fix unit test build regression fallout from global
     "missing-prototypes" change

   - Fix compatibility with devices that do not support interrupts

   - Fix overflow when calculating the capacity of large interleave sets"

* tag 'cxl-fixes-6.8-rc2' of git://git.kernel.org/pub/scm/linux/kernel/git/cxl/cxl:
  cxl/region：Fix overflow issue in alloc_hpa()
  cxl/pci: Skip irq features if MSI/MSI-X are not supported
  tools/testing/nvdimm: Disable "missing prototypes / declarations" warnings
  tools/testing/cxl: Disable "missing prototypes / declarations" warnings