kernel modules: hack up quick floating point example

README.adoc

@@ -5537,7 +5537,23 @@ This likely comes from the ifdef split at `init/main.c`:
 
 ==== Kernel module parameters
 
-The Linux kernel allows passing module parameters at insertion time <<myinsmod,through the `init_module` and `finit_module` system calls>>:
+The Linux kernel allows passing module parameters at insertion time <<myinsmod,through the `init_module` and `finit_module` system calls>>.
+
+The `insmod` tool exposes that as:
+
+....
+insmod params.ko i=3 j=4
+....
+
+Parameters are declared in the module as:
+
+....
+static u32 i = 0;
+module_param(i, int, S_IRUSR | S_IWUSR);
+MODULE_PARM_DESC(i, "my favorite int");
+....
+
+Automated test:
 
 ....
 ./params.sh
@@ -5890,6 +5906,52 @@ Source: link:kernel_modules/init_module.c[]
 
 TODO why were `module_init` and `module_exit` created? https://stackoverflow.com/questions/3218320/what-is-the-difference-between-module-init-and-init-module-in-a-linux-kernel-mod
 
+==== Floating point in kernel modules
+
+It is generally hard / impossible to use floating point operations in the kernel. TODO understand details.
+
+A quick (x86-only for now because lazy) example is shown at: link:kernel_modules/float.c[]
+
+Usage:
+
+....
+insmod float.ko myfloat=1 enable_fpu=1
+....
+
+We have to call: `kernel_fpu_begin()` before starting FPU operations, and `kernel_fpu_end()` when we are done. This particular example however did not blow up without it at lkmc 7f917af66b17373505f6c21d75af9331d624b3a9 + 1:
+
+....
+insmod float.ko myfloat=1 enable_fpu=0
+....
+
+The v5.1 documentation under link:https://github.com/cirosantilli/linux/blob/v5.1/arch/x86/include/asm/fpu/api.h#L15[arch/x86/include/asm/fpu/api.h] reads:
+
+....
+ * Use kernel_fpu_begin/end() if you intend to use FPU in kernel context. It
+ * disables preemption so be careful if you intend to use it for long periods
+ * of time.
+....
+
+The example sets in the link:kernel_modules/Makefile[]:
+
+....
+CFLAGS_REMOVE_float.o += -mno-sse -mno-sse2
+....
+
+to avoid:
+
+....
+error: SSE register return with SSE disabled
+....
+
+We found those flags with `./build-modules --verbose`.
+
+Bibliography:
+
+* https://stackoverflow.com/questions/13886338/use-of-floating-point-in-the-linux-kernel
+* https://stackoverflow.com/questions/15883947/why-am-i-able-to-perform-floating-point-operations-inside-a-linux-kernel-module/47056242
+* https://stackoverflow.com/questions/1556142/sse-register-return-with-sse-disabled
+
 === Kernel panic and oops
 
 To test out kernel panics and oops in controlled circumstances, try out the modules:

build-modules

@@ -8,6 +8,7 @@ import shutil
 
 import common
 from shell_helpers import LF
+import path_properties
 
 class Main(common.BuildCliFunction):
     def __init__(self):
@@ -32,13 +33,15 @@ Build the Linux kernel modules against the host kernel.
 Place the modules on a separate magic directory from non --host builds.
 ''',
         )
+        self._add_argument('--force-rebuild')
         self.add_argument(
             'kernel-modules',
             default=[],
-            help='Which kernel modules to build. Default: build all',
+            help='''\
+Which kernel modules to build. Default: build all.
+Can be either the path to the C file, or its basename without extension.''',
             nargs='*',
         )
-        self._add_argument('--force-rebuild')
 
     def build(self):
         build_dir = self.get_build_dir()
@@ -63,11 +66,14 @@ Place the modules on a separate magic directory from non --host builds.
         )
         all_kernel_modules = []
         for basename in os.listdir(self.env['kernel_modules_source_dir']):
-            src = os.path.join(self.env['kernel_modules_source_dir'], basename)
-            if os.path.isfile(src):
+            abspath = os.path.join(self.env['kernel_modules_source_dir'], basename)
+            if os.path.isfile(abspath):
                 noext, ext = os.path.splitext(basename)
                 if ext == self.env['c_ext']:
-                    all_kernel_modules.append(noext)
+                    relpath = abspath[len(self.env['root_dir']) + 1:]
+                    my_path_properties = path_properties.get(relpath)
+                    if my_path_properties.should_be_built(self.env):
+                        all_kernel_modules.append(noext)
         if self.env['kernel_modules'] == []:
             kernel_modules = all_kernel_modules
         else:

kernel_modules/Makefile

@@ -13,6 +13,11 @@ ccflags-y := \
   -Wno-declaration-after-statement \
   $(CCFLAGS)
 
+ifeq ($(ARCH),x86)
+	# https://github.com/cirosantilli/linux-kernel-module-cheat#floating-point-in-kernel-modules
+	CFLAGS_REMOVE_float.o += -mno-sse -mno-sse2
+endif
+
 .PHONY: all
 
 all:

kernel_modules/float.c

@@ -0,0 +1,31 @@
+/* https://github.com/cirosantilli/linux-kernel-module-cheat#floating-point-in-kernel-modules */
+
+#include <linux/kernel.h>
+#include <linux/module.h>
+#include <asm/fpu/api.h> /* kernel_fpu_begin, kernel_fpu_end */
+
+/* float params are not supported, so we just go with int. */
+static int myfloat = 1;
+static int enable_fpu = 1;
+module_param(myfloat, int, S_IRUSR | S_IWUSR);
+module_param(enable_fpu, int, S_IRUSR | S_IWUSR);
+
+static int myinit(void)
+{
+	if (enable_fpu) {
+		kernel_fpu_begin();
+	}
+	if ((float)myfloat + 1.5f == 2.5f) {
+		pr_info("magic value\n");
+	}
+	if (enable_fpu) {
+		kernel_fpu_end();
+	}
+	return 0;
+}
+
+static void myexit(void) {}
+
+module_init(myinit)
+module_exit(myexit)
+MODULE_LICENSE("GPL");

path_properties.py

@@ -128,18 +128,18 @@ class PathProperties:
                 self['allowed_archs'] is None or
                 env['arch'] in self['allowed_archs']
             ) and
-            (
+            not (
                 (
                     env['mode'] == 'userland' and
                     (
-                        self['userland'] and
-                        ext in env['build_in_exts']
+                        not self['userland'] or
+                        not ext in env['build_in_exts']
                     )
                 ) or
                 (
                     env['mode'] == 'baremetal' and (
-                        self['baremetal'] and
-                        ext in env['baremetal_build_in_exts']
+                        not self['baremetal'] or
+                        not ext in env['baremetal_build_in_exts']
                     )
                 )
             ) and
@@ -311,6 +311,12 @@ path_properties_tuples = (
                 'getchar.c': {'interactive': True},
             }
         ),
+        'kernel_modules': (
+            {},
+            {
+                'float.c': {'allowed_archs': 'x86_64'}
+            },
+        ),
         'lkmc.c': {
             'baremetal': True,
             'userland': True,