Host insane unsafe usage

README.md

@@ -4,9 +4,47 @@ Run one command, get a QEMU Buildroot BusyBox virtual machine with several minim
 
 ![](screenshot.png)
 
-## Getting started
+## Insane unsafe host super fast quickstart
 
-Usage:
+    cd kernel_module
+    ./make-host.sh
+
+If the compilation of any of the C files fails (because of kernel or toolchain differences that we don't control on the host), just rename it to remove the `.c` extension and try again:
+
+    mv broken.c broken.c~
+    ./build_host
+
+Once you manage to compile, try it out with:
+
+    sudo insmod hello.ko
+
+    # Our module is there.
+    sudo lsmod | grep hello
+
+    # Last message should be: hello init
+    dmest -T
+
+    sudo rmmod hello
+
+    # Last message should be: hello exit
+    dmesg -T
+
+    # Not present anymore
+    sudo lsmod | grep hello
+
+Why this is very bad and you should be ashamed:
+
+-   bugs can easily break you system. E.g.:
+    - segfaults can trivially lead to a kernel crash, and require a reboot
+    - your disk could get erased. Yes, this can also happen with `sudo` from userland. But you should not use `sudo` when developing newbie programs. And for the kernel you don't have the choice not to use `sudo`
+    - even more subtle problems like [not being able to rmmod](https://unix.stackexchange.com/questions/78858/cannot-remove-or-reinsert-kernel-module-after-error-while-inserting-it-without-r)
+-   can't control which kernel version to use. So some of the modules may simply not compile because of kernel API changes, since [the Linux kernel does not have a stable kernel module API](https://stackoverflow.com/questions/37098482/how-to-build-a-linux-kernel-module-so-that-it-is-compatible-with-all-kernel-rele/45429681#45429681).
+-   can't control which hardware is are using, notably the CPU architecture
+-   can't step debug it with GDB easily
+
+The only advantage of using your host machine, is that you don't have to wait 2 hours and use up 8 Gigs for the build. But you will soon find out that this is a very reasonable price to pay.
+
+## Do the right thing and use a virtual machine
 
     sudo apt-get build-dep qemu
     ./run

host/Makefile

@@ -6,7 +6,7 @@ ccflags-y := -Wno-declaration-after-statement -std=gnu99
 all: hello.ko
 
 hello.ko: hello.c
-	make -C '/lib/modules/$(shell uname -r)/build' M='$(PWD)' modules
+	$(MAKE) -C '/lib/modules/$(shell uname -r)/build' M='$(PWD)' modules
 
 clean:
-	make -C '/lib/modules/$(shell uname -r)/build' M='$(PWD)' clean
+	$(MAKE) -C '/lib/modules/$(shell uname -r)/build' M='$(PWD)' clean

host/README.md

@@ -1,35 +1,3 @@
 # Host
 
-Simple things that can be demonstrated by inserting a module into the currently running host. Tested on Ubuntu 16.04.
-
-1. [hello](hello.c)
-
-## Rationale
-
-This method easier to setup, but it is not recommended for development, as:
-
-- it may break your system
-- you can't control which kernel version to use
-
-Use VMs instead.
-
-## Usage
-
-We only use it for super simple examples.
-
-Build, insert and remove a hello world module:
-
-    make
-
-    sudo insmod hello.ko
-
-    # Our module should be there.
-    sudo lsmod | grep hello
-
-    # Last message should be: init_module
-    dmest -T
-
-    sudo rmmod hello
-
-    # Last message should be: cleanup_module
-    dmest -T
+Minimal host build system sanity check example.

host/hello.c

@@ -1,13 +1,17 @@
 #include <linux/module.h>
 #include <linux/kernel.h>
 
-int init_module(void)
+static int myinit(void)
 {
-	printk(KERN_INFO "init_module\n");
+	pr_info("hello init\n");
 	return 0;
 }
 
-void cleanup_module(void)
+static void myexit(void)
 {
-	printk(KERN_INFO "cleanup_module\n");
+	pr_info("hello exit\n");
 }
+
+module_init(myinit)
+module_exit(myexit)
+MODULE_LICENSE("GPL");

kernel_module/Makefile

@@ -1,10 +1,10 @@
-obj-m += $(addsuffix .o, $(notdir $(basename $(wildcard $(BR2_EXTERNAL_KERNEL_MODULE_PATH)/*.c))))
-ccflags-y := -DDEBUG -g -std=gnu99 -Werror -Wno-declaration-after-statement
+obj-m += $(addsuffix .o, $(notdir $(basename $(filter-out %.mod.c, $(wildcard $(BR2_EXTERNAL_KERNEL_MODULE_PATH)/*.c)))))
+ccflags-y := -DDEBUG -g -std=gnu99 -Werror -Wno-declaration-after-statement -Wframe-larger-than=1000000000
 
 .PHONY: all clean
 
 all:
-	$(MAKE) -C '$(LINUX_DIR)' M='$(PWD)' modules
+	$(MAKE) -C '/lib/modules/$(shell uname -r)/build' M='$(PWD)' modules
 
 clean:
 	$(MAKE) -C '$(LINUX_DIR)' M='$(PWD)' clean

kernel_module/make-host.sh

@@ -0,0 +1,15 @@
+#!/usr/bin/env bash
+
+# We can almost do everything from the Makefile itself by using default values for
+#
+#    LINUX_DIR ?= "/lib/modules/$(uname -r)/build"
+#    BR2_EXTERNAL_KERNEL_MODULE_PATH="$(pwd)"
+#
+# The problem with that is that if you define those variables in your environment,
+# the build breaks, so this is more portable.
+#
+# Trying to add `-i` to overcome incompatible modules will fail,
+# because any build failure prevents the generation of all `.mod.c` files.
+
+make -j $(($(nproc) - 2)) BR2_EXTERNAL_KERNEL_MODULE_PATH="$(pwd)" LINUX_DIR="/lib/modules/$(uname -r)/build" "$@"
+make -C user/ -j $(($(nproc) - 2)) "$@"

kernel_module/user/Makefile

@@ -1,6 +1,6 @@
-.PHONY: clean
+.PHONY: all clean
 
-CC ?= gcc -ggdb3 -O0 -std=c99 -Wall -Werror -Wextra
+CCC ?= gcc -ggdb3 -O0 -std=c99 -Wall -Werror -Wextra
 IN_EXT ?= .c
 OUT_EXT ?= .out
 
@@ -9,7 +9,7 @@ OUTS := $(addsuffix $(OUT_EXT), $(basename $(wildcard *$(IN_EXT))))
 all: $(OUTS)
 
 %$(OUT_EXT): %$(IN_EXT)
-	$(CC) -o '$@' '$<'
+	$(CCC) -o '$@' '$<'
 
 clean:
 	rm -f *'$(OUT_EXT)'

kernel_module/user/common.h

@@ -1,7 +1,7 @@
 #ifndef COMMON_H
 #define COMMON_H
 
-#define _POSIX_C_SOURCE 200809L
+#define _XOPEN_SOURCE 700
 #include <fcntl.h> /* open */
 #include <stdint.h> /* uint64_t  */
 #include <stdlib.h> /* size_t */

kernel_module/user/mmap.c

@@ -1,3 +1,4 @@
+#define _XOPEN_SOURCE 700
 #include <assert.h>
 #include <fcntl.h>
 #include <stdio.h>

kernel_module/user/pagemap_dump.c

@@ -11,7 +11,7 @@ Dump the page map of a given process PID.
 Data sources: /proc/PIC/{map,pagemap}
 */
 
-#define _POSIX_C_SOURCE 200809L
+#define _XOPEN_SOURCE 700
 #include <errno.h>
 #include <fcntl.h>
 #include <stdint.h>
@@ -63,7 +63,6 @@ int main(int argc, char **argv) {
 					size_t x = i - 1;
 					while (x && buffer[x] != '\n') x--;
 					if (buffer[x] == '\n') x++;
-					size_t beginning = x;
 					while (buffer[x] != '-' && x < sizeof buffer) {
 						char c = buffer[x++];
 						low *= 16;

kernel_module/user/poll.c

@@ -1,4 +1,5 @@
 #define _XOPEN_SOURCE 700
+#include <assert.h>
 #include <fcntl.h> /* creat, O_CREAT */
 #include <poll.h> /* poll */
 #include <stdio.h> /* printf, puts, snprintf */
@@ -6,11 +7,15 @@
 #include <unistd.h> /* read */
 
 int main(int argc, char **argv) {
-	char buf[1024], path[1024];
+	char buf[1024];
 	int fd, i, n;
 	short revents;
 	struct pollfd pfd;
 
+	if (argc < 2) {
+		fprintf(stderr, "usage: %s <poll-device>\n", argv[0]);
+		exit(EXIT_FAILURE);
+	}
 	fd = open(argv[1], O_RDONLY | O_NONBLOCK);
 	if (fd == -1) {
 		perror("open");
@@ -23,7 +28,7 @@ int main(int argc, char **argv) {
 		i = poll(&pfd, 1, -1);
 		if (i == -1) {
 			perror("poll");
-			exit(EXIT_FAILURE);
+			assert(0);
 		}
 		revents = pfd.revents;
 		if (revents & POLLIN) {

kernel_module/user/usermem.c

@@ -39,7 +39,7 @@ Yes!!! We read the correct value from the physical address.
 
 ## /dev/mem
 
-Firts up, this requires:
+Firt up, this requires:
 
 - CONFIG_STRICT_DEVMEM is not set.
 - nopat on kernel parameters

kernel_module/user/virt_to_phys_user.c

@@ -6,6 +6,7 @@ https://stackoverflow.com/questions/5748492/is-there-any-api-for-determining-the
 Test this out with usermem.c.
 */
 
+#define _XOPEN_SOURCE 700
 #include <stdio.h> /* printf */
 #include <stdlib.h> /* EXIT_SUCCESS, EXIT_FAILURE, strtoull */
 
@@ -14,7 +15,7 @@ Test this out with usermem.c.
 int main(int argc, char **argv)
 {
 	pid_t pid;
-	uintptr_t vaddr, paddr;
+	uintptr_t vaddr, paddr = 0;
 
 	if (argc < 3) {
 		printf("Usage: %s pid vaddr\n", argv[0]);