Import from internal tree

Author: jserv

.clang-format

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+BasedOnStyle: Chromium
+Language: Cpp
+MaxEmptyLinesToKeep: 3
+IndentCaseLabels: false
+AllowShortIfStatementsOnASingleLine: false
+AllowShortCaseLabelsOnASingleLine: false
+AllowShortLoopsOnASingleLine: false
+DerivePointerAlignment: false
+PointerAlignment: Right
+SpaceAfterCStyleCast: true
+TabWidth: 4
+UseTab: Never
+IndentWidth: 4
+BreakBeforeBraces: Linux
+AccessModifierOffset: -4
+ForEachMacros:   
+  - foreach
+  - Q_FOREACH
+  - BOOST_FOREACH
+  - list_for_each
+  - list_for_each_safe
+  - list_for_each_entry
+  - list_for_each_entry_safe
+  - hlist_for_each_entry
+  - rb_list_foreach
+  - rb_list_foreach_safe
+  - vec_foreach

.gitignore

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+# Prerequisites
+*.d
+
+# Object files
+*.o
+*.ko
+*.elf
+
+# Linker output
+*.map
+
+# Precompiled Headers
+*.gch
+*.pch
+
+# Libraries
+*.a
+
+# Kernel Module Compile Results
+*.mod*
+*.cmd
+.tmp_versions/
+modules.order
+Module.symvers
+Mkfile.old
+dkms.conf
+
+# test-oriented
+client
+out

Makefile

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+CONFIG_MODULE_SIG = n
+TARGET_MODULE := fibdrv
+
+obj-m := $(TARGET_MODULE).o
+ccflags-y := -std=gnu99 -Wno-declaration-after-statement
+
+KDIR := /lib/modules/$(shell uname -r)/build
+PWD := $(shell pwd)
+
+all: client
+	$(MAKE) -C $(KDIR) M=$(PWD) modules
+
+clean:
+	$(MAKE) -C $(KDIR) M=$(PWD) clean
+	$(RM) client out
+load:
+	sudo insmod $(TARGET_MODULE).ko
+unload:
+	sudo rmmod $(TARGET_MODULE) || true >/dev/null
+
+client: client.c
+	$(CC) -o $@ $^
+
+PRINTF = env printf
+PASS_COLOR = \e[32;01m
+NO_COLOR = \e[0m
+pass = $(PRINTF) "$(PASS_COLOR)$1 Passed [-]$(NO_COLOR)\n"
+
+check: all
+	$(MAKE) unload
+	$(MAKE) load
+	sudo ./client > out
+	$(MAKE) unload
+	@diff -u out expected.txt && $(call pass)

README.md

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+# fibdrv
+
+Linux kernel module that creates device /dev/fibonacci.  Writing to this device
+should have no effect, however reading at offset k should return the kth
+fibonacci number.
+
+## References
+
+* [Writing a simple device driver](https://www.apriorit.com/dev-blog/195-simple-driver-for-linux-os)
+* [Character device drivers](https://linux-kernel-labs.github.io/master/labs/device_drivers.html#open-and-release)
+* [cdev interface](https://lwn.net/Articles/195805/)
+* [Character device files](https://sysplay.in/blog/linux-device-drivers/2013/06/character-device-files-creation-operations/)

client.c

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+#include <fcntl.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/types.h>
+#include <unistd.h>
+
+#define FIB_DEV "/dev/fibonacci"
+
+int main()
+{
+    int fd;
+    long long sz;
+
+    char buf[1];
+    char write_buf[] = "testing writing";
+    int offset = 100;  // TODO: test something bigger than the limit
+    int i = 0;
+
+    fd = open(FIB_DEV, O_RDWR);
+
+    if (fd < 0) {
+        perror("Failed to open character device");
+        exit(1);
+    }
+
+    for (i = 0; i <= offset; i++) {
+        sz = write(fd, write_buf, strlen(write_buf));
+        printf("Writing to " FIB_DEV ", returned the sequence %lld\n", sz);
+    }
+
+    for (i = 0; i <= offset; i++) {
+        lseek(fd, i, SEEK_SET);
+        sz = read(fd, buf, 1);
+        printf("Reading from " FIB_DEV
+               " at offset %d, returned the sequence "
+               "%lld.\n",
+               i, sz);
+    }
+
+    for (i = offset; i >= 0; i--) {
+        lseek(fd, i, SEEK_SET);
+        sz = read(fd, buf, 1);
+        printf("Reading from " FIB_DEV
+               " at offset %d, returned the sequence "
+               "%lld.\n",
+               i, sz);
+    }
+
+    close(fd);
+    return 0;
+}