Add system calls under Linux kernel-5.8.1

    There are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. Please read Documentation/admin-guide/README.rst first.

    In order to build the documentation, use make htmldocs or make pdfdocs. The formatted documentation can also be read online at:

https://www.kernel.org/doc/html/latest/

    There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation.

    Please read the Documentation/process/changes.rst file, as it contains the requirements for building and running the kernel, and information about the problems which may result by upgrading your kernel.

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In the linux-5.8.1 directory

vim linux-5.8.1/Makefile

in line 1073 (add newSystemcall/)  kernel/ certs/ mm/ fs/ ipc/ security/ crypto/ block/ newSystemcall/

vim linux-5.8.1/include/linux/syscalls.h

add asmlinkage long sys_newSystemcall(pid_t user_pid, void* __user user_address); before ##endif

vim linux-5.8.1/arch/x86/entry/syscalls/syscall_64.tbl

(ID 440 is for sys_helloworld in my pc, 441 common newSystemcall) sys_newSystemcall

The nproc in my pc is 8.

"lab1" folder is in the home directory. "newSystemcall" folder is under the linux-5.8.1 directory.

"pics" folder is the execution record.

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To do work items:

Define file

1. Check the kernel version and determine where to add sys call.

uname -r

    It should show:

5.8.1 generic

    Anotehr member shows 5.11.0-27-generic.
    Below shows the execution process of the member.

2. The kernel is stored in /usr/src.

cd /usr/src/linux-headers-5.11.0-27-generic

3. Configure the system call table.
   Path： /usr/src/linux-headers-5.11.0-27-generic/arch/x86/kernel/

cd /usr/src/linux-headers-5.11.0-27-generic/arch/x86/kernel/

4. Add the system call file. (There is no system call file syscall_table_32.S in the directory of the member.)
   Therefore, she create a new system call file. nano syscall_table_32.S .long sys mycall /* yan add row */

5. Define new system call Path：/usr/src/linux-headers-5.11.0-27-generic/arch/x86/include/asm/unistd.h （Difference from kernel-2.6.28: there is no a file named unistd_32.h file, and the file name is changed to: unistd.h.)

cd /usr/src/linux-headers-5.11.0-27-generic/arch/x86/include/asm/

nano unistd.h

    Explanation: In the file unistd.h, add the number of system call, that is to add #define __NR_mycall, and it is defined as the 333rd system call number, it is __NR_syscall_max + 1 in the code. After adding the description, add #define NR_syscalls 334 by the way, it is __NR_syscall_max + 2 in the code. This line of description allows system call number to provide upper and lower bounds.

6. Define the function prototype of the system call Path: /usr/src/linux-headers-5.11.0-27-generic/arch/x86/include/asm/syscalls.h.
   Next, she added function prototype of asmlinkage int sys_mycall(void);
cd /usr/src/linux-headers-5.11.0-27-generic/arch/x86/include/asm/
   Next, nano syscalls.h
   Afterwards, add asmlinkage int sys_mycall(void);

7. Add the main code of system call
   Path: /usr/src/linux-headers-5.11.0-27-generic/kernel/
cd /usr/src/linux-headers-5.11.0-27-generic/kernel/
   The purpose here is to write the code for the new system call. Note that the name of the newly added code file must be the same as the name of the system call.
   For example, here you have to add a file named mycall.c. Otherwise, the os can't find the correct system call correspondence.
   nano mycall.c

8. Makefile configuration
   Path:/usr/src/linux-headers-5.11.0-27-generic/kernel/Makefile
cd /usr/src/linux-headers-5.11.0-27-generic/kernel/

        Add your own new file to be compiled after obj-y. The variable obj-y represents the set of target file names that need to be compiled into the kernel, and the following example adds mycall.o.
    To finish the action, do the following action: nano Makefile

Compile

Path: /usr/src/linux-headers-5.11.0-27-generic/
cd /usr/src/linux-headers-5.11.0-27-generic/

1. Delete intermediate files
   make mrproper
make clean

2. By using menuconfig
   My computer is missing the components:
   sudo apt-get install flex
apt-get install bison -y
   Start up command: make menuconfig
   Get stuck. Afterwards, I did it on my own under Linux kernel-5.8.1. And there are serval probelms we encountered. They are described below.

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    Discussed issues are below:
    What is asmlinkage? When the system call handler wants to call the corresponding system call routine, it pushes the value of the general-purpose scratchpad onto the stack. Therefore, the system call routine has to use the stack to read the parameters passed by the system call handler. Then, that's what the asmlinkage tag is for. After adding asmlinkage, C function will take parameters from stack instead of taking parameters from register.
#define asmlinkage CPP_ASMLINKAGE_attribute_((regparm(0)))

• regparm(0) means not to pass parameters from registers.
      What is SYSCALL_DEFINEx?
• VA_ARGS keyword, used to support defining variable number of parameters.
  After entering SYSCALL_DEFINEx, there are two functions in it.

1. SYSCALL_METADATA