Introduction to system calls

Introduction

This post opens up a new chapter in linux-insides book, and as you may understand from the title, this chapter will be devoted to the System call concept in the Linux kernel. The choice of topic for this chapter is not accidental. In the previous chapter we saw interrupts and interrupt handling. The concept of system calls is very similar to that of interrupts. This is because the most common way to implement system calls is as software interrupts. We will see many different aspects that are related to the system call concept. For example, we will learn what's happening when a system call occurs from userspace. We will see an implementation of a couple system call handlers in the Linux kernel, VDSO and vsyscall concepts and many many more.

Before we dive into Linux system call implementation, it is good to know some theory about system calls. Let's do it in the following paragraph.

System call. What is it?

A system call is just a userspace request of a kernel service. Yes, the operating system kernel provides many services. When your program wants to write to or read from a file, start to listen for connections on a socket, delete or create directory, or even to finish its work, a program uses a system call. In other words, a system call is just a C kernel space function that user space programs call to handle some request.

The Linux kernel provides a set of these functions and each architecture provides its own set. For example: the x86_64 provides 322 system calls and the x86 provides 358 different system calls. Ok, a system call is just a function. Let's look on a simple Hello world example that's written in the assembly programming language:

.data

msg:
    .ascii "Hello, world!\n"
    len = . - msg

.text
    .global _start

_start:
    movq  $1, %rax
    movq  $1, %rdi
    movq  $msg, %rsi
    movq  $len, %rdx
    syscall

    movq  $60, %rax
    xorq  %rdi, %rdi
    syscall

We can compile the above with the following commands:

$ gcc -c test.S
$ ld -o test test.o

and run it as follows:

./test
Hello, world!

Ok, what do we see here? This simple code represents Hello world assembly program for the Linux x86_64 architecture. We can see two sections here:

.data
.text

The first section - .data stores initialized data of our program (Hello world string and its length in our case). The second section - .text contains the code of our program. We can split the code of our program into two parts: first part will be before the first syscall instruction and the second part will be between first and second syscall instructions. First of all what does the syscall instruction do in our code and generally? As we can read in the 64-ia-32-architectures-software-developer-vol-2b-manual:

SYSCALL invokes an OS system-call handler at privilege level 0. It does so by
loading RIP from the IA32_LSTAR MSR (after saving the address of the instruction
following SYSCALL into RCX). (The WRMSR instruction ensures that the
IA32_LSTAR MSR always contain a canonical address.)
...
...
...
SYSCALL loads the CS and SS selectors with values derived from bits 47:32 of the
IA32_STAR MSR. However, the CS and SS descriptor caches are not loaded from the
descriptors (in GDT or LDT) referenced by those selectors.

Instead, the descriptor caches are loaded with fixed values. It is the respon-
sibility of OS software to ensure that the descriptors (in GDT or LDT) referenced
by those selector values correspond to the fixed values loaded into the descriptor
caches; the SYSCALL instruction does not ensure this correspondence.

To summarize, the syscall instruction jumps to the address stored in the MSR_LSTAR Model specific register (Long system target address register). The kernel is responsible for providing its own custom function for handling syscalls as well as writing the address of this handler function to the MSR_LSTAR register upon system startup. The custom function is entry_SYSCALL_64, which is defined in arch/x86/entry/entry_64.S. The address of this syscall handling function is written to the MSR_LSTAR register during startup in arch/x86/kernel/cpu/common.c.

wrmsrl(MSR_LSTAR, entry_SYSCALL_64);

So, the syscall instruction invokes a handler of a given system call. But how does it know which handler to call? Actually it gets this information from the general purpose registers. As you can see in the system call table, each system call has a unique number. In our example the first system call is write, which writes data to the given file. Let's look in the system call table and try to find the write system call. As we can see, the write system call has number 1. We pass the number of this system call through the rax register in our example. The next general purpose registers: %rdi, %rsi, and %rdx take the three parameters of the write syscall. In our case, they are:

File descriptor (1 is stdout in our case)
Pointer to our string
Size of data

Yes, you heard right. Parameters for a system call. As I already wrote above, a system call is a just C function in the kernel space. In our case first system call is write. This system call defined in the fs/read_write.c source code file and looks like:

SYSCALL_DEFINE3(write, unsigned int, fd, const char __user *, buf,
        size_t, count)
{
    ...
    ...
    ...
}

Or in other words:

ssize_t write(int fd, const void *buf, size_t nbytes);

Don't worry about the SYSCALL_DEFINE3 macro for now, we'll come back to it.

The second part of our example is the same, but we call another system call. In this case we call the exit system call. This system call gets only one parameter:

Return value

and handles the way our program exits. We can pass the program name of our program to the strace util and we will see our system calls:

$ strace test
execve("./test", ["./test"], [/* 62 vars */]) = 0
write(1, "Hello, world!\n", 14Hello, world!
)         = 14
_exit(0)                                = ?

+++ exited with 0 +++

In the first line of the strace output, we can see the execve system call that executes our program, and the second and third are system calls that we have used in our program: write and exit. Note that we pass the parameter through the general purpose registers in our example. The order of the registers is not accidental. The order of the registers is defined by the following agreement - x86-64 calling conventions. This, and the other agreement for the x86_64 architecture are explained in the special document - System V Application Binary Interface. PDF. In a general way, argument(s) of a function are placed either in registers or pushed on the stack. The right order is:

rdi
rsi
rdx
rcx
r8
r9

for the first six parameters of a function. If a function has more than six arguments, the remaining parameters will be placed on the stack.

We do not use system calls in our code directly, but our program uses them when we want to print something, check access to a file or just write or read something to it.

For example:

#include <stdio.h>

int main(int argc, char **argv)
{
   FILE *fp;
   char buff[255];

   fp = fopen("test.txt", "r");
   fgets(buff, 255, fp);
   printf("%s\n", buff);
   fclose(fp);

   return 0;
}

There are no fopen, fgets, printf, and fclose system calls in the Linux kernel, but open, read, write, and close instead. I think you know that fopen, fgets, printf, and fclose are defined in the C standard library. Actually, these functions are just wrappers for the system calls. We do not call system calls directly in our code, but instead use these wrapper functions from the standard library. The main reason of this is simple: a system call must be performed quickly, very quickly. As a system call must be quick, it must be small. The standard library takes responsibility to perform system calls with the correct parameters and makes different checks before it will call the given system call. Let's compile our program with the following command:

$ gcc test.c -o test

and examine it with the ltrace util:

$ ltrace ./test
__libc_start_main([ "./test" ] <unfinished ...>
fopen("test.txt", "r")                                             = 0x602010
fgets("Hello World!\n", 255, 0x602010)                             = 0x7ffd2745e700
puts("Hello World!\n"Hello World!

)                                                                  = 14
fclose(0x602010)                                                   = 0
+++ exited (status 0) +++

The ltrace util displays a set of userspace calls of a program. The fopen function opens the given text file, the fgets function reads file content to the buf buffer, the puts function prints the buffer to stdout, and the fclose function closes the file given by the file descriptor. And as I already wrote, all of these functions call an appropriate system call. For example, puts calls the write system call inside, we can see it if we will add -S option to the ltrace program:

write@SYS(1, "Hello World!\n\n", 14) = 14

Yes, system calls are ubiquitous. Each program needs to open/write/read files and network connections, allocate memory, and many other things that can be provided only by the kernel. The proc file system contains special files in a format: /proc/${pid}/syscall that exposes the system call number and argument registers for the system call currently being executed by the process. For example, pid 1 is systemd for me:

$ sudo cat /proc/1/comm
systemd

$ sudo cat /proc/1/syscall
232 0x4 0x7ffdf82e11b0 0x1f 0xffffffff 0x100 0x7ffdf82e11bf 0x7ffdf82e11a0 0x7f9114681193

the system call with number - 232 which is epoll_wait system call that waits for an I/O event on an epoll file descriptor. Or for example emacs editor where I'm writing this part:

$ ps ax | grep emacs
2093 ?        Sl     2:40 emacs

$ sudo cat /proc/2093/comm
emacs

$ sudo cat /proc/2093/syscall
270 0xf 0x7fff068a5a90 0x7fff068a5b10 0x0 0x7fff068a59c0 0x7fff068a59d0 0x7fff068a59b0 0x7f777dd8813c

the system call with the number 270 which is sys_pselect6 system call that allows emacs to monitor multiple file descriptors.

Now we know a little about system call, what is it and why we need in it. So let's look at the write system call that our program used.

Implementation of write system call

Let's look at the implementation of this system call directly in the source code of the Linux kernel. As we already know, the write system call is defined in the fs/read_write.c source code file and looks like this:

SYSCALL_DEFINE3(write, unsigned int, fd, const char __user *, buf,
        size_t, count)
{
    struct fd f = fdget_pos(fd);
    ssize_t ret = -EBADF;

    if (f.file) {
        loff_t pos = file_pos_read(f.file);
        ret = vfs_write(f.file, buf, count, &pos);
        if (ret >= 0)
            file_pos_write(f.file, pos);
        fdput_pos(f);
    }

    return ret;
}

First of all, the SYSCALL_DEFINE3 macro is defined in the include/linux/syscalls.h header file and expands to the definition of the sys_name(...) function. Let's look at this macro:

#define SYSCALL_DEFINE3(name, ...) SYSCALL_DEFINEx(3, _##name, __VA_ARGS__)

#define SYSCALL_DEFINEx(x, sname, ...)                \
        SYSCALL_METADATA(sname, x, __VA_ARGS__)       \
        __SYSCALL_DEFINEx(x, sname, __VA_ARGS__)

As we can see the SYSCALL_DEFINE3 macro takes name parameter which will represent name of a system call and variadic number of parameters. This macro just expands to the SYSCALL_DEFINEx macro that takes the number of the parameters the given system call, the _##name stub for the future name of the system call (more about tokens concatenation with the ## you can read in the documentation of gcc). Next we can see the SYSCALL_DEFINEx macro. This macro expands to the two following macros:

SYSCALL_METADATA;
__SYSCALL_DEFINEx.

Implementation of the first macro SYSCALL_METADATA depends on the CONFIG_FTRACE_SYSCALLS kernel configuration option. As we can understand from the name of this option, it allows to enable tracer to catch the syscall entry and exit events. If this kernel configuration option is enabled, the SYSCALL_METADATA macro executes initialization of the syscall_metadata structure that defined in the include/trace/syscall.h header file and contains different useful fields as name of a system call, number of a system call in the system call table, number of parameters of a system call, list of parameter types and etc:

#define SYSCALL_METADATA(sname, nb, ...)                             \
    ...                                                              \
    ...                                                              \
    ...                                                              \
    struct syscall_metadata __used                                   \
              __syscall_meta_##sname = {                             \
                    .name           = "sys"#sname,                   \
                    .syscall_nr     = -1,                            \
                    .nb_args        = nb,                            \
                    .types          = nb ? types_##sname : NULL,     \
                    .args           = nb ? args_##sname : NULL,      \
                    .enter_event    = &event_enter_##sname,          \
                    .exit_event     = &event_exit_##sname,           \
                    .enter_fields   = LIST_HEAD_INIT(__syscall_meta_##sname.enter_fields), \
             };                                                                            \

    static struct syscall_metadata __used                           \
              __attribute__((section("__syscalls_metadata")))       \
             *__p_syscall_meta_##sname = &__syscall_meta_##sname;

If the CONFIG_FTRACE_SYSCALLS kernel option is not enabled during kernel configuration, the SYSCALL_METADATA macro expands to an empty string:

#define SYSCALL_METADATA(sname, nb, ...)

The second macro __SYSCALL_DEFINEx expands to the definition of the five following functions:

#define __SYSCALL_DEFINEx(x, name, ...)                                 \
        asmlinkage long sys##name(__MAP(x,__SC_DECL,__VA_ARGS__))       \
                __attribute__((alias(__stringify(SyS##name))));         \
                                                                        \
        static inline long SYSC##name(__MAP(x,__SC_DECL,__VA_ARGS__));  \
                                                                        \
        asmlinkage long SyS##name(__MAP(x,__SC_LONG,__VA_ARGS__));      \
                                                                        \
        asmlinkage long SyS##name(__MAP(x,__SC_LONG,__VA_ARGS__))       \
        {                                                               \
                long ret = SYSC##name(__MAP(x,__SC_CAST,__VA_ARGS__));  \
                __MAP(x,__SC_TEST,__VA_ARGS__);                         \
                __PROTECT(x, ret,__MAP(x,__SC_ARGS,__VA_ARGS__));       \
                return ret;                                             \
        }                                                               \
                                                                        \
        static inline long SYSC##name(__MAP(x,__SC_DECL,__VA_ARGS__))

The first sys##name is definition of the syscall handler function with the given name - sys_system_call_name. The __SC_DECL macro takes the __VA_ARGS__ and combines call input parameter system type and the parameter name, because the macro definition is unable to determine the parameter types. And the __MAP macro applies __SC_DECL macro to the __VA_ARGS__ arguments. The other functions that are generated by the __SYSCALL_DEFINEx macro are need to protect from the CVE-2009-0029 and we will not dive into details about this here. Ok, as result of the SYSCALL_DEFINE3 macro, we will have:

asmlinkage long sys_write(unsigned int fd, const char __user * buf, size_t count);

Now we know a little about the system call's definition and we can go back to the implementation of the write system call. Let's look on the implementation of this system call again:

SYSCALL_DEFINE3(write, unsigned int, fd, const char __user *, buf,
        size_t, count)
{
    struct fd f = fdget_pos(fd);
    ssize_t ret = -EBADF;

    if (f.file) {
        loff_t pos = file_pos_read(f.file);
        ret = vfs_write(f.file, buf, count, &pos);
        if (ret >= 0)
            file_pos_write(f.file, pos);
        fdput_pos(f);
    }

    return ret;
}

As we already know and can see from the code, it takes three arguments:

fd - file descriptor;
buf - buffer to write;
count - length of buffer to write.

and writes data from a buffer declared by the user to a given device or a file. Note that the second parameter buf, defined with the __user attribute. The main purpose of this attribute is for checking the Linux kernel code with the sparse util. It is defined in the include/linux/compiler.h header file and depends on the __CHECKER__ definition in the Linux kernel. That's all about useful meta-information related to our sys_write system call, let's try to understand how this system call is implemented. As we can see it starts from the definition of the f structure that has fd structure type that represents file descriptor in the Linux kernel and we put the result of the call of the fdget_pos function. The fdget_pos function defined in the same source code file and just expands the call of the __to_fd function:

static inline struct fd fdget_pos(int fd)
{
        return __to_fd(__fdget_pos(fd));
}

The main purpose of the fdget_pos is to convert the given file descriptor which is just a number to the fd structure. Through the long chain of function calls, the fdget_pos function gets the file descriptor table of the current process, current->files, and tries to find a corresponding file descriptor number there. As we got the fd structure for the given file descriptor number, we check it and return if it does not exist. We get the current position in the file with the call of the file_pos_read function that just returns f_pos field of our file:

static inline loff_t file_pos_read(struct file *file)
{
        return file->f_pos;
}

and calls the vfs_write function. The vfs_write function defined in the fs/read_write.c source code file and does the work for us - writes given buffer to the given file starting from the given position. We will not dive into details about the vfs_write function, because this function is weakly related to the system call concept but mostly about Virtual file system concept which we will see in another chapter. After the vfs_write has finished its work, we check the result and if it was finished successfully we change the position in the file with the file_pos_write function:

if (ret >= 0)
    file_pos_write(f.file, pos);

that just updates f_pos with the given position in the given file:

static inline void file_pos_write(struct file *file, loff_t pos)
{
        file->f_pos = pos;
}

At the end of the our write system call handler, we can see the call of the following function:

fdput_pos(f);

unlocks the f_pos_lock mutex that protects file position during concurrent writes from threads that share file descriptor.

That's all.

We have seen the partial implementation of one system call provided by the Linux kernel. Of course we have missed some parts in the implementation of the write system call, because as I mentioned above, we will see only system calls related stuff in this chapter and will not see other stuff related to other subsystems, such as Virtual file system.

Conclusion

This concludes the first part covering system call concepts in the Linux kernel. We have covered the theory of system calls so far and in the next part we will continue to dive into this topic, touching Linux kernel code related to system calls.

If you have questions or suggestions, feel free to ping me in twitter 0xAX, drop me email or just create issue.

Please note that English is not my first language and I am really sorry for any inconvenience. If you found any mistakes please send me PR to linux-insides.

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Last updated 4 years ago