How the Linux kernel runs a program
How does the Linux kernel run a program
This is the fourth part of the chapter that describes system calls in the Linux kernel and as I wrote in the conclusion of the previous - this part will be last in this chapter. In the previous part we stopped at the two new concepts:
vsyscall
;vDSO
;
that are related and very similar on system call concept.
This part will be last part in this chapter and as you can understand from the part's title - we will see what does occur in the Linux kernel when we run our programs. So, let's start.
how do we launch our programs?
There are many different ways to launch an application from a user perspective. For example we can run a program from the shell or double-click on the application icon. It does not matter. The Linux kernel handles application launch regardless how we do launch this application.
In this part we will consider the way when we just launch an application from the shell. As you know, the standard way to launch an application from shell is the following: We just launch a terminal emulator application and just write the name of the program and pass or not arguments to our program, for example:
Let's consider what does occur when we launch an application from the shell, what does shell do when we write program name, what does Linux kernel do etc. But before we will start to consider these interesting things, I want to warn that this book is about the Linux kernel. That's why we will see Linux kernel insides related stuff mostly in this part. We will not consider in details what does shell do, we will not consider complex cases, for example subshells etc.
My default shell is - bash, so I will consider how do bash shell launches a program. So let's start. The bash
shell as well as any program that written with C programming language starts from the main function. If you will look on the source code of the bash
shell, you will find the main
function in the shell.c source code file. This function makes many different things before the main thread loop of the bash
started to work. For example this function:
checks and tries to open
/dev/tty
;check that shell running in debug mode;
parses command line arguments;
reads shell environment;
loads
.bashrc
,.profile
and other configuration files;and many many more.
After all of these operations we can see the call of the reader_loop
function. This function defined in the eval.c source code file and represents main thread loop or in other words it reads and executes commands. As the reader_loop
function made all checks and read the given program name and arguments, it calls the execute_command
function from the execute_cmd.c source code file. The execute_command
function through the chain of the functions calls:
makes different checks like do we need to start subshell
, was it builtin bash
function or not etc. As I already wrote above, we will not consider all details about things that are not related to the Linux kernel. In the end of this process, the shell_execve
function calls the execve
system call:
The execve
system call has the following signature:
and executes a program by the given filename, with the given arguments and environment variables. This system call is the first in our case and only, for example:
So, a user application (bash
in our case) calls the system call and as we already know the next step is Linux kernel.
execve system call
We saw preparation before a system call called by a user application and after a system call handler finished its work in the second part of this chapter. We stopped at the call of the execve
system call in the previous paragraph. This system call defined in the fs/exec.c source code file and as we already know it takes three arguments:
Implementation of the execve
is pretty simple here, as we can see it just returns the result of the do_execve
function. The do_execve
function defined in the same source code file and do the following things:
Initialize two pointers on a userspace data with the given arguments and environment variables;
return the result of the
do_execveat_common
.
We can see its implementation:
The do_execveat_common
function does main work - it executes a new program. This function takes similar set of arguments, but as you can see it takes five arguments instead of three. The first argument is the file descriptor that represent directory with our application, in our case the AT_FDCWD
means that the given pathname is interpreted relative to the current working directory of the calling process. The fifth argument is flags. In our case we passed 0
to the do_execveat_common
. We will check in a next step, so will see it later.
First of all the do_execveat_common
function checks the filename
pointer and returns if it is NULL
. After this we check flags of the current process that limit of running processes is not exceeded:
If these two checks were successful we unset PF_NPROC_EXCEEDED
flag in the flags of the current process to prevent fail of the execve
. You can see that in the next step we call the unshare_files
function that defined in the kernel/fork.c and unshares the files of the current task and check the result of this function:
We need to call this function to eliminate potential leak of the execve'd binary's file descriptor. In the next step we start preparation of the bprm
that represented by the struct linux_binprm
structure (defined in the include/linux/binfmts.h header file). The linux_binprm
structure is used to hold the arguments that are used when loading binaries. For example it contains vma
field which has vm_area_struct
type and represents single memory area over a contiguous interval in a given address space where our application will be loaded, mm
field which is memory descriptor of the binary, pointer to the top of memory and many other different fields.
First of all we allocate memory for this structure with the kzalloc
function and check the result of the allocation:
After this we start to prepare the binprm
credentials with the call of the prepare_bprm_creds
function:
Initialization of the binprm
credentials in other words is initialization of the cred
structure that stored inside of the linux_binprm
structure. The cred
structure contains the security context of a task for example real uid of the task, real guid of the task, uid
and guid
for the virtual file system operations etc. In the next step as we executed preparation of the bprm
credentials we check that now we can safely execute a program with the call of the check_unsafe_exec
function and set the current process to the in_execve
state.
After all of these operations we call the do_open_execat
function that checks the flags that we passed to the do_execveat_common
function (remember that we have 0
in the flags
) and searches and opens executable file on disk, checks that our we will load a binary file from noexec
mount points (we need to avoid execute a binary from filesystems that do not contain executable binaries like proc or sysfs), initializes file
structure and returns pointer on this structure. Next we can see the call the sched_exec
after this:
The sched_exec
function is used to determine the least loaded processor that can execute the new program and to migrate the current process to it.
After this we need to check file descriptor of the give executable binary. We try to check does the name of the our binary file starts from the /
symbol or does the path of the given executable binary is interpreted relative to the current working directory of the calling process or in other words file descriptor is AT_FDCWD
(read above about this).
If one of these checks is successful we set the binary parameter filename:
Otherwise if the filename is empty we set the binary parameter filename to the /dev/fd/%d
or /dev/fd/%d/%s
depends on the filename of the given executable binary which means that we will execute the file to which the file descriptor refers:
Note that we set not only the bprm->filename
but also bprm->interp
that will contain name of the program interpreter. For now we just write the same name there, but later it will be updated with the real name of the program interpreter depends on binary format of a program. You can read above that we already prepared cred
for the linux_binprm
. The next step is initialization of other fields of the linux_binprm
. First of all we call the bprm_mm_init
function and pass the bprm
to it:
The bprm_mm_init
defined in the same source code file and as we can understand from the function's name, it makes initialization of the memory descriptor or in other words the bprm_mm_init
function initializes mm_struct
structure. This structure defined in the include/linux/mm_types.h header file and represents address space of a process. We will not consider implementation of the bprm_mm_init
function because we do not know many important stuff related to the Linux kernel memory manager, but we just need to know that this function initializes mm_struct
and populate it with a temporary stack vm_area_struct
.
After this we calculate the count of the command line arguments which were passed to our executable binary, the count of the environment variables and set it to the bprm->argc
and bprm->envc
respectively:
As you can see we do this operations with the help of the count
function that defined in the same source code file and calculates the count of strings in the argv
array. The MAX_ARG_STRINGS
macro defined in the include/uapi/linux/binfmts.h header file and as we can understand from the macro's name, it represents maximum number of strings that were passed to the execve
system call. The value of the MAX_ARG_STRINGS
:
After we calculated the number of the command line arguments and environment variables, we call the prepare_binprm
function. We already call the function with the similar name before this moment. This function is called prepare_binprm_cred
and we remember that this function initializes cred
structure in the linux_bprm
. Now the prepare_binprm
function:
fills the linux_binprm
structure with the uid
from inode and read 128
bytes from the binary executable file. We read only first 128
from the executable file because we need to check a type of our executable. We will read the rest of the executable file in the later step. After the preparation of the linux_bprm
structure we copy the filename of the executable binary file, command line arguments and environment variables to the linux_bprm
with the call of the copy_strings_kernel
function:
And set the pointer to the top of new program's stack that we set in the bprm_mm_init
function:
The top of the stack will contain the program filename and we store this filename to the exec
field of the linux_bprm
structure.
Now we have filled linux_bprm
structure, we call the exec_binprm
function:
First of all we store the pid and pid
that seen from the namespace of the current task in the exec_binprm
:
and call the:
function. This function goes through the list of handlers that contains different binary formats. Currently the Linux kernel supports the following binary formats:
binfmt_script
- support for interpreted scripts that are starts from the #! line;binfmt_misc
- support different binary formats, according to runtime configuration of the Linux kernel;binfmt_elf
- support elf format;binfmt_aout
- support a.out format;binfmt_flat
- support for flat format;
So, the search_binary_handler
tries to call the load_binary
function and pass linux_binprm
to it. If the binary handler supports the given executable file format, it starts to prepare the executable binary for execution:
Where the load_binary
for example for the elf checks the magic number (each elf
binary file contains magic number in the header) in the linux_bprm
buffer (remember that we read first 128
bytes from the executable binary file): and exit if it is not elf
binary:
If the given executable file is in elf
format, the load_elf_binary
continues to execute. The load_elf_binary
does many different things to prepare on execution executable file. For example it checks the architecture and type of the executable file:
and exit if there is wrong architecture and executable file non executable non shared. Tries to load the program header table
:
that describes segments. Read the program interpreter
and libraries that linked with the our executable binary file from disk and load it to memory. The program interpreter
specified in the .interp
section of the executable file and as you can read in the part that describes Linkers it is - /lib64/ld-linux-x86-64.so.2
for the x86_64
. It setups the stack and map elf
binary into the correct location in memory. It maps the bss and the brk sections and does many many other different things to prepare executable file to execute.
In the end of the execution of the load_elf_binary
we call the start_thread
function and pass three arguments to it:
These arguments are:
Set of registers for the new task;
Address of the entry point of the new task;
Address of the top of the stack for the new task.
As we can understand from the function's name, it starts new thread, but it is not so. The start_thread
function just prepares new task's registers to be ready to run. Let's look on the implementation of this function:
As we can see the start_thread
function just makes a call of the start_thread_common
function that will do all for us:
The start_thread_common
function fills fs
segment register with zero and es
and ds
with the value of the data segment register. After this we set new values to the instruction pointer, cs
segments etc. In the end of the start_thread_common
function we can see the force_iret
macro that forces a system call return via iret
instruction. Ok, we prepared new thread to run in userspace and now we can return from the exec_binprm
and now we are in the do_execveat_common
again. After the exec_binprm
will finish its execution we release memory for structures that was allocated before and return.
After we returned from the execve
system call handler, execution of our program will be started. We can do it, because all context related information is already configured for this purpose. As we saw the execve
system call does not return control to a process, but code, data and other segments of the caller process are just overwritten of the program segments. The exit from our application will be implemented through the exit
system call.
That's all. From this point our program will be executed.
Conclusion
This is the end of the fourth part of the about the system calls concept in the Linux kernel. We saw almost all related stuff to the system call
concept in these four parts. We started from the understanding of the system call
concept, we have learned what is it and why do users applications need in this concept. Next we saw how does the Linux handle a system call from a user application. We met two similar concepts to the system call
concept, they are vsyscall
and vDSO
and finally we saw how does Linux kernel run a user program.
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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