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In this section we will discuss in detail how to use our own system call to debug the application.
First, principle
To make a system call, of course, the prime minister has to understand the process of system calls:
We take the open function to give an example: When the user space to perform the open function, will eventually call sys_open function through the role of the glibc library, sys_open function will eventually call our specific registered open function! So what's the main thing here is the glibc library? In fact, its role is to perform a swi #val instruction when the user space executes the open function. This instruction causes the cpu to behave abnormally, and jumps to the exception vector entry: vector_swi to execute, after which the code will be triggered. Abnormal instructions to take out the parameters, and call the corresponding processing function based on this parameter! Sys_open, sys_read, sys_write these functions are placed in an array, which is based on the val value of the sys_open function to find the index!
That is a bit messy, we have to reason: app call open-"swi #val-" throw cpu exception - "jump to the exception vector entry-" according to the instruction that caused the exception to call the corresponding processing function!
Then we need to implement two points when we make system calls:
1, write an application function: swi #val
2, in the kernel inside sys_xxx write a function, into the array!
The former is used to throw an exception, the latter is used for concrete realization!
Second, to achieve
1, the kernel function
(1) Add an item at the end of the CALL() list in the arch/arm/kernel/call.S file, such as: CALL(sys_hello)
Here is used to call sys_hello
(2) Add the following code in fs/read_write.c file:
Asmlinkage void sys_hello(const char __user * buf, size_t count)
{
Char ker_buf[100];
If(buf)
{
// The goal is to copy data from user space to kernel space, failing to return the number of bytes that were not copied, successfully returning 0
Third, steps
1, modify the application's executable file, replace the code of a place for the swi val
2, the implementation of the program
3, enter sys_hello-> print information in sys_hello -> execute the original instruction -> return
Fourth, the specific realization
Our application is:
//file:test_sc.c
#include
Int cnt = 0;
Void C(void)
{
Int i = 0;
While (1)
{
Printf("Hello, cnt = %d, i = %d", cnt, i);
Cnt++;
i = i + 2;
Sleep(5);
}
}
Void B(void)
{
C();
}
Void A(void)
{
B();
}
Int main(int argc, char **argv)
{
A();
Return 0;
}
Specific steps:
(1) compile: arm-linux-gcc test_sc.c -o test_sc
(2) Disassembler: arm-linux-objdump -D test_sc > test_sc.dis
(3) We open the above executable and disassembled files
For example, if we want to breakpoint at the i=i+2; point of the C function, we first find the corresponding instruction in the disassembly file:
84d4: e2833002 add r3, r3, #2 ; 0x2
Among them: e2833002 is a machine code, which is what we need!
We go to the executable file and search for the machine code. The corresponding machine code in the executable file should be: 02 30 83 e2
We changed this machine code to the machine code of the swi instruction!
We can disassemble the files in the previous section, and then get the swi machine code: ef900160
(4) After the above modification, when the program is executed to i=i+2; this instruction will generate a system call and finally execute the sys_hello function.
Inside the sys_hello function, we can do the most necessary work. The specific procedure is as follows: 
You also need to change the function declaration in the file include/linux/syscalls.h to: asmlinkage void sys_hello(const char __user * buf, int count);
(5) compile the kernel, start with a new kernel
(6) Run the test program (before modifying the test program's permission: chmod 777 test_sc_swi), the output information is as follows:
Hello, cnt = 0, i = 0
Sys_hello: cnt = 1
Sys_hello: i = 0
Hello, cnt = 1, i = 2
Sys_hello: cnt = 2
Sys_hello: i = 2
Hello, cnt = 2, i = 4
Sys_hello: cnt = 3
Sys_hello: i = 4
Test success!
The debugging method described in this section is rather obscure and generally not used!
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