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modifying_the_linux_kernel [2020/09/07 14:22] misterjei [Adding a System Call] |
modifying_the_linux_kernel [2021/09/17 11:36] (current) misterjei [Adding a System Call] |
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==== Adding a System Call ==== | ==== Adding a System Call ==== | ||
There are three major steps to adding a system call to the Linux kernel: | There are three major steps to adding a system call to the Linux kernel: | ||
- | -Adding a new entry to the system call table for the appropriate architecture | + | -Entry to the system call table for the appropriate architecture |
- | -Marking the function | + | -Prototype of system call & marking |
- | -Adding the system call functions themselves | + | -Definition of system call (implementation) |
- | === Adding an Entry to the System Call Table === | + | === 1. Entry in System Call Table === |
Every system call must have an associated system call number which corresponds to its position in the system call table. This table is transformed / imported into the source as part of the build process. The correct table varies by architecture, | Every system call must have an associated system call number which corresponds to its position in the system call table. This table is transformed / imported into the source as part of the build process. The correct table varies by architecture, | ||
- | / | + | '' |
- | For example, in the x64 version of Linux, < | + | For example, in the x64 version of Linux, |
The table has four columns: | The table has four columns: | ||
- | < | + | **//< |
* When the syscall machine instruction executes, it loads a unique **call number** to identify the function. | * When the syscall machine instruction executes, it loads a unique **call number** to identify the function. | ||
Line 23: | Line 23: | ||
* Finally, the **entry point** is the assembly signature for the call (which uses the arch-specific prefix). | * Finally, the **entry point** is the assembly signature for the call (which uses the arch-specific prefix). | ||
- | This example uses the " | + | This example uses the " |
- | 435 common sample_syscall __x64_sys_sample_syscall | + | < |
+ | === 2. Prototype of the System Call === | ||
+ | The kernel has no " | ||
+ | |||
+ | < | ||
+ | |||
+ | === 3. Definition of the System Call === | ||
+ | Finally, we need to add the system call definition. Thise will need to be compiled into the kernel as well. The kernel has lots of examples along these lines, which is for a system call with one parameter ('' | ||
+ | |||
+ | < | ||
+ | { | ||
+ | return sample_param + 1; // Adds 1 to the parameter and returns it | ||
+ | }</ | ||
+ | |||
+ | The '' |