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modifying_the_linux_kernel [2020/09/07 13:14] misterjei |
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 ==== | ||
- | Test here | + | There are three major steps to adding a system call to the Linux kernel: |
+ | -Entry to the system call table for the appropriate architecture | ||
+ | -Prototype of system call & marking so parameters do not reside in registers (only the CPU stack) | ||
+ | -Definition of system call (implementation) | ||
+ | |||
+ | === 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, | ||
+ | |||
+ | '' | ||
+ | |||
+ | For example, in the x64 version of Linux, **//< | ||
+ | |||
+ | The table has four columns: | ||
+ | **//< | ||
+ | |||
+ | * When the syscall machine instruction executes, it loads a unique **call number** to identify the function. | ||
+ | * The **ABI** (application binary interface) for ABI-specific calls. Most calls use " | ||
+ | * A unique **call name** is the programmer-readable version of the system call used to define it later. | ||
+ | * Finally, the **entry point** is the assembly signature for the call (which uses the arch-specific prefix). | ||
+ | |||
+ | This example uses the " | ||
+ | < | ||
+ | |||
+ | === 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 '' |