As we can see, the `sys_call_table` is an array of `__NR_syscall_max + 1` size where the `__NR_syscall_max` macro represents the maximum number of system calls for the given [architecture](https://en.wikipedia.org/wiki/List_of_CPU_architectures). This book is about the [x86_64](https://en.wikipedia.org/wiki/X86-64) architecture, so for our case the `__NR_syscall_max` is `322` and this is the correct number at the time of writing (current Linux kernel version is `4.2.0-rc8+`). We can see this macro in the header file generated by [Kbuild](https://www.kernel.org/doc/Documentation/kbuild/makefiles.txt) during kernel compilation - include/generated/asm-offsets.h`:
As we can see, the `sys_call_table` is an array of `__NR_syscall_max + 1` size where the `__NR_syscall_max` macro represents the maximum number of system calls for the given [architecture](https://en.wikipedia.org/wiki/List_of_CPU_architectures). This book is about the [x86_64](https://en.wikipedia.org/wiki/X86-64) architecture, so for our case the `__NR_syscall_max` is `547` and this is the correct number at the time of writing (current Linux kernel version is `5.0.0-rc7`). We can see this macro in the header file generated by [Kbuild](https://www.kernel.org/doc/Documentation/kbuild/makefiles.txt) during kernel compilation - include/generated/asm-offsets.h`:
```C
#define __NR_syscall_max 322
#define __NR_syscall_max 547
```
There will be the same number of system calls in the [arch/x86/entry/syscalls/syscall_64.tbl](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/entry/syscalls/syscall_64.tbl#L331) for the `x86_64`. There are two important topics here; the type of the `sys_call_table` array, and the initialization of elements in this array. First of all, the type. The `sys_call_ptr_t` represents a pointer to a system call table. It is defined as [typedef](https://en.wikipedia.org/wiki/Typedef) for a function pointer that returns nothing and does not take arguments:
0xAX
5 years ago
committed by