![]() ![]() ![]() However, sysenter does absolutely no state saving, so the user stack pointer and return address both have to either be well-known values, or have to be saved by the user-space code leading up to the sysenter. The sysenter entry point will have the kernel stack set already. It allows a faster switch from user mode to kernel mode, by limiting the overhead of changing mode. On Intel CPU, starting from the Pentium II, a new instruction pair sysenter/sysexit has appeared. Sysenter/Sysexit (Intel) Main article: Sysenter If there is a hole in the table, fill it with a pointer to a function returning an error code! Note that this assumes the syscall table to be NULL free. This can be simplified into something like this: Linux proceeds to construct a pt_regs structure on the stack and passes a pointer to it to a C function to handle the call itself. The ABI however places all arguments in reverse order on the stack. It is therefore necessary to translate from one to the other.įor example, on i386, the Linux kernel gets its arguments in eax, ebx, ecx, edx, esi, edi, and ebp in that order. This is because your system call ABI will likely not correspond to the normal ABI the compiler supports. To do this, you will have to create your interrupt handler in Assembly. Other systems may have a fixed system call vector (e.g. Linux traditionally uses interrupt 0x80 for this purpose on x86. It is probably the most portable way to implement system calls. The most common way to implement system calls is using a software interrupt. ![]() Possible methods to make a System Call Interrupts 1 Possible methods to make a System Call. ![]()
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