Hi,
I'm trying to figure out the entry for 'syscall' in Fiasco, I already know that Fiasco will set MSR_STAR, MSR_LSTAR and MSR_CSTAR in Cpu::setup_sysenter.
However, things don't seem right when I run a simple program of my own, which is compiled statically outside in Ubuntu with gcc 5.4.0. This program directly issues a single 'syscall' instruction (instead of the syscall() function), which is equivalent to getpid(). Here is the program, which is compiled with gcc -static -o syscall_single -O2 -std=gnu99 syscall_single.c -------------------- BEGIN --------------------- //syscall_single.c //gcc -static -o syscall_single -O2 -std=gnu99 syscall_single.c #include <stdio.h>
void printres(unsigned long res) { printf("Done, id = %d\n", res); }
int main() { asm volatile( "mov $39, %%rax\n" "syscall\n" "mov %%rax, %%rdi\n" "call printres" : : : "cc" ); return 0; } --------------------- END ------------------------
Since Fiasco has specified MSR_LSTAR, I expect every normal system call to goto the 'entry_sys_fast_ipc_c' in src/kern/ia32/64/entry-native.S, therefore I add such line at the beginning of entry_sys_fast_ipc_c -------------------- BEGIN --------------------- .extern unsigned long test_rax; entry_sys_fast_ipc_c: movq %rax, test_rax //mov rax to the global variable test_rax --------------------- END ------------------------ And I print this variable in 'sys_ipc_wrapper'.
Now strange things happen, when I type './syscall_single' in the shell, test_rax is never 39 at any time! But yet it can get the right result, and there is output in 'dispatch_system_call' in l4linux side(manually added by me).
My questions are (1) What is the entrypoint and actual path if a user program directly issues a syscall instruction? (2) I find dispatch_system_call judges whether there is a system call based on the 'trap_nr' field of thread_struct, does this mean it is implemented through exception?
Thanks.