On Thu, Apr 01, 2004 at 19:36:29 -0600, Zhen Liu wrote: > >On Thu, Apr 01, 2004 at 16:41:29 +0200, Eric wrote: > >> Hi, > >> > >> > >> "All system calls are marked with the asmlinkage tag, so they all look > >> to the stack for arguments" > >> From kernelnewbies.org on the FAQ at "What is asmlinkage ?". > >> > >> I find this a bit strange :) > >> Let's take the exemple of the system call dup(0). > >> The C funtion dup() will call the real dup system call (by tne Interrupt > >> Software int 0x80). But the arguments of the real system call have to be > >> passed on registers (0 on ebx for the example). > >> Becoz when we call a system call, we're generally in user space. So the > >> stack is different. > >> So why system calls are marked with the asmlinkage tag ? I don't > >> understand. > > >So they all look to the stack for arguments! > > >> Or perhaps there's or i make a confusion between system call and C > >> function wrapper of system calls. > > >You are probably confusing the user-space and the kernel-space side of > >the thing. On the user-side, the wrapper takes arguments from the stack > >and loads them to the registers. And calls the trap. Now the kernel > >parts comes to play. It does some setup and calls the sys_* function. > >But the arguments are still in registers. So the asmlinkage tells the > >compiler, that the function will have the args in registers and not on > >the stack, as would be expected for C function. > > Correct me if I am wrong. > As my understanding, "asmlinkage" tells the compiler that the function > should find the args on stack instead of register. The reason to do so is to > disable some of confusion caused by compiler optimization, since the > optimization may use the register to pass args instead of stack. You can > find a piece of code in entry.S. Before calling the system call, the kernel > saves all register onto stack using "SAVE_ALL", so that the system call > functions can always find their args on the stack. You are right. Just checked the definition. It says: __attribute__((regparm(0))) So yes, the syscall gets arguments from the stack, really. Actualy, it is only defined that way on i386. On ia64 it says __attribute__((syscall_linkage)) and for all other architectures, it is just empty. ------------------------------------------------------------------------------- Jan 'Bulb' Hudec <bulb@ucw.cz>
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