2011/9/8 rohan puri <rohan.puri15@gmail.com>:
On Thu, Sep 8, 2011 at 9:50 AM, Mulyadi Santosa <mulyadi.santosa@gmail.com> wrote:
Hi :)
On Thu, Sep 8, 2011 at 10:40, rohan puri <rohan.puri15@gmail.com> wrote:
Hi Mulyadi,
Thanks for the response. I do agree with the issue concerning the no.of registers available are less for 32 bit to that of 64 bit.
But even in this case if parameters are more than the no of register for 32 bit, then also before executing int 80h interrupt it stores the parameters in the register and after that system call dispatcher puts them on the kernel stack of that process. So, how the case where more than no of registers are the parameters to the system call is handled.
Please don't top post :)
Regarding passing the parameters when it is more than number of registers used in x86 32 bit, AFAIK it's using EDX to contain the address of user space memory that contains the data, later it will be copied to kernel stack IIRC.
-- regards,
Mulyadi Santosa Freelance Linux trainer and consultant
blog: the-hydra.blogspot.com training: mulyaditraining.blogspot.com
Sorry for top posting. :)
Got it now. Thanks.
Regards, Rohan.
_______________________________________________ Kernelnewbies mailing list Kernelnewbies@kernelnewbies.org http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
Kernel newbies website crashed? can not login, only get the snapshot as below. http://kernelnewbies.org/FAQ/asmlinkage FAQ/asmlinkage What is asmlinkage? The asmlinkage tag is one other thing that we should observe about this simple function. This is a #define for some gcc magic that tells the compiler that the function should not expect to find any of its arguments in registers (a common optimization), but only on the CPU's stack. Recall our earlier assertion that system_call consumes its first argument, the system call number, and allows up to four more arguments that are passed along to the real system call. system_call achieves this feat simply by leaving its other arguments (which were passed to it in registers) on the stack. All system calls are marked with the asmlinkage tag, so they all look to the stack for arguments. Of course, in sys_ni_syscall's case, this doesn't make any difference, because sys_ni_syscall doesn't take any arguments, but it's an issue for most other system calls. And, because you'll be seeing asmlinkage in front of many other functions, I thought you should know what it was about. It is also used to allow calling a function from assembly files. There also have explicit explanation in the kernel maillist archive. Date: Sat, 18 Jun 2005 16:29:01 +0200 To: Roy Lee <roylee17@gmail.com> On Sat, 18 Jun 2005 22:00:39 +0800 Roy Lee <roylee17@gmail.com> wrote:
It says that "To tell a compiler not to use the argument in the registers". but the syscall's argument does pass the arguments though registers, doesn't it?
yes, user space programs put arguments in registers (ebx, ecx, edx...) but see what happens later... 1) user space programs do something like this: mov $SYSCALLNUMER, %eax mov $ARG1, %ebx mov $ARG2, %ecx ... int $0x80 ------ go to kernel syscall handler ---> ---> arch(i386/kernel/entry.S: .... # system call handler stub ENTRY(system_call) pushl %eax # save orig_eax SAVE_ALL GET_THREAD_INFO(%ebp) # system call tracing in operation /* Note, _TIF_SECCOMP is bit number 8, and so it needs testw and not testb */ testw $(_TIF_SYSCALL_TRACE|_TIF_SYSCALL_AUDIT|_TIF_SECCOMP),TI_flags(%ebp) jnz syscall_trace_entry cmpl $(nr_syscalls), %eax jae syscall_badsys syscall_call: call *sys_call_table(,%eax,4) movl %eax,EAX(%esp) # store the return value ... notice just these 2 things: 1) SAVE_ALL ---> it's a MACRO that do this: #define SAVE_ALL \ cld; \ pushl %es; \ pushl %ds; \ pushl %eax; \ pushl %ebp; \ pushl %edi; \ pushl %esi; \ .... pushl %edx; \ > ARG3 < pushl %ecx; \ > ARG2 < pushl %ebx; \ > ARG1 < movl $(__USER_DS), %edx; \ movl %edx, %ds; \ movl %edx, %es; 2) call *sys_call_table(,%eax,4) this calls the "C" system call function So, since the arguments are passed on the stack (see SAVE_ALL) the "C" function MUST take them from the stack. -- Best Regards Lin