On Sat, 18 Jun 2005 22:00:39 +0800
Roy Lee <[email protected]> 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.
>
> While tracing the code, I found the asmlinkage was a #define of a extern
> "C", and the only usage of extern "C" that I know is to avoid the name
> mangling while calling a C function in C++. Does the asmlinkage here have
> connection with that?
#define asmlinkage CPP_ASMLINKAGE __attribute__((regparm(0)))
SEE "__attribute__((regparm(0)))"... it tells GCC that the arguments are on
the STACK even if "-mregparm=NUMER" option is used.
--
Paolo Ornati
Linux 2.6.12 on x86_64
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to [email protected]
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
[Index of Archives]
[Kernel Newbies]
[Netfilter]
[Bugtraq]
[Photo]
[Stuff]
[Gimp]
[Yosemite News]
[MIPS Linux]
[ARM Linux]
[Linux Security]
[Linux RAID]
[Video 4 Linux]
[Linux for the blind]
[Linux Resources]
