Messages in this thread | | | Date | Fri, 7 Jul 2000 11:42:04 -0700 (PDT) | From | Chris Lattner <> | Subject | Question about kernel_thread/sys_clone on i386 (and others) |
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Okay, this really has me stumped. :) I'm trying to figure out how kernel_thread could possible work. Here's the code for it [truncated]:
int kernel_thread(int (*fn)(void *), void *arg, unsigned long flags) { long retval, d0;
__asm__ __volatile__( "movl %%esp,%%esi\n\t" "int $0x80\n\t" /* Linux/i386 system call */ "cmpl %%esp,%%esi\n\t" /* child or parent? */ "je 1f\n\t" /* parent - jump */
/* do child stuff here */ "1:\t" :"=&a" (retval), "=&S" (d0) :"0" (__NR_clone), "i" (__NR_exit), "r" (arg), "r" (fn), "b" (flags | CLONE_VM) : "memory");
Which seems to make sense... we're using sys_clone to do the work. The problem is that when it's compiled by gcc, I get code like this:
0x1006d584 [kernel_thread]: pushl %ebp 0x1006d585 [kernel_thread+0x1]: movl %esp,%ebp 0x1006d587 [kernel_thread+0x3]: pushl %esi 0x1006d588 [kernel_thread+0x4]: pushl %ebx 0x1006d589 [kernel_thread+0x5]: movl 8(%ebp),%edx 0x1006d58c [kernel_thread+0x8]: movl 16(%ebp),%ebx 0x1006d58f [kernel_thread+0xb]: orb $0x1,%bh 0x1006d592 [kernel_thread+0xe]: movl $0x78,%eax 0x1006d597 [kernel_thread+0x13]: movl 12(%ebp),%ecx 0x1006d59a [kernel_thread+0x16]: movl %esp,%esi 0x1006d59c [kernel_thread+0x18]: int $0x80 0x1006d59e [kernel_thread+0x1a]: cmpl %esp,%esi 0x1006d5a0 [kernel_thread+0x1c]: je 0xc <1006d5ae> ...
So the "newesp" argument is getting set to 12(%ebp) which is the 'arg' parameter to the kernel_thread. Okay, this is wierd. On other platforms [sparc] it gets set to 0, which is fine because clone assumes you want to return on the same stack that you left on... that's fine as well, except that the 'cmpl %esp,%esi' would seem to preclude returning on the same stack. Why isn't %eax checked for the return value of clone?
This is very mysterious code... and maybe my version of gcc is not happy, but I could swear nothing says to set ecx to 0... and when it does get to zero, how does kernel_thread know that it should allocate a new stack for the process? (which is what I assume is the only possible thing to do...)
Thanks all,
-Chris
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