CODEWITHSUNDEEP
linuxx86stackkernelstate
Caleb Stewart
Caleb Stewart

Reputation: 640

What is the Linux process kernel stack state at process creation?

I can't find this information anywhere. Everywhere I look, I find things referring to how the stack looks once you hit "main" (whatever your entry point is), which would be the program arguments, and environment, but what I'm looking for is how the system sets up the stack to cooperate with the switch_to macro. The first time the task gets switched to, it would need to have EFLAGS, EBP, the registers that GCC saves, and the return address from the schedule() function on the stack pointed to by "tsk->thread->esp", but what I can't figure out is how the kernel sets up this stack, since it lets GCC save the general purpose registers (using the output parameters for inline assembly).

I am referring to x86 PCs only. I am researching the Linux scheduler/process system for my own small kernel I am (attempting) to write, and I can't get my head around what I'm missing. I know I'm missing something since the fact that Slackware is running on my computer is a testament to the fact that the scheduler works :P

EDIT: I seem to have worded this badly. I am looking for information on how the tasks kernel stack is setup not how the tasks user task is setup. More specifically, the stack which tsk->thread->esp points to, and that "switch_to" switches to.

Upvotes: 11

Views: 1974

Answers (3)

caf
caf

Reputation: 239341

The initial kernel stack for a new process is set in copy_thread(), which is an arch-specific function. The x86 version, for example, starts out like this:

int copy_thread(unsigned long clone_flags, unsigned long sp,
unsigned long unused,
struct task_struct *p, struct pt_regs *regs)
{
    struct pt_regs *childregs;
    struct task_struct *tsk;
    int err;

    childregs = task_pt_regs(p);
    *childregs = *regs;
    childregs->ax = 0;
    childregs->sp = sp;

    p->thread.sp = (unsigned long) childregs;
    p->thread.sp0 = (unsigned long) (childregs+1);

    p->thread.ip = (unsigned long) ret_from_fork;

p->thread.sp and p->thread.ip are the new thread's kernel stack pointer and instruction pointer respectively.

Note that it does not place a saved %eflags, %ebp etc there, because when a newly-created thread of execution is first switched to, it starts out executing at ret_from_fork (this is where __switch_to() returns to for a new thread), which means that it doesn't execute the second-half of the switch_to() routine.

Upvotes: 6

Employed Russian
Employed Russian

Reputation: 213955

Google for "linux stack layout process startup" gives this link: "Startup state of a Linux/i386 ELF binary", which describes the set up that the kernel performs just before transferring control to the libc startup code.

Upvotes: 2

Basile Starynkevitch
Basile Starynkevitch

Reputation: 1

The state of the stack at process creation is described in the X86-64 SVR4 ABI supplement (for AMD64, ie x86-64 64 bits machines). The equivalent for 32 bits Intel processor is probably ABI i386. I strongly recommend reading also Assembly HOWTO. And of course, you should perhaps read the relevant Linux kernel file.

Upvotes: 2

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