I am in the process of creating a fiber threading system in C, following https://graphitemaster.github.io/fibers/ . I have a function to set and restore context, and what i am trying to accomplish is launching a function as a fiber with its own stack. Linux, x86_64 SysV ABI.

extern void restore_context(struct fiber_context*);
extern void create_context(struct fiber_context*);

void foo_fiber()
{
    printf("Called as a fiber");
    exit(0);
}

int main()
{
    const uint32_t stack_size = 4096 * 16;
    const uint32_t red_zone_abi = 128;

    char* stack = aligned_alloc(16, stack_size);
    char* sp = stack   stack_size - red_zone_abi;

    struct fiber_context c = {0};
    c.rip = (void*)foo_fiber;
    c.rsp = (void*)sp;

    restore_context(&c);
}

where restore_context code is as follows:

.type restore_context, @function
.global restore_context
restore_context:
  movq 8*0(%rdi), %r8

  # Load new stack pointer.
  movq 8*1(%rdi), %rsp

  # Load preserved registers.
  movq 8*2(%rdi), %rbx
  movq 8*3(%rdi), %rbp
  movq 8*4(%rdi), %r12
  movq 8*5(%rdi), %r13
  movq 8*6(%rdi), %r14
  movq 8*7(%rdi), %r15

  # Push RIP to stack for RET.
  pushq %r8

  xorl