从exec系统调用返回时使用iret

Question

我注意到在exec大部分工作完成后调用的start_thread函数的末尾，有一个对force_iret的调用：

    static void
    start_thread_common(struct pt_regs *regs, unsigned long new_ip,
        unsigned long new_sp,
        unsigned int _cs, unsigned int _ss, unsigned int _ds)
    {
         loadsegment(fs, 0);
         loadsegment(es, _ds);
         loadsegment(ds, _ds);
         load_gs_index(0);
         regs->ip       = new_ip;
         regs->sp       = new_sp;
         regs->cs       = _cs;
         regs->ss       = _ss;
         regs->flags        = X86_EFLAGS_IF;
         force_iret();
    }    

我认为这样做是为了确保不使用sysexit返回用户空间。 那么，为什么从exec返回时必须使用iret呢？

Answer 1

该函数修改了sysret / sysexit无法恢复的寄存器。

这是arch/x86/include/asm/thread_info.h ：

/*
 * Force syscall return via IRET by making it look as if there was
 * some work pending. IRET is our most capable (but slowest) syscall
 * return path, which is able to restore modified SS, CS and certain
 * EFLAGS values that other (fast) syscall return instructions
 * are not able to restore properly.
 */
#define force_iret() set_thread_flag(TIF_NOTIFY_RESUME)