从中断处理程序中的用户堆栈弹出值

Pop values from user stack inside interrupt handler

我正在尝试在 Pintos 中实现系统调用的处理程序。在引发中断之前,系统调用的参数按以下方式推送:

/* Invokes syscall NUMBER, passing argument ARG0, and returns the
   return value as an `int'. */
#define syscall1(NUMBER, ARG0)                                           \
        ({                                                               \
          int retval;                                                    \
          asm volatile                                                   \
            ("pushl %[arg0]; pushl %[number]; int [=10=]x30; addl , %%esp" \
               : "=a" (retval)                                           \
               : [number] "i" (NUMBER),                                  \
                 [arg0] "g" (ARG0)                                       \
               : "memory");                                              \
          retval;                                                        \
        })

/* Invokes syscall NUMBER, passing arguments ARG0 and ARG1, and
   returns the return value as an `int'. */
#define syscall2(NUMBER, ARG0, ARG1)                            \
        ({                                                      \
          int retval;                                           \
          asm volatile                                          \
            ("pushl %[arg1]; pushl %[arg0]; "                   \
             "pushl %[number]; int [=10=]x30; addl , %%esp"      \
               : "=a" (retval)                                  \
               : [number] "i" (NUMBER),                         \
                 [arg0] "g" (ARG0),                             \
                 [arg1] "g" (ARG1)                              \
               : "memory");                                     \
          retval;                                               \
        })

/* Invokes syscall NUMBER, passing arguments ARG0, ARG1, and
   ARG2, and returns the return value as an `int'. */
#define syscall3(NUMBER, ARG0, ARG1, ARG2)                      \
        ({                                                      \
          int retval;                                           \
          asm volatile                                          \
            ("pushl %[arg2]; pushl %[arg1]; pushl %[arg0]; "    \
             "pushl %[number]; int [=10=]x30; addl , %%esp"      \
               : "=a" (retval)                                  \
               : [number] "i" (NUMBER),                         \
                 [arg0] "g" (ARG0),                             \
                 [arg1] "g" (ARG1),                             \
                 [arg2] "g" (ARG2)                              \
               : "memory");                                     \
          retval;                                               \
        })

我有一个可用的结构,它包含所有被压入的寄存器,还有一个指向用户级堆栈的指针(系统调用号和参数被压入)。

/* Interrupt stack frame. */
struct intr_frame
  {
    /* Pushed by intr_entry in intr-stubs.S.
       These are the interrupted task's saved registers. */
    uint32_t edi;               /* Saved EDI. */
    uint32_t esi;               /* Saved ESI. */
    uint32_t ebp;               /* Saved EBP. */
    uint32_t esp_dummy;         /* Not used. */
    uint32_t ebx;               /* Saved EBX. */
    uint32_t edx;               /* Saved EDX. */
    uint32_t ecx;               /* Saved ECX. */
    uint32_t eax;               /* Saved EAX. */
    uint16_t gs, :16;           /* Saved GS segment register. */
    uint16_t fs, :16;           /* Saved FS segment register. */
    uint16_t es, :16;           /* Saved ES segment register. */
    uint16_t ds, :16;           /* Saved DS segment register. */

    /* Pushed by intrNN_stub in intr-stubs.S. */
    uint32_t vec_no;            /* Interrupt vector number. */

    /* Sometimes pushed by the CPU,
       otherwise for consistency pushed as 0 by intrNN_stub.
       The CPU puts it just under `eip', but we move it here. */
    uint32_t error_code;        /* Error code. */

    /* Pushed by intrNN_stub in intr-stubs.S.
       This frame pointer eases interpretation of backtraces. */
    void *frame_pointer;        /* Saved EBP (frame pointer). */

    /* Pushed by the CPU.
       These are the interrupted task's saved registers. */
    void (*eip) (void);         /* Next instruction to execute. */
    uint16_t cs, :16;           /* Code segment for eip. */
    uint32_t eflags;            /* Saved CPU flags. */
    void *esp;                  /* Saved stack pointer. */
    uint16_t ss, :16;           /* Data segment for esp. */
  };

我现在想得到这些参数。堆栈上的所有指针都是 4 字节大小,所以我想我可以简单地将一个参数(取消引用的指针)转换为相应的类型,然后将堆栈指针增加 4 并转换下一个指针。

我有以下问题:

pushl 指令将值压入堆栈正确吗?所以我应该能够简单地通过取消引用指向堆栈的指针来获得这些值?例如,要获得第一个参数(假设这是一个 int),我会使用 (int) *(f->esp + 4),其中 f 是指向结构 intr_frame 的指针,我添加 4,因为系统调用number 是堆栈中的第一个元素。现在的问题是 C 中不允许对 void 指针进行指针运算,并且参数可以是不同的类型,所以有人可以就如何从堆栈中弹出这些参数提出任何建议吗?

是的,您可以通过取消引用用户 esp 来获取参数值。与任何 void * 一样,您必须在取消引用或索引之前将其转换为合适的指针类型。在这种情况下,uint32_t * 是合适的,因此您可以使用

 *(((uint32_t *)f->esp) + 1)

请注意 +1 而不是 +4,因为索引是根据指向的对象的大小缩放的。如果你想使用实际的字节偏移量,你需要两次转换

*(uint32_t *)(((uint8_t *)f->esp) + 4)