Question

我试图从以下代码中删除堆栈依赖项。

void  myfunction(struct kprobe *p, struct pt_regs *regs)
 {
         register void *rregs asm("r1") = regs;
         register void *rfn asm("lr") = p->ainsn.insn_fn;

         __asm__ __volatile__ (
                 "stmdb  sp!, {%[regs], r11}     \n\t"
                 "ldmia  %[regs], {r0-r12}       \n\t"    
                 "blx    %[fn]                   \n\t"
                 "ldr    lr, [sp], #4            \n\t" /* lr = regs */
                 "stmia  lr, {r0-r12}            \n\t"
                 "ldr    r11, [sp], #4           \n\t"

                 : [regs] "=r" (rregs), [fn] "=r" (rfn)
                 : "" (rregs), "1" (rfn)
                 : "r0", "r2", "r3", "r4", "r5", "r6", "r7",
                   "r8", "r9", "r10", "r12", "memory", "cc"
                 );
 }

在上面的函数中，stmdb sp!, {%[regs], r11}将r1和r11推入堆栈，然后重新启动。

就我而言，我应该避免在这里使用堆栈。所以我重写了

void myfunction(struct kprobe *p, struct pt_regs *regs)
{
        int r1_bk = 0, r11_bk = 0;
        register void *rregs asm("r1") = regs;
        register void *rfn asm("lr") = p->ainsn.insn_fn;
        register void *r1b_c asm("r1") = &r1_bk;
        register void *r11b_c asm("r11") = &r11_bk;    

      __asm__ __volatile__ (
                "ldr    %[r1b], r1      \n\t"
                "ldr    %[r11b], r11    \n\t"
                "ldmia  %[regs], {r0-r12}       \n\t"
                "blx    %[fn]                   \n\t"    
                "ldr    lr, %[r1b]              \n\t" /* lr = regs */
                "stmia  lr, {r0-r12}            \n\t"
                "ldr    r11, %[r11b]            \n\t"

                : [regs] "=r" (rregs), [fn] "=r" (rfn), [r1b] "=r" (r1b_c), [r11b] "=r" (r11b_c)
                : "0" (rregs), "1" (rfn)
                : "r0", "r2", "r3", "r4", "r5", "r6", "r7",
                  "r8", "r9", "r10", "r12", "memory", "cc"
                );
}

当我编译时，出现以下错误。

/tmp/ccJMefdC.s: Assembler messages:
/tmp/ccJMefdC.s:579: Error: internal_relocation (type: OFFSET_IMM) not fixed up
/tmp/ccJMefdC.s:580: Error: internal_relocation (type: OFFSET_IMM) not fixed up
/tmp/ccJMefdC.s:583: Error: internal_relocation (type: OFFSET_IMM) not fixed up
/tmp/ccJMefdC.s:585: Error: internal_relocation (type: OFFSET_IMM) not fixed up

我在这里提到internal relocation not fixed up。但它并没有给出明确的想法。请分享您对此的了解。

Answer 1

您的内联asm几乎所有寄存器都被调用，并且通过volatile指令明确告知编译器它不应跳过或尝试移动调用以优化寄存器使用。这意味着编译器在生成myfunction的等效指令时需要在发出内联汇编之前将寄存器保存到某处。

让我向你证明：

$ cat asm_vol.c 
void f() {
    asm volatile("" : : : "r0", "r2", "r3", "r4", "r5", "r6", "r7",
                  "r8", "r9", "r10", "r12", "memory", "cc");
}
$ arm-linux-gnueabihf-gcc -c -O2 asm_vol.c
$ arm-linux-gnueabihf-objdump -d asm_vol.o

asm_vol.o:     file format elf32-littlearm


Disassembly of section .text:

00000000 <f>:
   0:   e92d 07f0   stmdb   sp!, {r4, r5, r6, r7, r8, r9, sl}
   4:   e8bd 07f0   ldmia.w sp!, {r4, r5, r6, r7, r8, r9, sl}
   8:   4770        bx  lr
   a:   bf00        nop

Answer 2

错误消息的原因是ldr接受寄存器和内存引用，您提供两次相同的寄存器。汇编程序然后将寄存器名称解释为内存位置，因此抱怨它未在同一文件中定义。

由于您已经用完了寄存器，因此只能使用全局变量来避免堆栈使用。

从汇编代码中删除堆栈依赖项

2 个答案: