c - 在BPF程序上传中破译“无效的内存访问map_ptr”

我正在编写一个自定义应用程序，它读取C代码，调用LLVM从该C代码生成BPF字节码，然后重新定位任何BPF映射符号并将其上载到内核。我可以成功地上载和运行不使用BPF映射的程序，但只要将程序重新定位为使用BPF映射，就会出现以下错误：

invalid mem access 'map_ptr'

详情如下：
向LLVM提供以下输入：

// Placeholder values for user-requested maps
void *a;

#include <linux/ptrace.h>
#include <uapi/linux/bpf.h>
#include "bpf_helpers.h"

int kprobe__blk_start_request(struct pt_regs *ctx) {
  long rq = PT_REGS_PARM1(ctx);
  u64 val = bpf_ktime_get_ns();
  bpf_map_update_elem(a, &rq, &val, BPF_ANY);
  return 0;
}

它生成以下字节码

Found function: kprobe__blk_start_request
  size: 120 bytes
  addr: 0
relocate: a @ 32

/tmp/bpf7990/bpf.o: file format ELF64-BPF

Disassembly of section .text:
kprobe__blk_start_request:
       0:   79 11 70 00 00 00 00 00     r1 = *(u64 *)(r1 + 112)
       1:   7b 1a f8 ff 00 00 00 00     *(u64 *)(r10 - 8) = r1
       2:   85 00 00 00 05 00 00 00     call 5
       3:   7b 0a f0 ff 00 00 00 00     *(u64 *)(r10 - 16) = r0
       4:   18 01 00 00 00 00 00 00 00 00 00 00 00 00 00 00     r1 = 0ll
       6:   79 11 00 00 00 00 00 00     r1 = *(u64 *)(r1 + 0)
       7:   bf a2 00 00 00 00 00 00     r2 = r10
       8:   07 02 00 00 f8 ff ff ff     r2 += -8
       9:   bf a3 00 00 00 00 00 00     r3 = r10
      10:   07 03 00 00 f0 ff ff ff     r3 += -16
      11:   b7 04 00 00 00 00 00 00     r4 = 0
      12:   85 00 00 00 02 00 00 00     call 2
      13:   b7 00 00 00 00 00 00 00     r0 = 0
      14:   95 00 00 00 00 00 00 00     exit

“a”符号在指令4中重新定位如下：

Relocating instruction 4
    from code:0x18 dst_reg:1 src_reg:0 off:0x0 imm:0x0
      to code:0x18 dst_reg:1 src_reg:1 off:0x0 imm:0x4
kprobe__blk_start_request:
       0:   79 11 70 00 00 00 00 00
       1:   7b 1a f8 ff 00 00 00 00
       2:   85 00 00 00 05 00 00 00
       3:   7b 0a f0 ff 00 00 00 00
       4:   18 11 00 00 04 00 00 00
       5:   00 00 00 00 00 00 00 00
       6:   79 11 00 00 00 00 00 00
       7:   bf a2 00 00 00 00 00 00
       8:   07 02 00 00 f8 ff ff ff
       9:   bf a3 00 00 00 00 00 00
      10:   07 03 00 00 f0 ff ff ff
      11:   b7 04 00 00 00 00 00 00
      12:   85 00 00 00 02 00 00 00
      13:   b7 00 00 00 00 00 00 00
      14:   95 00 00 00 00 00 00 00

指令4中的“04”是符号“a”映射的FD。
在调用BPF_PROG_LOAD之后，我在内核的错误日志中得到以下信息。

Failed to load kprobe__blk_start_request BPF code
0: R1=ctx(id=0,off=0,imm=0) R10=fp0
0: (79) r1 = *(u64 *)(r1 +112)
1: R1=inv(id=0) R10=fp0
1: (7b) *(u64 *)(r10 -8) = r1
2: R1=inv(id=0) R10=fp0
2: (85) call bpf_ktime_get_ns#5
3: R0=inv(id=0) R10=fp0
3: (7b) *(u64 *)(r10 -16) = r0
4: R0=inv(id=0) R10=fp0
4: (18) r1 = 0xffff88042be0b000
6: R0=inv(id=0) R1=map_ptr(id=0,off=0,ks=8,vs=8) R10=fp0
6: (79) r1 = *(u64 *)(r1 +0)
R1 invalid mem access 'map_ptr'

Errno: 13 (Permission denied)

我无法破译这个错误日志。内核想告诉我什么？

最佳答案

错误解释
invalid mem access 'map_ptr'表示试图从无效的内存位置读取数据，特别是映射指针指向的内存位置。
实际上，参考你的字节码：

   4:   18 01 00 00 00 00 00 00 00 00 00 00 00 00 00 00     r1 = 0ll
   6:   79 11 00 00 00 00 00 00     r1 = *(u64 *)(r1 + 0)

首先在r1中加载一个立即数，然后读取r1指向的内存位置。但是，内核验证器将BPF-LD-IMM指令标识为映射指针加载。所以，it tags r1 with the map_ptr type。验证器然后拒绝指令6because it tries to read the memory location pointed to by the map pointer（您只应将其传递给map helpers，而不是在BPF程序中使用它）。
基本上，指令6既无效又不需要。没有它，你的程序应该通过验证程序。
可能修复
我没有你的重定位代码，所以我无法复制，但我猜这个无效的字节码与你如何声明映射有关。如果您查看BPF samples in the Linux kernel，您将看到映射通常声明为全局结构，第一个映射帮助器参数指向该全局结构：

// Placeholder values for user-requested maps
struct bpf_map_def a = {
    .type = BPF_MAP_TYPE_ARRAY,
    .key_size = sizeof(u32),
    .value_size = sizeof(int),
    .max_entries = 1024,
};

#include <linux/ptrace.h>
#include <uapi/linux/bpf.h>
#include "bpf_helpers.h"

int kprobe__blk_start_request(struct pt_regs *ctx) {
  long rq = PT_REGS_PARM1(ctx);
  u64 val = bpf_ktime_get_ns();
  bpf_map_update_elem(&a, &rq, &val, BPF_ANY);
  return 0;
}