加载将缓冲区复制到 BPF 堆栈的 BPF 程序时出错

Question

我正在尝试加载一个 BPF 程序，它只是将 tty_write 的 buf 参数复制到 BPF 堆栈。我的程序如下：

#define BUFSIZE 256

SEC("kprobe/tty_write")
int kprobe__tty_write(struct pt_regs *ctx, struct file *file, const char __user *buf, size_t count)
{
  char buffer[BUFSIZE];
  bpf_probe_read(buffer, BUFSIZE, (void *)buf);

  return 0;
}

请注意，我正在使用 bpf_helpers.h from tcptracer-bpf to define the SEC macro. In my real program I would actually use buffer for something, but I have not shown that part here. When I try to load the program (from an ELF file using gobpf) 我收到以下错误：

error while loading "kprobe/tty_write" (permission denied):
0: (bf) r1 = r10
1: (07) r1 += -256
2: (b7) r2 = 256
3: (85) call bpf_probe_read#4
R3 !read_ok

这是为什么？我的程序改编自 ttysnoop.py，所以我知道可以做我想做的事情。我的程序完整反汇编如下：

Disassembly of section kprobe/tty_write:
kprobe__tty_write:
       0:   bf a1 00 00 00 00 00 00     r1 = r10
       1:   07 01 00 00 00 ff ff ff     r1 += -256
       2:   b7 02 00 00 00 01 00 00     r2 = 256
       3:   85 00 00 00 04 00 00 00     call 4
       4:   b7 00 00 00 00 00 00 00     r0 = 0
       5:   95 00 00 00 00 00 00 00     exit

uname -a: Linux ubuntu1710 4.13.0-32-generic #35-Ubuntu SMP Thu Jan 25 09:13:46 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux

编辑：

作为实验，我尝试加载一个类似于 the example program 的程序，该程序描述了 bpf_probe_read_str 作为辅助函数引入时的描述：

#define BUFSIZE 256

SEC("kprobe/sys_open")
void bpf_sys_open(struct pt_regs *ctx)
{
  char buf[BUFSIZE];
  bpf_probe_read(buf, sizeof(buf), (void *)ctx->di);
}

加载没有问题并给出以下程序集：

Disassembly of section kprobe/sys_open:
bpf_sys_open:
       0:   79 13 70 00 00 00 00 00     r3 = *(u64 *)(r1 + 112)
       1:   bf a1 00 00 00 00 00 00     r1 = r10
       2:   07 01 00 00 00 ff ff ff     r1 += -256
       3:   b7 02 00 00 00 01 00 00     r2 = 256
       4:   85 00 00 00 04 00 00 00     call 4
       5:   95 00 00 00 00 00 00 00     exit

所以我的 tty_write 程序似乎在 kprobe 被触发后将第三个寄存器直接传递给对 bpf_probe_read 的调用；这可能是我看到的错误的原因，但我不确定。

Answer 1

正如您自己发现的那样，问题出在 kprobe__tty_write 中使用了额外的参数。这在 ttysnoop 中有效，因为它使用 bcc 来编译和加载 BPF 程序。 bcc 实际上将附加参数 重写为ctx->xx 解引用。您可以通过以下代码片段看到这一点：

from bcc import BPF
BPF(text="""
#include <linux/ptrace.h>
int kprobe__tty_write(struct pt_regs *ctx, struct file *file, const char __user *buf, size_t count) {
return 0;
}
""", debug=4)

感谢debug=4，我们可以在执行时看到重写：

$ sudo python tmp.py
clang -cc1 -triple x86_64-unknown-linux-gnu -emit-llvm-bc -emit-llvm-uselists -disable-free -disable-llvm-verifier -discard-value-names -main-file-name main.c -mrelocation-model static -mthread-model posix -fmath-errno -masm-verbose -mconstructor-aliases -fuse-init-array -target-cpu x86-64 -momit-leaf-frame-pointer -dwarf-column-info -debugger-tuning=gdb -coverage-notes-file /usr/src/linux-headers-4.4.0-112-generic/main.gcno -nostdsysteminc -nobuiltininc -resource-dir lib/clang/5.0.1 -isystem /virtual/lib/clang/include -include ./include/linux/kconfig.h -include /virtual/include/bcc/bpf.h -include /virtual/include/bcc/helpers.h -isystem /virtual/include -I /home/paul/bcc2 -I ./arch/x86/include -I arch/x86/include/generated/uapi -I arch/x86/include/generated -I include -I ./arch/x86/include/uapi -I arch/x86/include/generated/uapi -I ./include/uapi -I include/generated/uapi -D __KERNEL__ -D __HAVE_BUILTIN_BSWAP16__ -D __HAVE_BUILTIN_BSWAP32__ -D __HAVE_BUILTIN_BSWAP64__ -O2 -Wno-deprecated-declarations -Wno-gnu-variable-sized-type-not-at-end -Wno-pragma-once-outside-header -Wno-address-of-packed-member -Wno-unknown-warning-option -Wno-unused-value -Wno-pointer-sign -fdebug-compilation-dir /usr/src/linux-headers-4.4.0-112-generic -ferror-limit 19 -fmessage-length 168 -fobjc-runtime=gcc -fdiagnostics-show-option -vectorize-loops -vectorize-slp -o main.bc -x c /virtual/main.c

#include <linux/ptrace.h>
__attribute__((section(".bpf.fn.kprobe__tty_write")))
int kprobe__tty_write(struct pt_regs *ctx) { struct file *file = ctx->di; const char __user *buf = ctx->si; size_t count = ctx->dx;
return 0;
}

以同样的方式，bcc从kprobe__tty_write中提取函数名并自动将BPF程序附加到tty_write。

加载将缓冲区复制到 BPF 堆栈的 BPF 程序时出错

Error while loading a BPF program that copies a buffer to the BPF stack

c

bpf

ebpf