android - 如何在 Android 的 ndk-gdb 中调用 malloc()

Question

当我尝试调用 ndk-gdb 中的 malloc() 函数时

(gdb) call malloc(4)

gdb报告失败

Program received signal SIGSEGV, Segmentation fault.
0x4017a8f4 in ?? () from /Users/chyhfj/Desktop/hello-jni/obj/local/armeabi/linker

错误日志出现在日志 View 中:

F/linker  (32355): ERROR: malloc called from the dynamic linker!

我做了一些研究，研究了 android 仿生链接器 source code ，并发现它实现了 malloc() 函数以避免在链接器中调用 malloc() 。

// You shouldn't try to call memory-allocating functions in the dynamic linker.
// Guard against the most obvious ones.
#define DISALLOW_ALLOCATION(return_type, name, ...) \
    return_type name __VA_ARGS__ \
    { \
        const char* msg = "ERROR: " #name " called from the dynamic linker!\n"; \
        __libc_format_log(ANDROID_LOG_FATAL, "linker", "%s", msg); \
        write(2, msg, strlen(msg)); \
        abort(); \
    }
DISALLOW_ALLOCATION(void*, malloc, (size_t u __unused));
DISALLOW_ALLOCATION(void, free, (void* u __unused));
DISALLOW_ALLOCATION(void*, realloc, (void* u1 __unused, size_t u2 __unused));
DISALLOW_ALLOCATION(void*, calloc, (size_t u1 __unused, size_t u2 __unused));

然后我检查加载的共享库:

(gdb) info shared library
From        To          Syms Read   Shared Object Library
0x40175a40  0x40180da8  Yes (*)     /Users/chyhfj/Desktop/hello-jni/obj/local/armeabi/linker
0x400feac0  0x4012e36c  Yes (*)     /Users/chyhfj/Desktop/hello-jni/obj/local/armeabi/libc.so

linker 和 libc.so 都已加载。

---

更新:作为Richard MA引用了一些 gdb 命令，这里有一些更多的信息

(gdb) info functions malloc
0x40177664  malloc
0x400ffb58  malloc

(gdb) info symbol malloc
malloc in section .text of /Users/chyhfj/Desktop/hello-jni/obj/local/armeabi/linker
(gdb) info symbol 0x40177664
malloc in section .text of /Users/chyhfj/Desktop/hello-jni/obj/local/armeabi/linker
(gdb) info symbol 0x400ffb58
malloc in section .text of /Users/chyhfj/Desktop/hello-jni/obj/local/armeabi/libc.so

---

我想做的这个破事:

• 在 gdb 下，我不能调用任何内部调用 malloc() 的函数

所以，问题:

为什么gdb调用linker中实现的malloc()函数而不是libc.so,

以及如何调用libc.so的malloc()，以及内部调用malloc()的函数

Answer 1

示例:

(gdb) info symbol malloc
malloc in section .text of /lib64/ld-linux-x86-64.so.2

(gdb) info addr malloc
Symbol "malloc" is at 0x7ffff7df07a0 in a file compiled without debugging.

malloc的原型(prototype)是:void *malloc(size_t size)

然后，投出指针

(gdb) x (void (*)(size_t))(0x7ffff7df07a0)(4)
0x603030:   0x00000000

0x6003030是结果