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在继续之前,我知道将函数命名为error
通常是不好的做法,因为它可能会与libc中的类似函数冲突,但这是我在使用一些第三方软件时遇到的问题,我对此几乎无法控制。另外,我真的很想知道这个错误是从哪里来的:-)
问题所在
我遇到的问题是,下面的代码在通过Python解释器执行时,实际上是调用error
的error
函数(如下面的GDB堆栈跟踪所示),而不是调用libC函数的本地实现。
当在另一个C程序中简单地编译相同的代码时,我不会遇到这样的问题。有人知道这是从哪里来的吗?这与Python加载共享库的方式有关吗?
MCVE
#include <stdio.h>
#include <Python.h>
static PyObject* call_error(PyObject *self, PyObject *args);
static PyMethodDef module_methods[] = {
{"error", call_error, METH_NOARGS, "call error"},
{NULL, NULL, 0, NULL}
};
static struct PyModuleDef module_defs = {
PyModuleDef_HEAD_INIT,
"Test", "Test", -1, module_methods, NULL, NULL, NULL, NULL};
PyObject* PyInit_Test(void)
{
PyObject *module = PyModule_Create(&module_defs);
return module;
}
void error(const char* fmt, ...);
PyObject* call_error(PyObject *self, PyObject *args)
{
error("Error!");
Py_RETURN_NONE;
}
void error(const char* fmt, ...)
{
va_list ap;
va_start(ap, fmt);
vprintf(fmt, ap);
va_end(ap);
}
GDB输出
下面是在GDB中使用python3 -c "import Test; Test.error()"
导入并运行上述代码的输出。
GNU gdb (Ubuntu 8.1-0ubuntu3) 8.1.0.20180409-git
Copyright (C) 2018 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law. Type "show copying"
and "show warranty" for details.
This GDB was configured as "x86_64-linux-gnu".
Type "show configuration" for configuration details.
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>.
Find the GDB manual and other documentation resources online at:
<http://www.gnu.org/software/gdb/documentation/>.
For help, type "help".
Type "apropos word" to search for commands related to "word"...
Reading symbols from python3...(no debugging symbols found)...done.
(gdb) r -c 'import Test; Test.error()'
Starting program: /usr/bin/python3 -c 'import Test; Test.error()'
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
/usr/bin/python3:
Program received signal SIGSEGV, Segmentation fault.
__strchrnul_sse2 () at ../sysdeps/x86_64/multiarch/../strchr.S:32
32 ../sysdeps/x86_64/multiarch/../strchr.S: No such file or directory.
(gdb) where
#0 __strchrnul_sse2 () at ../sysdeps/x86_64/multiarch/../strchr.S:32
#1 0x00007ffff6c2c432 in __find_specmb (format=0x4 <error: Cannot access memory at
# address 0x4>) at printf-parse.h:108
#2 _IO_vfprintf_internal (s=0x7fffffffae60, format=0x4 <error: Cannot access memory at
# address 0x4>, ap=0x7fffffffd5b0) at vfprintf.c:1320
#3 0x00007ffff6c2f680 in buffered_vfprintf (s=s@entry=0x7ffff6fbd680 <_IO_2_1_stderr_>,
# format=format@entry=0x4 <error: Cannot access memory at address 0x4>,
# args=args@entry=0x7fffffffd5b0) at vfprintf.c:2329
#4 0x00007ffff6c2c726 in _IO_vfprintf_internal (s=0x7ffff6fbd680 <_IO_2_1_stderr_>,
# format=format@entry=0x4 <error: Cannot access memory at address 0x4>,
# ap=ap@entry=0x7fffffffd5b0) at vfprintf.c:1301
#5 0x00007ffff6cef9bb in error_tail (status=status@entry=-161613509,
# errnum=errnum@entry=0, message=message@entry=0x4 <error: Cannot access memory at
# address 0x4>, args=args@entry=0x7fffffffd5b0) at error.c:271
#6 0x00007ffff6cefb3d in __error (status=-161613509, errnum=0, message=0x4
# <error: Cannot access memory at address 0x4>) at error.c:321
#7 0x00007ffff65df82e in call_error (self=0x7ffff67f3548, args=0x0) at test.c:24
#8 0x00000000004c5352 in _PyCFunction_FastCallKeywords ()
#9 0x000000000054ffe4 in ?? ()
#10 0x00000000005546cf in _PyEval_EvalFrameDefault ()
#11 0x000000000054fbe1 in ?? ()
#12 0x0000000000550b93 in PyEval_EvalCode ()
#13 0x000000000042c4ca in PyRun_SimpleStringFlags ()
#14 0x0000000000441918 in Py_Main ()
#15 0x0000000000421ff4 in main ()
编辑
我确实考虑过导入Python模块的dlopen
问题,实际上,以下代码可以很好地编译和运行,并打印出来:
> ./main
Hi there
main.c
#include <stdio.h>
#include <stdlib.h>
#include <dlfcn.h>
#include <errno.h>
#include <stdarg.h>
typedef void*(*arbitrary)();
extern void error(const char* fmt, ...);
int main(int argc, char **argv)
{
void *handle;
arbitrary my_function;
handle = dlopen("./libtest.so", RTLD_LAZY | RTLD_GLOBAL);
if (!handle) {
fprintf(stderr, "%s\n", dlerror());
exit(EXIT_FAILURE);
}
dlerror(); /* Clear any existing error */
*(void**)(&my_function) = dlsym(handle,"foo");
(void) my_function();
// Note: binding using dlsym(handle, "error") works too
dlclose(handle);
exit(EXIT_SUCCESS);
}
test.c
#include <stdio.h>
#include <stdarg.h>
extern void error(const char* fmt, ...);
extern void foo(void);
void foo(void)
{
error("Hi there\n");
}
void error(const char* fmt, ...)
{
va_list ap;
va_start(ap, fmt);
vprintf(fmt, ap);
va_end(ap);
}
https://stackoverflow.com/questions/51619095
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