16.29 导出符号的版本控制

https://scz.617.cn/unix/201507241413.txt

A: Masanori Goto 2015-07-24 14:13

$ gcc --version
gcc (Debian 4.9.2-10) 4.9.2

$ vi test_0.c

--------------------------------------------------------------------------
#include <stdio.h>

int max ( int a, int b )
{
    int ret = ( a > b ? a : b );

    printf( "max_0( %d, %d ) = %d\n", a, b, ret );
    return( ret );
}  /* end of max_0 */
--------------------------------------------------------------------------

$ vi test_1.c

--------------------------------------------------------------------------
#include <stdio.h>

extern int max ( int a, int b );

int main ( int argc, char * argv[] )
{
    int ret = max( 1, 2 );

    printf( "max() = %d\n", ret );
    return( ret );
}  /* end of main */
--------------------------------------------------------------------------

$ gcc -Wl,-soname,libtest.so -fPIC -shared -Wall -pipe -O0 -s -o libtest.so.0 test_0.c
$ ln -s libtest.so.0 libtest.so
$ gcc -Wall -pipe -O0 -s -o test_1 test_1.c -L. -ltest
$ LD_LIBRARY_PATH=. ./test_1
max_0( 1, 2 ) = 2
max() = 2

$ vi test_2.c

--------------------------------------------------------------------------
#include <stdio.h>

int max_0 ( int a, int b )
{
    int ret = ( a > b ? a : b );

    printf( "max_0( %d, %d ) = %d\n", a, b, ret );
    return( ret );
}  /* end of max_0 */

int max_2 ( int a, int b, int c )
{
    int d   = ( a > b ? a : b );
    int ret = ( d > c ? d : c );

    printf( "max_2( %d, %d, %d ) = %d\n", a, b, c, ret );
    return( ret );
}  /* end of max_0 */

__asm__                         \
("                              \
.symver max_0,max@LIBTEST_1.0  ;\
.symver max_2,max@@LIBTEST_2.0 ;\
");
--------------------------------------------------------------------------

$ vi test_2.map

--------------------------------------------------------------------------
LIBTEST_1.0
{
    global  :

        max;

    local   :

        *;
};

LIBTEST_2.0
{
    global  :

        max;

} LIBTEST_1.0;
--------------------------------------------------------------------------

$ vi test_3.c

--------------------------------------------------------------------------
#include <stdio.h>

extern int max ( int a, int b, int c );

int main ( int argc, char * argv[] )
{
    int ret = max( 1, 2, 3 );

    printf( "max() = %d\n", ret );
    return( ret );
}  /* end of main */
--------------------------------------------------------------------------

$ gcc -Wl,--version-script,test_2.map -Wl,-soname,libtest.so -fPIC -shared -Wall -pipe -O0 -s -o libtest.so.2 test_2.c
$ ln -sf libtest.so.2 libtest.so
$ gcc -Wall -pipe -O0 -s -o test_3 test_3.c -L. -ltest
$ LD_LIBRARY_PATH=. ldd ./test_3 | grep libtest.so
        libtest.so => ./libtest.so (0xb773e000)
$ LD_LIBRARY_PATH=. ldd ./test_1 | grep libtest.so
        libtest.so => ./libtest.so (0xb775a000)
$ ls -l libtest.so
lrwxrwxrwx 1 root root 12 Jul 24 15:52 libtest.so -> libtest.so.2*

$ LD_LIBRARY_PATH=. ./test_1
max_0( 1, 2 ) = 2
max() = 2
$ LD_LIBRARY_PATH=. ./test_3
max_2( 1, 2, 3 ) = 3
max() = 3

$ readelf -Ws test_1 | grep max
     3: 00000000     0 FUNC    GLOBAL DEFAULT  UND max
$ nm -D test_1 | grep max
         U max

$ readelf -Ws test_3 | grep max
     1: 00000000     0 FUNC    GLOBAL DEFAULT  UND max@LIBTEST_2.0 (2)
$ nm -D test_3 | grep max
         U max

$ readelf -Ws libtest.so.0 | grep max
     8: 00000525    65 FUNC    GLOBAL DEFAULT   11 max
$ nm -D libtest.so.0 | grep max
00000525 T max

$ readelf -Ws libtest.so.2 | grep max
     7: 00000586    86 FUNC    GLOBAL DEFAULT   12 max@@LIBTEST_2.0
     8: 00000545    65 FUNC    GLOBAL DEFAULT   12 max@LIBTEST_1.0
$ nm -D libtest.so.2 | grep max
00000586 T max
00000545 T max
$ readelf -WV libtest.so.2
...
Version definition section '.gnu.version_d' contains 3 entries:
  Addr: 0x00000000000002c8  Offset: 0x0002c8  Link: 4 (.dynstr)  000000: Rev: 1  Flags: BASE   Index: 1  Cnt: 1  Name: libtest.so
  0x001c: Rev: 1  Flags: none  Index: 2  Cnt: 1  Name: LIBTEST_1.0
  0x0038: Rev: 1  Flags: none  Index: 3  Cnt: 2  Name: LIBTEST_2.0
  0x0054: Parent 1: LIBTEST_1.0
...

test_1找max，但libtest.so.2中没有max，只有带版本的max，这种情况下用
.gnu.version_d中Index为2的那个版本。

test_3找"max@LIBTEST_2.0 (2)"，libtest.so.2中有max@@LIBTEST_2.0。这个(2)不
知啥意思？

$ vi test_4.c

--------------------------------------------------------------------------
#include <stdio.h>

int max_0 ( int a, int b )
{
    int ret = ( a > b ? a : b );

    printf( "max_0( %d, %d ) = %d\n", a, b, ret );
    return( ret );
}  /* end of max_0 */

int max_1 ( int a, int b )
{
    int ret = ( a > b ? a : b );

    printf( "max_1( %d, %d ) = %d\n", a, b, ret );
    return( ret );
}  /* end of max_1 */

int max_1_5 ( int a, int b )
{
    int ret = ( a > b ? a : b );

    printf( "max_1_5( %d, %d ) = %d\n", a, b, ret );
    return( ret );
}  /* end of max_1_5 */

int max_2 ( int a, int b, int c )
{
    int d   = ( a > b ? a : b );
    int ret = ( d > c ? d : c );

    printf( "max_2( %d, %d, %d ) = %d\n", a, b, c, ret );
    return( ret );
}  /* end of max_0 */

__asm__                             \
("                                  \
.symver max_0,max@                 ;\
.symver max_1,max@LIBTEST_1.0      ;\
.symver max_1_5,max@LIBTEST_1.5    ;\
.symver max_2,max@@LIBTEST_2.0     ;\
");
--------------------------------------------------------------------------

$ vi test_4.map

--------------------------------------------------------------------------
LIBTEST_1.0
{
    global  :

        max;

    local   :

        *;
};

LIBTEST_1.5
{
    global  :

        max;
} LIBTEST_1.0;

LIBTEST_2.0
{
    global  :

        max;

} LIBTEST_1.5;
--------------------------------------------------------------------------

$ gcc -Wl,--version-script,test_4.map -Wl,-soname,libtest.so -fPIC -shared -Wall -pipe -O0 -s -o libtest.so.4 test_4.c
$ ln -sf libtest.so.4 libtest.so
$ LD_LIBRARY_PATH=. ./test_1
max_0( 1, 2 ) = 2
max() = 2
$ LD_LIBRARY_PATH=. ./test_3
max_2( 1, 2, 3 ) = 3
max() = 3

$ readelf -Ws libtest.so.4 | grep max
     7: 00000688    86 FUNC    GLOBAL DEFAULT   12 max@@LIBTEST_2.0
     8: 00000647    65 FUNC    GLOBAL DEFAULT   12 max@LIBTEST_1.5
     9: 000005c5    65 FUNC    GLOBAL DEFAULT   12 max
    10: 00000606    65 FUNC    GLOBAL DEFAULT   12 max@LIBTEST_1.0
$ nm -D libtest.so.4 | grep max
00000688 T max
00000647 T max
000005c5 T max
00000606 T max
$ readelf -WV libtest.so.4
...
Version definition section '.gnu.version_d' contains 4 entries:
  Addr: 0x0000000000000318  Offset: 0x000318  Link: 4 (.dynstr)  000000: Rev: 1  Flags: BASE   Index: 1  Cnt: 1  Name: libtest.so
  0x001c: Rev: 1  Flags: none  Index: 2  Cnt: 1  Name: LIBTEST_1.0
  0x0038: Rev: 1  Flags: none  Index: 3  Cnt: 2  Name: LIBTEST_1.5
  0x0054: Parent 1: LIBTEST_1.0
  0x005c: Rev: 1  Flags: none  Index: 4  Cnt: 2  Name: LIBTEST_2.0
  0x0078: Parent 1: LIBTEST_1.5
...

test_1找max，libtest.so.4中有max。

test_3找"max@LIBTEST_2.0 (2)"，libtest.so.4中有max@@LIBTEST_2.0。

$ vi test_5.c

--------------------------------------------------------------------------
#include <stdio.h>

extern int max ( int a, int b );

int main ( int argc, char * argv[] )
{
    int ret = max( 1, 2 );

    printf( "max() = %d\n", ret );
    return( ret );
}  /* end of main */

__asm__                         \
("                              \
.symver max,max@LIBTEST_1.0    ;\
");
--------------------------------------------------------------------------

$ gcc -Wall -pipe -O0 -s -o test_5 test_5.c -L. -ltest
$ LD_LIBRARY_PATH=. ./test_5
max_1( 1, 2 ) = 2
max() = 2

$ readelf -Ws test_5 | grep max
     5: 00000000     0 FUNC    GLOBAL DEFAULT  UND max@LIBTEST_1.0 (3)

test_5找"max@LIBTEST_1.0 (3)"，libtest.so.4中有max@LIBTEST_1.0。这个(3)不
知啥意思？

使用.symver时，max@@LIBTEST_2.0有两个@，即@@，表示这是缺省版本，一般对应当
前最新版本。假设动态链接库导出多个版本的max，主程序与之动态链接时使用缺省
版本的max，除非主程序中明确指定使用哪个版本的max。

假设动态链接库使用.symver导出多个版本的同名符号，缺省版本必不可少。

$ ls -l libtest.so
lrwxrwxrwx 1 root root 12 Jul 24 18:09 libtest.so -> libtest.so.4*
$ gcc -Wall -pipe -O0 -s -o test_6 test_1.c -L. -ltest
$ LD_LIBRARY_PATH=. ./test_6
max_2( 1, 2, 134518692 ) = 134518692
max() = 134518692
$ readelf -Ws test_6 | grep max
     1: 00000000     0 FUNC    GLOBAL DEFAULT  UND max@LIBTEST_2.0 (2)

test_6与test_1对应同一份源码(test_1.c)，但链接时使用了不同的动态库，分别是
libtest.so.4与libtest.so.0，前者导出多个版本的max，后者只导出一个max。链接
结果出现差异，test_6导入max@LIBTEST_2.0(缺省版本)，test_1导入max。此时
test_6的执行结果已经不是我们期望的结果，将不属于自己栈帧的栈中数据拿来参与
运算，幸运的是，max_2()是__cdecl调用风格，test_1.c中的extern声明确保不会出
现栈失衡，使得test_6正常结束。

$ objdump -dj .text test_1 | grep -B1 'call.*__libc_start_main' | awk '/push.*0x/ { print $NF }'
$0x804854b
$ objdump -d test_1 | grep -A 11 "804854b:"
 804854b:       8d 4c 24 04             lea    0x4(%esp),%ecx
 804854f:       83 e4 f0                and    $0xfffffff0,%esp
 8048552:       ff 71 fc                pushl  -0x4(%ecx)
 8048555:       55                      push   %ebp
 8048556:       89 e5                   mov    %esp,%ebp
 8048558:       51                      push   %ecx
 8048559:       83 ec 14                sub    $0x14,%esp
 804855c:       83 ec 08                sub    $0x8,%esp
 804855f:       6a 02                   push   $0x2
 8048561:       6a 01                   push   $0x1
 8048563:       e8 b8 fe ff ff          call   8048420 <max@plt>
 8048568:       83 c4 10                add    $0x10,%esp
$ objdump -dj .text test_6 | grep -B1 'call.*__libc_start_main' | awk '/push.*0x/ { print $NF }'
$0x80484db
$ objdump -d test_6 | grep -A 11 "80484db:"
 80484db:       8d 4c 24 04             lea    0x4(%esp),%ecx
 80484df:       83 e4 f0                and    $0xfffffff0,%esp
 80484e2:       ff 71 fc                pushl  -0x4(%ecx)
 80484e5:       55                      push   %ebp
 80484e6:       89 e5                   mov    %esp,%ebp
 80484e8:       51                      push   %ecx
 80484e9:       83 ec 14                sub    $0x14,%esp
 80484ec:       83 ec 08                sub    $0x8,%esp
 80484ef:       6a 02                   push   $0x2
 80484f1:       6a 01                   push   $0x1
 80484f3:       e8 a8 fe ff ff          call   80483a0 <max@plt>
 80484f8:       83 c4 10                add    $0x10,%esp

前例说明，即使主程序源码相同、编译链接命令行相同，链接库升级仍要谨慎，尤其
是主程序需要重新编译的时候。

排查多版本同名符号导致的问题时，应该用"readelf -Ws"，而不是"nm -D"，因为后
者看不到同名符号的版本信息。

前面关于(2)、(3)的疑问，参看:

http://dev.gentoo.org/~dberkholz/articles/toolchain/symbol-versioning

好像对应vn_version成员，指出"Version of structure"。

$ readelf -Ws test_3 | grep max
     1: 00000000     0 FUNC    GLOBAL DEFAULT  UND max@LIBTEST_2.0 (2)
$ readelf -WV test_3
...
Version needs section '.gnu.version_r' contains 2 entries:
 Addr: 0x0000000008048300  Offset: 0x000300  Link: 6 (.dynstr)
  000000: Version: 1  File: libc.so.6  Cnt: 1
  0x0010:   Name: GLIBC_2.0  Flags: none  Version: 3
  0x0020: Version: 1  File: libtest.so  Cnt: 1
  0x0030:   Name: LIBTEST_2.0  Flags: none  Version: 2  // (2)好像对应Version

$ readelf -Ws test_5 | grep max
     5: 00000000     0 FUNC    GLOBAL DEFAULT  UND max@LIBTEST_1.0 (3)
$ readelf -WV test_5
...
Version needs section '.gnu.version_r' contains 2 entries:
 Addr: 0x0000000008048300  Offset: 0x000300  Link: 6 (.dynstr)
  000000: Version: 1  File: libtest.so  Cnt: 1
  0x0010:   Name: LIBTEST_1.0  Flags: none  Version: 3  // (3)好像对应Version
  0x0020: Version: 1  File: libc.so.6  Cnt: 1
  0x0030:   Name: GLIBC_2.0  Flags: none  Version: 2