Skip gdb.base/relativedebug.exp if libc doesn't have debug info

Skip gdb.base/relativedebug.exp if libc doesn't have debug info

Hi,
I see the fail in gdb.base/relativedebug.exp on aarch64 box on which
glibc doesn't have debug info,

 bt^M
 #0 0x0000002000061a88 in raise () from /lib/aarch64-linux-gnu/libc.so.6^M
 #1 0x0000002000064efc in abort () from /lib/aarch64-linux-gnu/libc.so.6^M
 #2 0x0000000000400640 in handler (signo=14) at ../../../binutils-gdb/gdb/testsuite/gdb.base/relativedebug.c:25^M
 #3 <signal handler called>^M
 #4 0x00000020000cc478 in ?? () from /lib/aarch64-linux-gnu/libc.so.6^M
 #5 0x0000000000400664 in main () at ../../../binutils-gdb/gdb/testsuite/gdb.base/relativedebug.c:32^M
 (gdb) FAIL: gdb.base/relativedebug.exp: pause found in backtrace

if glibc has debug info, this test doesn't fail.

In sysdeps/unix/sysv/linux/generic/pause.c, __libc_pause calls
__syscall_pause,

  static int
  __syscall_pause (void)
  {
    sigset_t set;

    int rc =
      INLINE_SYSCALL (rt_sigprocmask, 4, SIG_BLOCK, NULL, &set, _NSIG / 8);
    if (rc == 0)
      rc = INLINE_SYSCALL (rt_sigsuspend, 2, &set, _NSIG / 8);

    return rc;
  }

  int
  __libc_pause (void)
  {
    if (SINGLE_THREAD_P)
      return __syscall_pause ();     <--- tail call

    int oldtype = LIBC_CANCEL_ASYNC ();

    int result = __syscall_pause ();

    LIBC_CANCEL_RESET (oldtype);

    return result;
  }

and GDB unwinder is confused by the GCC optimized code,

(gdb) disassemble pause
Dump of assembler code for function pause:
   0x0000007fb7f274c4 <+0>:     stp     x29, x30, [sp,#-32]!
   0x0000007fb7f274c8 <+4>:     mov     x29, sp
   0x0000007fb7f274cc <+8>:     adrp    x0, 0x7fb7fd2000
   0x0000007fb7f274d0 <+12>:    ldr     w0, [x0,#364]
   0x0000007fb7f274d4 <+16>:    stp     x19, x20, [sp,#16]
   0x0000007fb7f274d8 <+20>:    cbnz    w0, 0x7fb7f274e8 <pause+36>

   0x0000007fb7f274dc <+24>:    ldp     x19, x20, [sp,#16]
   0x0000007fb7f274e0 <+28>:    ldp     x29, x30, [sp],#32
   0x0000007fb7f274e4 <+32>:    b       0x7fb7f27434    <---- __syscall_pause

   0x0000007fb7f274e8 <+36>:    bl      0x7fb7f5e080

Note that the program stops in __syscall_pause, but its symbol is
stripped in glibc, so GDB doesn't know where the program stops.
__syscall_pause is a tail call in __libc_pause, so it returns to main
instead of __libc_pause.  As a result, the backtrace is like,

 #0  0x0000007fb7ebca88 in raise () from /lib/aarch64-linux-gnu/libc.so.6
 #1  0x0000007fb7ebfefc in abort () from /lib/aarch64-linux-gnu/libc.so.6
 #2  0x0000000000400640 in handler (signo=14) at ../../../binutils-gdb/gdb/testsuite/gdb.base/relativedebug.c:25
 #3  <signal handler called>
 #4  0x0000007fb7f27478 in ?? () from /lib/aarch64-linux-gnu/libc.so.6   <-- [in __syscall_pause]
 #5  0x0000000000400664 in main () at ../../../binutils-gdb/gdb/testsuite/gdb.base/relativedebug.c:32

looks GDB does nothing wrong here.  I looked back at the test case
gdb.base/relativedebug.exp, which was added
https://sourceware.org/ml/gdb-patches/2006-10/msg00305.html
This test was indented to test the problem that "backtraces no longer
display some libc functions" after separate debug info is installed.
IOW, it makes few sense to test against libc which doesn't have debug
info at all, such as my case.

This patch is to tweak the test case to catch the output of
"info shared", if "(*)" is found for libc.so, which means libc doesn't
have debug info, then skip the test.

gdb/testsuite:

2015-04-30  Yao Qi  <yao.qi@linaro.org>

	* gdb.base/relativedebug.exp: Invoke gdb command
	"info sharedlibrary", and if libc.so doesn't have debug info,
	skip the test.