Fix x86-64 GNU/Linux crashes

Fix x86-64 GNU/Linux crashes

Ref: https://sourceware.org/ml/gdb-patches/2017-07/msg00162.html

Debugging x86-64 GNU/Linux programs currently crashes GDB in
tdesc_use_registers during gdbarch initialization:

  Program received signal SIGSEGV, Segmentation fault.
  0x0000000001093eaf in htab_remove_elt_with_hash (htab=0x2ef9fa0, element=0x26af960, hash=557151073) at src/libiberty/hashtab.c:728
  728       if (*slot == HTAB_EMPTY_ENTRY)
  (top-gdb) p slot
  $1 = (void **) 0x0
  (top-gdb) bt
  #0  0x0000000001093eaf in htab_remove_elt_with_hash (htab=0x2ef9fa0, element=0x26af960, hash=557151073) at src/libiberty/hashtab.c:728
  #1  0x0000000001093e79 in htab_remove_elt (htab=0x2ef9fa0, element=0x26af960) at src/libiberty/hashtab.c:714
  #2  0x00000000009121b0 in tdesc_use_registers (gdbarch=0x3001240, target_desc=0x2659cb0, early_data=0x2881cb0)
      at src/gdb/target-descriptions.c:1328
  #3  0x000000000047c93e in i386_gdbarch_init (info=..., arches=0x0) at src/gdb/i386-tdep.c:8634
  #4  0x0000000000818d5f in gdbarch_find_by_info (info=...) at src/gdb/gdbarch.c:5394
  #5  0x00000000007198a8 in set_gdbarch_from_file (abfd=0x2f48250) at src/gdb/arch-utils.c:618
  #6  0x00000000007f21cb in exec_file_attach (filename=0x7fffffffddb0 "/home/pedro/gdb/tests/threads", from_tty=1) at src/gdb/exec.c:380
  #7  0x0000000000865c18 in catch_command_errors_const (command=0x7f1d83 <exec_file_attach(char const*, int)>, arg=0x7fffffffddb0 "/home/pedro/gdb/tests/threads",
      from_tty=1) at src/gdb/main.c:403
  #8  0x00000000008669cf in captured_main_1 (context=0x7fffffffd860) at src/gdb/main.c:1035
  #9  0x0000000000866de2 in captured_main (data=0x7fffffffd860) at src/gdb/main.c:1142
  #10 0x0000000000866e24 in gdb_main (args=0x7fffffffd860) at src/gdb/main.c:1160
  #11 0x000000000041312d in main (argc=3, argv=0x7fffffffd968) at src/gdb/gdb.c:32

The direct cause of the crash is that we tried to remove an element
from the hash which supposedly exists, but does not.  (htab_remove_elt
shouldn't really crash in this case, but that's secondary.)

The real problem is that early_data passed to tdesc_use_registers
includes regs from a target description that is not the target_desc,
which violates its assumptions.  The registers in question are the
fs_base/gs_base registers, added by amd64_init_abi:

      tdesc_numbered_register (feature, tdesc_data_segments,
		       AMD64_FSBASE_REGNUM, "fs_base");
      tdesc_numbered_register (feature, tdesc_data_segments,
		       AMD64_GSBASE_REGNUM, "gs_base");

and that happens because amd64_linux_init_abi uses amd64_init_abi as
helper, but they don't coordinate on which fallback tdesc to use.

amd64_init_abi does:

  if (! tdesc_has_registers (tdesc))
    tdesc = tdesc_amd64;

and then adds the fs_base/gs_base registers of the "tdesc_amd64" tdesc
to the tdesc_arch_data.

After amd64_init_abi returns, amd64_linux_init_abi does:

  if (! tdesc_has_registers (tdesc))
    tdesc = tdesc_amd64_linux;
  tdep->tdesc = tdesc;

and we end up tdesc_amd64_linux installed in tdep->tdesc.

The fix is to make sure that amd64_linux_init_abi and amd64_init_abi
agree on default tdesc, by adding a "default tdesc" parameter to
amd64_init_abi, instead of having amd64_init_abi hardcode a default.
With this, amd64_init_abi creates the fs_base/gs_base registers using
the tdesc_amd64_linux tdesc.

Tested on x86-64 GNU/Linux, -m64.  I don't have an x32 setup handy.

Thanks to John Baldwin, Yao Qi and Simon Marchi for the investigation.

gdb/ChangeLog:
2017-07-13  Pedro Alves  <palves@redhat.com>

	* amd64-darwin-tdep.c (x86_darwin_init_abi_64): Pass tdesc_amd64
	as default tdesc.
	* amd64-dicos-tdep.c (amd64_dicos_init_abi):
	* amd64-fbsd-tdep.c (amd64fbsd_init_abi):
	* amd64-linux-tdep.c (amd64_linux_init_abi): Pass
	tdesc_amd64_linux as default tdesc.  Get final tdesc from the
	tdep.
	(amd64_x32_linux_init_abi): Pass tdesc_x32_linux as default tdesc.
	Get final tdesc from the tdep.
	* amd64-nbsd-tdep.c (amd64nbsd_init_abi): Pass tdesc_amd64 as
	default tdesc.
	* amd64-obsd-tdep.c (amd64obsd_init_abi): Likewise.
	* amd64-sol2-tdep.c (amd64_sol2_init_abi): Likewise.
	* amd64-tdep.c (amd64_init_abi): Add 'default_tdesc' parameter.
	Use it as default tdesc.
	(amd64_x32_init_abi): Add 'default_tdesc' parameter, and pass it
	down to amd_init_abi.  No longer handle fallback tdesc here.
	* amd64-tdep.h (tdesc_x32): Declare.
	(amd64_init_abi, amd64_x32_init_abi): Add 'default_tdesc'
	parameter.
	* amd64-windows-tdep.c (amd64_windows_init_abi): Pass tdesc_amd64
	as default tdesc.