gdb/python: avoid throwing an exception over libopcodes code

gdb/python: avoid throwing an exception over libopcodes code

Bug gdb/29712 identifies a problem with the Python disassembler API.
In some cases GDB will try to throw an exception through the
libopcodes disassembler code, however, not all targets include
exception unwind information when compiling C code, for targets that
don't include this information GDB will terminate when trying to pass
the exception through libopcodes.

To explain what GDB is trying to do, consider the following trivial
use of the Python disassembler API:

  class ExampleDisassembler(gdb.disassembler.Disassembler):

      class MyInfo(gdb.disassembler.DisassembleInfo):
          def __init__(self, info):
              super().__init__(info)

          def read_memory(self, length, offset):
              return super().read_memory(length, offset)

      def __init__(self):
          super().__init__("ExampleDisassembler")

      def __call__(self, info):
          info = self.MyInfo(info)
          return gdb.disassembler.builtin_disassemble(info)

This disassembler doesn't add any value, it defers back to GDB to do
all the actual work, but it serves to allow us to discuss the problem.

The problem occurs when a Python exception is raised by the
MyInfo.read_memory method.  The MyInfo.read_memory method is called
from the C++ function gdbpy_disassembler::read_memory_func.  The C++
stack at the point this function is called looks like this:

  #0  gdbpy_disassembler::read_memory_func (memaddr=4198805, buff=0x7fff9ab9d2a8 "\220ӹ\232\377\177", len=1, info=0x7fff9ab9d558) at ../../src/gdb/python/py-disasm.c:510
  #1  0x000000000104ba06 in fetch_data (info=0x7fff9ab9d558, addr=0x7fff9ab9d2a9 "ӹ\232\377\177") at ../../src/opcodes/i386-dis.c:305
  #2  0x000000000104badb in ckprefix (ins=0x7fff9ab9d100) at ../../src/opcodes/i386-dis.c:8571
  #3  0x000000000104e28e in print_insn (pc=4198805, info=0x7fff9ab9d558, intel_syntax=-1) at ../../src/opcodes/i386-dis.c:9548
  #4  0x000000000104f4d4 in print_insn_i386 (pc=4198805, info=0x7fff9ab9d558) at ../../src/opcodes/i386-dis.c:9949
  #5  0x00000000004fa7ea in default_print_insn (memaddr=4198805, info=0x7fff9ab9d558) at ../../src/gdb/arch-utils.c:1033
  #6  0x000000000094fe5e in i386_print_insn (pc=4198805, info=0x7fff9ab9d558) at ../../src/gdb/i386-tdep.c:4072
  #7  0x0000000000503d49 in gdbarch_print_insn (gdbarch=0x5335560, vma=4198805, info=0x7fff9ab9d558) at ../../src/gdb/gdbarch.c:3351
  #8  0x0000000000bcc8c6 in disasmpy_builtin_disassemble (self=0x7f2ab07f54d0, args=0x7f2ab0789790, kw=0x0) at ../../src/gdb/python/py-disasm.c:324

  ### ... snip lots of frames as we pass through Python itself ...

  #22 0x0000000000bcd860 in gdbpy_print_insn (gdbarch=0x5335560, memaddr=0x401195, info=0x7fff9ab9e3c8) at ../../src/gdb/python/py-disasm.c:783
  #23 0x00000000008995a5 in ext_lang_print_insn (gdbarch=0x5335560, address=0x401195, info=0x7fff9ab9e3c8) at ../../src/gdb/extension.c:939
  #24 0x0000000000741aaa in gdb_print_insn_1 (gdbarch=0x5335560, vma=0x401195, info=0x7fff9ab9e3c8) at ../../src/gdb/disasm.c:1078
  #25 0x0000000000741bab in gdb_disassembler::print_insn (this=0x7fff9ab9e3c0, memaddr=0x401195, branch_delay_insns=0x0) at ../../src/gdb/disasm.c:1101

So gdbpy_disassembler::read_memory_func is called from the libopcodes
disassembler to read memory, this C++ function then calls into user
supplied Python code to do the work.

If the user supplied Python code raises an gdb.MemoryError exception
indicating the memory read failed, this is fine.  The C++ code
converts this exception back into a return value that libopcodes can
understand, and returns to libopcodes.

However, if the user supplied Python code raises some other exception,
what we want is for this exception to propagate through GDB and appear
as if raised by the call to gdb.disassembler.builtin_disassemble.  To
achieve this, when gdbpy_disassembler::read_memory_func spots an
unknown Python exception, we must pass the information about this
exception from frame #0 to frame #8 in the above backtrace.  Frame #8
is the C++ implementation of gdb.disassembler.builtin_disassemble, and
so it is this function that we want to re-raise the unknown Python
exception, so the user can, if they want, catch the exception in their
code.

The previous mechanism by which the exception was passed was to pack
the details of the Python exception into a C++ exception, then throw
the exception from frame #0, and catch the exception in frame #8,
unpack the details of the Python exception, and re-raise it.

However, this relies on the exception passing through frames #1 to #7,
some of which are in libopcodes, which is C code, and so, might not be
compiled with exception support.

This commit proposes an alternative solution that does not rely on
throwing a C++ exception.

When we spot an unhandled Python exception in frame #0, we will store
the details of this exception within the gdbpy_disassembler object
currently in use.  Then we return to libopcodes a value indicating
that the memory_read failed.

libopcodes will now continue to disassemble as though that memory read
failed (with one special case described below), then, when we
eventually return to disasmpy_builtin_disassemble we check to see if
there is an exception stored in the gdbpy_disassembler object.  If
there is then this exception can immediately be installed, and then we
return back to Python, when the user will be able to catch the
exception.

There is one extra change in gdbpy_disassembler::read_memory_func.
After the first call that results in an exception being stored on the
gdbpy_disassembler object, any future calls to the ::read_memory_func
function will immediately return as if the read failed.  This avoids
any additional calls into user supplied Python code.

My thinking here is that should the first call fail with some unknown
error, GDB should not keep trying with any additional calls.  This
maintains the illusion that the exception raised from
MyInfo.read_memory is immediately raised by
gdb.disassembler.builtin_disassemble.  I have no tests for this change
though - to trigger this issue would rely on a libopcodes disassembler
that will try to read further memory even after the first failed
read.  I'm not aware of any such disassembler that currently does
this, but that doesn't mean such a disassembler couldn't exist in the
future.

With this change in place the gdb.python/py-disasm.exp test should now
pass on AArch64.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29712

Approved-By: Simon Marchi <simon.marchi@efficios.com>