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binutils-gdb/gdb/common/common-exceptions.h
Pedro Alves 89525768cd Propagate GDB/C++ exceptions across readline using sj/lj-based TRY/CATCH 
If we map GDB'S TRY/CATCH macros to C++ try/catch, GDB breaks on
systems where readline isn't built with exceptions support.  The
problem is that readline calls into GDB through the callback
interface, and if GDB's callback throws a C++ exception/error, the
system unwinder won't manage to unwind past the readline frame, and
ends up calling std::terminate(), which aborts the process:

 (gdb) whatever-command-that-causes-an-error
 terminate called after throwing an instance of 'gdb_exception_RETURN_MASK_ERROR'
 Aborted
 $

This went unnoticed for so long because:

- the x86-64 ABI requires -fasynchronous-unwind-tables, making it
  possible for exceptions to cross readline with no special handling.
  But e.g., on ARM or AIX, unless you build readline with
  -fexceptions, you trip on the problem.

- TRY/CATCH was mapped to setjmp/longjmp, even in C++ mode, until
  quite recently.

The fix is to catch and save any GDB exception that is thrown inside
the GDB readline callback, and then once the callback returns back to
the GDB code that called into readline in the first place, rethrow the
saved GDB exception.

This is similar in spirit to how we catch/map GDB exceptions at the
GDB/Python and GDB/Guile API boundaries.

The next question is then: if we intercept all exceptions within GDB's
readline callback, should we simply return normally to readline?  The
callback prototype has no way to signal an error back to readline (*).
The answer is no -- if we return normally, we'll be returning to a
loop inside rl_callback_read_char that continues processing pending
input, calling into GDB again, redisplaying the prompt, etc.  Thus if
we want to error out of rl_callback_read_char, we need to long jump
across it, just like we always did before TRY/CATCH were ever mapped
to C++ exceptions.

My first approach built a specialized API to handle this, with a
couple macros to hide the setjmp/longjmp and the struct gdb_exception
saving/rethrowing.

However, I realized that we need to:

 - Handle multiple active rl_callback_read_char invocations.  If,
   while processing input something triggers a secondary prompt, we
   end up in a nested rl_callback_read_char call, through
   gdb_readline_wrapper.

 - Propagate a struct gdb_exception along with the longjmp.

... and that this is exactly what the setjmp/longjmp-based TRY/CATCH
does.

So the fix makes the setjmp/longjmp TRY/CATCH always available under
new TRY_SJLJ/CATCH_SJLJ aliases, even when TRY/CATCH is mapped to C++
try/catch, and then uses TRY_SJLJ/CATCH_SJLJ to propagate GDB
exceptions across the readline callback.

This turns out to be a much better looking fix than my bespoke API
attempt, even.  We'll probably be able to simplify TRY_SJLJ/CATCH_SJLJ
when we finally get rid of TRY/CATCH all over the tree, but until
then, this reuse seems quite nice for avoiding a second parallel
setjmp/longjmp mechanism.

(*) - maybe we could propose a readline API change, but we still need
      to handle current readline, anyway.

gdb/ChangeLog:
2016-04-22  Pedro Alves  <palves@redhat.com>

	* common/common-exceptions.c (enum catcher_state, struct catcher)
	(current_catcher): Define in C++ mode too.
	(exceptions_state_mc_catch): Call throw_exception_sjlj instead of
	throw_exception.
	(throw_exception_sjlj, throw_exception_cxx): New functions,
	factored out from throw_exception.
	(throw_exception): Reimplement.
	* common/common-exceptions.h (exceptions_state_mc_init)
	(exceptions_state_mc_action_iter)
	(exceptions_state_mc_action_iter_1, exceptions_state_mc_catch):
	Declare in C++ mode too.
	(TRY): Rename to ...
	(TRY_SJLJ): ... this.
	(CATCH): Rename to ...
	(CATCH_SJLJ): ... this.
	(END_CATCH): Rename to ...
	(END_CATCH_SJLJ): ... this.
	[GDB_XCPT == GDB_XCPT_SJMP] (TRY, CATCH, END_CATCH): Map to SJLJ
	equivalents.
	(throw_exception): Update comments.
	(throw_exception_sjlj): Declare.
	* event-top.c (gdb_rl_callback_read_char_wrapper): Extend intro
	comment.  Wrap body in TRY_SJLJ/CATCH_SJLJ and rethrow any
	intercepted exception.
	(gdb_rl_callback_handler): New function.
	(gdb_rl_callback_handler_install): Always install
	gdb_rl_callback_handler as readline callback.
2016-04-22 16:20:04 +01:00

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/* Exception (throw catch) mechanism, for GDB, the GNU debugger.
Copyright (C) 1986-2016 Free Software Foundation, Inc.
This file is part of GDB.
This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 3 of the License, or
(at your option) any later version.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program. If not, see <http://www.gnu.org/licenses/>. */
#ifndef COMMON_EXCEPTIONS_H
#define COMMON_EXCEPTIONS_H
#include <setjmp.h>
/* Reasons for calling throw_exceptions(). NOTE: all reason values
must be less than zero. enum value 0 is reserved for internal use
as the return value from an initial setjmp(). The function
catch_exceptions() reserves values >= 0 as legal results from its
wrapped function. */
enum return_reason
{
/* User interrupt. */
RETURN_QUIT = -2,
/* Any other error. */
RETURN_ERROR
};
#define RETURN_MASK(reason) (1 << (int)(-reason))
typedef enum
{
RETURN_MASK_QUIT = RETURN_MASK (RETURN_QUIT),
RETURN_MASK_ERROR = RETURN_MASK (RETURN_ERROR),
RETURN_MASK_ALL = (RETURN_MASK_QUIT | RETURN_MASK_ERROR)
} return_mask;
/* Describe all exceptions. */
enum errors {
GDB_NO_ERROR,
/* Any generic error, the corresponding text is in
exception.message. */
GENERIC_ERROR,
/* Something requested was not found. */
NOT_FOUND_ERROR,
/* Thread library lacks support necessary for finding thread local
storage. */
TLS_NO_LIBRARY_SUPPORT_ERROR,
/* Load module not found while attempting to find thread local storage. */
TLS_LOAD_MODULE_NOT_FOUND_ERROR,
/* Thread local storage has not been allocated yet. */
TLS_NOT_ALLOCATED_YET_ERROR,
/* Something else went wrong while attempting to find thread local
storage. The ``struct gdb_exception'' message field provides
more detail. */
TLS_GENERIC_ERROR,
/* Problem parsing an XML document. */
XML_PARSE_ERROR,
/* Error accessing memory. */
MEMORY_ERROR,
/* Value not available. E.g., a register was not collected in a
traceframe. */
NOT_AVAILABLE_ERROR,
/* Value was optimized out. Note: if the value was a register, this
means the register was not saved in the frame. */
OPTIMIZED_OUT_ERROR,
/* DW_OP_GNU_entry_value resolving failed. */
NO_ENTRY_VALUE_ERROR,
/* Target throwing an error has been closed. Current command should be
aborted as the inferior state is no longer valid. */
TARGET_CLOSE_ERROR,
/* An undefined command was executed. */
UNDEFINED_COMMAND_ERROR,
/* Requested feature, method, mechanism, etc. is not supported. */
NOT_SUPPORTED_ERROR,
/* The number of candidates generated during line completion has
reached the user's specified limit. This isn't an error, this exception
is used to halt searching for more completions, but for consistency
"_ERROR" is appended to the name. */
MAX_COMPLETIONS_REACHED_ERROR,
/* Add more errors here. */
NR_ERRORS
};
struct gdb_exception
{
enum return_reason reason;
enum errors error;
const char *message;
};
/* The different exception mechanisms that TRY/CATCH can map to. */
/* Make GDB exceptions use setjmp/longjmp behind the scenes. This is
the only mode supported when GDB is built as a C program. */
#define GDB_XCPT_SJMP 1
/* Make GDB exceptions use try/catch behind the scenes. Can't be made
the default until we handle exceptions crossing foreign frames
(gdb -> readline callback -> gdb -> error). */
#define GDB_XCPT_TRY 2
/* Specify this mode to build with TRY/CATCH mapped directly to raw
try/catch. GDB won't work correctly, but building that way catches
code tryin to break/continue out of the try block, along with
spurious code between the TRY and the CATCH block. */
#define GDB_XCPT_RAW_TRY 3
/* Always use setjmp/longmp, even in C++ mode. */
#define GDB_XCPT GDB_XCPT_SJMP
/* Functions to drive the sjlj-based exceptions state machine. Though
declared here by necessity, these functions should be considered
internal to the exceptions subsystem and not used other than via
the TRY/CATCH (or TRY_SJLJ/CATCH_SJLJ) macros defined below. */
extern jmp_buf *exceptions_state_mc_init (void);
extern int exceptions_state_mc_action_iter (void);
extern int exceptions_state_mc_action_iter_1 (void);
extern int exceptions_state_mc_catch (struct gdb_exception *, int);
/* Same, but for the C++ try/catch-based TRY/CATCH mechanism. */
#if GDB_XCPT != GDB_XCPT_SJMP
extern void *exception_try_scope_entry (void);
extern void exception_try_scope_exit (void *saved_state);
extern void exception_rethrow (void);
#endif
/* Macro to wrap up standard try/catch behavior.
The double loop lets us correctly handle code "break"ing out of the
try catch block. (It works as the "break" only exits the inner
"while" loop, the outer for loop detects this handling it
correctly.) Of course "return" and "goto" are not so lucky.
For instance:
*INDENT-OFF*
TRY
{
}
CATCH (e, RETURN_MASK_ERROR)
{
switch (e.reason)
{
case RETURN_ERROR: ...
}
}
END_CATCH
Note that the SJLJ version of the macros are actually named
TRY_SJLJ/CATCH_SJLJ in order to make it possible to call them even
when TRY/CATCH are mapped to C++ try/catch. The SJLJ variants are
needed in some cases where gdb exceptions need to cross third-party
library code compiled without exceptions support (e.g.,
readline). */
#define TRY_SJLJ \
{ \
jmp_buf *buf = \
exceptions_state_mc_init (); \
setjmp (*buf); \
} \
while (exceptions_state_mc_action_iter ()) \
while (exceptions_state_mc_action_iter_1 ())
#define CATCH_SJLJ(EXCEPTION, MASK) \
{ \
struct gdb_exception EXCEPTION; \
if (exceptions_state_mc_catch (&(EXCEPTION), MASK))
#define END_CATCH_SJLJ \
}
#if GDB_XCPT == GDB_XCPT_SJMP
/* If using SJLJ-based exceptions for all exceptions, then provide
standard aliases. */
#define TRY TRY_SJLJ
#define CATCH CATCH_SJLJ
#define END_CATCH END_CATCH_SJLJ
#endif /* GDB_XCPT_SJMP */
#if GDB_XCPT == GDB_XCPT_TRY || GDB_XCPT == GDB_XCPT_RAW_TRY
/* Prevent error/quit during TRY from calling cleanups established
prior to here. This pops out the scope in either case of normal
exit or exception exit. */
struct exception_try_scope
{
exception_try_scope ()
{
saved_state = exception_try_scope_entry ();
}
~exception_try_scope ()
{
exception_try_scope_exit (saved_state);
}
void *saved_state;
};
#if GDB_XCPT == GDB_XCPT_TRY
/* We still need to wrap TRY/CATCH in C++ so that cleanups and C++
exceptions can coexist. The TRY blocked is wrapped in a
do/while(0) so that break/continue within the block works the same
as in C. */
#define TRY \
try \
{ \
exception_try_scope exception_try_scope_instance; \
do \
{
#define CATCH(EXCEPTION, MASK) \
} while (0); \
} \
catch (struct gdb_exception ## _ ## MASK &EXCEPTION)
#define END_CATCH \
catch (...) \
{ \
exception_rethrow (); \
}
#else
#define TRY try
#define CATCH(EXCEPTION, MASK) \
catch (struct gdb_exception ## _ ## MASK &EXCEPTION)
#define END_CATCH
#endif
/* The exception types client code may catch. They're just shims
around gdb_exception that add nothing but type info. Which is used
is selected depending on the MASK argument passed to CATCH. */
struct gdb_exception_RETURN_MASK_ALL : public gdb_exception
{
};
struct gdb_exception_RETURN_MASK_ERROR : public gdb_exception_RETURN_MASK_ALL
{
};
struct gdb_exception_RETURN_MASK_QUIT : public gdb_exception_RETURN_MASK_ALL
{
};
#endif /* GDB_XCPT_TRY || GDB_XCPT_RAW_TRY */
/* *INDENT-ON* */
/* Throw an exception (as described by "struct gdb_exception"). When
GDB is built as a C program, executes a LONG JUMP to the inner most
containing exception handler established using TRY/CATCH. When
built as a C++ program, throws a C++ exception, using "throw". */
extern void throw_exception (struct gdb_exception exception)
ATTRIBUTE_NORETURN;
/* Throw an exception by executing a LONG JUMP to the inner most
containing exception handler established using TRY_SJLJ. Works the
same regardless of whether GDB is built as a C program or a C++
program. Necessary in some cases where we need to throw GDB
exceptions across third-party library code (e.g., readline). */
extern void throw_exception_sjlj (struct gdb_exception exception)
ATTRIBUTE_NORETURN;
/* Convenience wrappers around throw_exception that throw GDB
errors. */
extern void throw_verror (enum errors, const char *fmt, va_list ap)
ATTRIBUTE_NORETURN ATTRIBUTE_PRINTF (2, 0);
extern void throw_vquit (const char *fmt, va_list ap)
ATTRIBUTE_NORETURN ATTRIBUTE_PRINTF (1, 0);
extern void throw_error (enum errors error, const char *fmt, ...)
ATTRIBUTE_NORETURN ATTRIBUTE_PRINTF (2, 3);
extern void throw_quit (const char *fmt, ...)
ATTRIBUTE_NORETURN ATTRIBUTE_PRINTF (1, 2);
/* A pre-defined non-exception. */
extern const struct gdb_exception exception_none;
#endif /* COMMON_EXCEPTIONS_H */
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