source: libcfa/src/exception.c @ 6d43cdde

//
// Cforall Version 1.0.0 Copyright (C) 2016 University of Waterloo
//
// The contents of this file are covered under the licence agreement in the
// file "LICENCE" distributed with Cforall.
//
// exception.c --
//
// Author           : Andrew Beach
// Created On       : Mon Jun 26 15:13:00 2017
// Last Modified By : Andrew Beach
// Last Modified On : Fri Mar 27 10:19:00 2020
// Update Count     : 12
//

// Normally we would get this from the CFA prelude.
#include <stddef.h> // for size_t

#include "exception.h"

// Implementation of the secret header is hardware dependent.
#if !( defined( __x86_64 ) || defined( __i386 ) )
#error Exception Handling: No known architecture detected.
#endif

#include <stdlib.h>
#include <stdio.h>
#include <unwind.h>
#include <bits/debug.hfa>

// FIX ME: temporary hack to keep ARM build working
#ifndef _URC_FATAL_PHASE1_ERROR
#define _URC_FATAL_PHASE1_ERROR 2
#endif // ! _URC_FATAL_PHASE1_ERROR
#ifndef _URC_FATAL_PHASE2_ERROR
#define _URC_FATAL_PHASE2_ERROR 2
#endif // ! _URC_FATAL_PHASE2_ERROR

#include "lsda.h"


// Base exception vtable is abstract, you should not have base exceptions.
struct __cfaehm_base_exception_t_vtable
                ___cfaehm_base_exception_t_vtable_instance = {
        .parent = NULL,
        .size = 0,
        .copy = NULL,
        .free = NULL,
        .msg = NULL
};


// Temperary global exception context. Does not work with concurency.
struct exception_context_t {
    struct __cfaehm_try_resume_node * top_resume;
    struct __cfaehm_try_resume_node * current_resume;

    exception_t * current_exception;
    int current_handler_index;
} static shared_stack = {NULL, NULL, NULL, 0};

// Get the current exception context.
// There can be a single global until multithreading occurs, then each stack
// needs its own. It will have to be updated to handle that.
struct exception_context_t * this_exception_context() {
        return &shared_stack;
}


// RESUMPTION ================================================================

void __cfaehm_throw_resume(exception_t * except) {
        struct exception_context_t * context = this_exception_context();

        __cfaabi_dbg_print_safe("Throwing resumption exception\n");

        struct __cfaehm_try_resume_node * original_head = context->current_resume;
        struct __cfaehm_try_resume_node * current =
                (original_head) ? original_head->next : context->top_resume;

        for ( ; current ; current = current->next) {
                context->current_resume = current;
                if (current->handler(except)) {
                        context->current_resume = original_head;
                        return;
                }
        }

        __cfaabi_dbg_print_safe("Unhandled exception\n");
        context->current_resume = original_head;

        // Fall back to termination:
        __cfaehm_throw_terminate(except);
        // TODO: Default handler for resumption.
}

// Do we control where exceptions get thrown even with concurency?
// If not these are not quite thread safe, the cleanup hook has to
// be added after the node is built but before it is made the top node.

void __cfaehm_try_resume_setup(struct __cfaehm_try_resume_node * node,
                        _Bool (*handler)(exception_t * except)) {
        struct exception_context_t * context = this_exception_context();
        node->next = context->top_resume;
        node->handler = handler;
        context->top_resume = node;
}

void __cfaehm_try_resume_cleanup(struct __cfaehm_try_resume_node * node) {
        struct exception_context_t * context = this_exception_context();
        context->top_resume = node->next;
}


// TERMINATION ===============================================================

// MEMORY MANAGEMENT (still for integers)
// May have to move to cfa for constructors and destructors (references).

struct __cfaehm_node {
        struct __cfaehm_node * next;
};

#define NODE_TO_EXCEPT(node) ((exception_t *)(1 + (node)))
#define EXCEPT_TO_NODE(except) ((struct __cfaehm_node *)(except) - 1)

// Creates a copy of the indicated exception and sets current_exception to it.
static void __cfaehm_allocate_exception( exception_t * except ) {
        struct exception_context_t * context = this_exception_context();

        // Allocate memory for the exception.
        struct __cfaehm_node * store = malloc(
                sizeof( struct __cfaehm_node ) + except->virtual_table->size );

        if ( ! store ) {
                // Failure: cannot allocate exception. Terminate thread.
                abort(); // <- Although I think it might be the process.
        }

        // Add the node to the list:
        store->next = EXCEPT_TO_NODE(context->current_exception);
        context->current_exception = NODE_TO_EXCEPT(store);

        // Copy the exception to storage.
        except->virtual_table->copy( context->current_exception, except );
}

// Delete the provided exception, unsetting current_exception if relivant.
static void __cfaehm_delete_exception( exception_t * except ) {
        struct exception_context_t * context = this_exception_context();

        __cfaabi_dbg_print_safe("Deleting Exception\n");

        // Remove the exception from the list.
        struct __cfaehm_node * to_free = EXCEPT_TO_NODE(except);
        struct __cfaehm_node * node;

        if ( context->current_exception == except ) {
                node = to_free->next;
                context->current_exception = (node) ? NODE_TO_EXCEPT(node) : 0;
        } else {
                node = EXCEPT_TO_NODE(context->current_exception);
                // It may always be in the first or second position.
                while ( to_free != node->next ) {
                        node = node->next;
                }
                node->next = to_free->next;
        }

        // Free the old exception node.
        except->virtual_table->free( except );
        free( to_free );
}

// If this isn't a rethrow (*except==0), delete the provided exception.
void __cfaehm_cleanup_terminate( void * except ) {
        if ( *(void**)except ) __cfaehm_delete_exception( *(exception_t **)except );
}


// We need a piece of storage to raise the exception
struct _Unwind_Exception this_exception_storage;

// Function needed by force unwind
// It basically says to unwind the whole stack and then exit when we reach the end of the stack
static _Unwind_Reason_Code _Stop_Fn(
                int version,
                _Unwind_Action actions,
                _Unwind_Exception_Class exception_class,
                struct _Unwind_Exception * unwind_exception,
                struct _Unwind_Context * unwind_context,
                void * stop_param) {
        if ( actions & _UA_END_OF_STACK  ) exit(1);
        if ( actions & _UA_CLEANUP_PHASE ) return _URC_NO_REASON;

        return _URC_FATAL_PHASE2_ERROR;
}

// The exception that is being thrown must already be stored.
static __attribute__((noreturn)) void __cfaehm_begin_unwind(void) {
        if ( ! this_exception_context()->current_exception ) {
                printf("UNWIND ERROR missing exception in begin unwind\n");
                abort();
        }


        // Call stdlibc to raise the exception
        _Unwind_Reason_Code ret = _Unwind_RaiseException( &this_exception_storage );

        // If we reach here it means something happened. For resumption to work we need to find a way
        // to return back to here. Most of them will probably boil down to setting a global flag and
        // making the phase 1 either stop or fail. Causing an error on purpose may help avoiding
        // unnecessary work but it might have some weird side effects. If we just pretend no handler
        // was found that would work but may be expensive for no reason since we will always search
        // the whole stack.

        if ( ret == _URC_END_OF_STACK ) {
                // No proper handler was found. This can be handled in many ways, C++ calls std::terminate.
                // Here we force unwind the stack, basically raising a cancellation.
                printf("Uncaught exception %p\n", &this_exception_storage);

                ret = _Unwind_ForcedUnwind( &this_exception_storage, _Stop_Fn, (void*)0x22 );
                printf("UNWIND ERROR %d after force unwind\n", ret);
                abort();
        }

        // We did not simply reach the end of the stack without finding a handler. This is an error.
        printf("UNWIND ERROR %d after raise exception\n", ret);
        abort();
}

void __cfaehm_throw_terminate( exception_t * val ) {
        __cfaabi_dbg_print_safe("Throwing termination exception\n");

        __cfaehm_allocate_exception( val );
        __cfaehm_begin_unwind();
}

void __cfaehm_rethrow_terminate(void) {
        __cfaabi_dbg_print_safe("Rethrowing termination exception\n");

        __cfaehm_begin_unwind();
}

// This is our personality routine. For every stack frame annotated with
// ".cfi_personality 0x3,__gcfa_personality_v0" this function will be called twice when unwinding.
//  Once in the search phase and once in the cleanup phase.
_Unwind_Reason_Code __gcfa_personality_v0(
                int version,
                _Unwind_Action actions,
                unsigned long long exception_class,
                struct _Unwind_Exception * unwind_exception,
                struct _Unwind_Context * unwind_context)
{

        //__cfaabi_dbg_print_safe("CFA: 0x%lx\n", _Unwind_GetCFA(context));
        __cfaabi_dbg_print_safe("Personality function (%d, %x, %llu, %p, %p):",
                        version, actions, exception_class, unwind_exception, unwind_context);

        // If we've reached the end of the stack then there is nothing much we can do...
        if (actions & _UA_END_OF_STACK) return _URC_END_OF_STACK;

        if (actions & _UA_SEARCH_PHASE) {
                __cfaabi_dbg_print_safe(" lookup phase");
        }
        else if (actions & _UA_CLEANUP_PHASE) {
                __cfaabi_dbg_print_safe(" cleanup phase");
        }
        // Just in case, probably can't actually happen
        else {
                printf(" error\n");
                return _URC_FATAL_PHASE1_ERROR;
        }

        // Get a pointer to the language specific data from which we will read what we need
        const unsigned char * lsd = _Unwind_GetLanguageSpecificData( unwind_context );

        if ( !lsd ) {   //Nothing to do, keep unwinding
                printf(" no LSD");
                goto UNWIND;
        }

        // Get the instuction pointer and a reading pointer into the exception table
        lsda_header_info lsd_info;
        const unsigned char * cur_ptr = parse_lsda_header(unwind_context, lsd, &lsd_info);
        _Unwind_Ptr instruction_ptr = _Unwind_GetIP(unwind_context);

        struct exception_context_t * context = this_exception_context();

        // Linearly search the table for stuff to do
        while ( cur_ptr < lsd_info.action_table ) {
                _Unwind_Ptr callsite_start;
                _Unwind_Ptr callsite_len;
                _Unwind_Ptr callsite_landing_pad;
                _uleb128_t  callsite_action;

                // Decode the common stuff we have in here
                cur_ptr = read_encoded_value(0, lsd_info.call_site_encoding, cur_ptr, &callsite_start);
                cur_ptr = read_encoded_value(0, lsd_info.call_site_encoding, cur_ptr, &callsite_len);
                cur_ptr = read_encoded_value(0, lsd_info.call_site_encoding, cur_ptr, &callsite_landing_pad);
                cur_ptr = read_uleb128(cur_ptr, &callsite_action);

                // Have we reach the correct frame info yet?
                if ( lsd_info.Start + callsite_start + callsite_len < instruction_ptr ) {
#ifdef __CFA_DEBUG_PRINT__
                        void * ls = (void*)lsd_info.Start;
                        void * cs = (void*)callsite_start;
                        void * cl = (void*)callsite_len;
                        void * bp = (void*)lsd_info.Start + callsite_start;
                        void * ep = (void*)lsd_info.Start + callsite_start + callsite_len;
                        void * ip = (void*)instruction_ptr;
                        __cfaabi_dbg_print_safe("\nfound %p - %p (%p, %p, %p), looking for %p\n",
                                        bp, ep, ls, cs, cl, ip);
#endif // __CFA_DEBUG_PRINT__
                        continue;
                }

                // Have we gone too far?
                if ( lsd_info.Start + callsite_start > instruction_ptr ) {
                        printf(" gone too far");
                        break;
                }

                // Check for what we must do:
                if ( 0 == callsite_landing_pad ) {
                        // Nothing to do, move along
                        __cfaabi_dbg_print_safe(" no landing pad");
                } else if (actions & _UA_SEARCH_PHASE) {
                        // In search phase, these means we found a potential handler we must check.

                        // We have arbitrarily decided that 0 means nothing to do and 1 means there is
                        // a potential handler. This doesn't seem to conflict the gcc default behavior.
                        if (callsite_action != 0) {
                                // Now we want to run some code to see if the handler matches
                                // This is the tricky part where we want to the power to run arbitrary code
                                // However, generating a new exception table entry and try routine every time
                                // is way more expansive than we might like
                                // The information we have is :
                                //  - The GR (Series of registers)
                                //    GR1=GP Global Pointer of frame ref by context
                                //  - The instruction pointer
                                //  - The instruction pointer info (???)
                                //  - The CFA (Canonical Frame Address)
                                //  - The BSP (Probably the base stack pointer)

                                // The current apprach uses one exception table entry per try block
                                _uleb128_t imatcher;
                                // Get the relative offset to the {...}?
                                cur_ptr = read_uleb128(cur_ptr, &imatcher);

#                               if defined( __x86_64 )
                                _Unwind_Word match_pos = _Unwind_GetCFA(unwind_context) + 8;
#                               elif defined( __i386 )
                                _Unwind_Word match_pos = _Unwind_GetCFA(unwind_context) + 24;
#                               endif
                                int (*matcher)(exception_t *) = *(int(**)(exception_t *))match_pos;

                                int index = matcher(context->current_exception);
                                _Unwind_Reason_Code ret = (0 == index)
                                        ? _URC_CONTINUE_UNWIND : _URC_HANDLER_FOUND;
                                context->current_handler_index = index;

                                // Based on the return value, check if we matched the exception
                                if (ret == _URC_HANDLER_FOUND) {
                                        __cfaabi_dbg_print_safe(" handler found\n");
                                } else {
                                        __cfaabi_dbg_print_safe(" no handler\n");
                                }
                                return ret;
                        }

                        // This is only a cleanup handler, ignore it
                        __cfaabi_dbg_print_safe(" no action");
                } else if (actions & _UA_CLEANUP_PHASE) {
                        // In clean-up phase, no destructors here but this could be the handler.

                        if ( (callsite_action != 0) && !(actions & _UA_HANDLER_FRAME) ){
                                // If this is a potential exception handler
                                // but not the one that matched the exception in the seach phase,
                                // just ignore it
                                goto UNWIND;
                        }

                        // We need to run some clean-up or a handler
                        // These statment do the right thing but I don't know any specifics at all
                        _Unwind_SetGR( unwind_context, __builtin_eh_return_data_regno(0),
                                (_Unwind_Ptr)unwind_exception );
                        _Unwind_SetGR( unwind_context, __builtin_eh_return_data_regno(1), 0 );

                        // I assume this sets the instruction pointer to the adress of the landing pad
                        // It doesn't actually set it, it only state the value that needs to be set once we
                        // return _URC_INSTALL_CONTEXT
                        _Unwind_SetIP( unwind_context, ((lsd_info.LPStart) + (callsite_landing_pad)) );

                        __cfaabi_dbg_print_safe(" action\n");

                        // Return have some action to run
                        return _URC_INSTALL_CONTEXT;
                }
        }
        // No handling found
        __cfaabi_dbg_print_safe(" table end reached\n");

        UNWIND:
        __cfaabi_dbg_print_safe(" unwind\n");

        // Keep unwinding the stack
        return _URC_CONTINUE_UNWIND;
}

#pragma GCC push_options
#pragma GCC optimize("O0")

// Try statements are hoisted out see comments for details. While this could probably be unique
// and simply linked from libcfa but there is one problem left, see the exception table for details
__attribute__((noinline))
void __cfaehm_try_terminate(void (*try_block)(),
                void (*catch_block)(int index, exception_t * except),
                __attribute__((unused)) int (*match_block)(exception_t * except)) {
        //! volatile int xy = 0;
        //! printf("%p %p %p %p\n", &try_block, &catch_block, &match_block, &xy);

        // Setup the personality routine and exception table.
        // Unforturnately these clobber gcc cancellation support which means we can't get access to
        // the attribute cleanup tables at the same time. We would have to inspect the assembly to
        // create a new set ourselves.
#ifdef __PIC__
        asm volatile (".cfi_personality 0x9b,CFA.ref.__gcfa_personality_v0");
        asm volatile (".cfi_lsda 0x1b, .LLSDACFA2");
#else
        asm volatile (".cfi_personality 0x3,__gcfa_personality_v0");
        asm volatile (".cfi_lsda 0x3, .LLSDACFA2");
#endif

        // Label which defines the start of the area for which the handler is setup.
        asm volatile (".TRYSTART:");

        // The actual statements of the try blocks
        try_block();

        // asm statement to prevent deadcode removal
        asm volatile goto ("" : : : : CATCH );

        // Normal return for when there is no throw.
        return;

        // Exceptionnal path
        CATCH : __attribute__(( unused ));
        // Label which defines the end of the area for which the handler is setup.
        asm volatile (".TRYEND:");
        // Label which defines the start of the exception landing pad. Basically what is called when
        // the exception is caught. Note, if multiple handlers are given, the multiplexing should be
        // done by the generated code, not the exception runtime.
        asm volatile (".CATCH:");

        // Exception handler
        // Note: Saving the exception context on the stack breaks termination exceptions.
        catch_block( this_exception_context()->current_handler_index,
                     this_exception_context()->current_exception );
}

// Exception table data we need to generate. While this is almost generic, the custom data refers
// to {*}try_terminate, which is no way generic. Some more works need to be done if we want to
// have a single call to the try routine.

#ifdef __PIC__
asm (
        // HEADER
        ".LFECFA1:\n"
        "       .globl  __gcfa_personality_v0\n"
        "       .section        .gcc_except_table,\"a\",@progbits\n"
        // TABLE HEADER (important field is the BODY length at the end)
        ".LLSDACFA2:\n"
        "       .byte   0xff\n"
        "       .byte   0xff\n"
        "       .byte   0x1\n"
        "       .uleb128 .LLSDACSECFA2-.LLSDACSBCFA2\n"
        // BODY (language specific data)
        // This uses language specific data and can be modified arbitrarily
        // We use handled area offset, handled area length,
        // handler landing pad offset and 1 (action code, gcc seems to use 0).
        ".LLSDACSBCFA2:\n"
        "       .uleb128 .TRYSTART-__cfaehm_try_terminate\n"
        "       .uleb128 .TRYEND-.TRYSTART\n"
        "       .uleb128 .CATCH-__cfaehm_try_terminate\n"
        "       .uleb128 1\n"
        ".LLSDACSECFA2:\n"
        // TABLE FOOTER
        "       .text\n"
        "       .size   __cfaehm_try_terminate, .-__cfaehm_try_terminate\n"
);

// Somehow this piece of helps with the resolution of debug symbols.
__attribute__((unused)) static const int dummy = 0;

asm (
        // Add a hidden symbol which points at the function.
        "       .hidden CFA.ref.__gcfa_personality_v0\n"
        "       .weak   CFA.ref.__gcfa_personality_v0\n"
        // No clue what this does specifically
        "       .section        .data.rel.local.CFA.ref.__gcfa_personality_v0,\"awG\",@progbits,CFA.ref.__gcfa_personality_v0,comdat\n"
        "       .align 8\n"
        "       .type CFA.ref.__gcfa_personality_v0, @object\n"
        "       .size CFA.ref.__gcfa_personality_v0, 8\n"
        "CFA.ref.__gcfa_personality_v0:\n"
#if defined( __x86_64 )
        "       .quad __gcfa_personality_v0\n"
#else // then __i386
        "   .long __gcfa_personality_v0\n"
#endif
);
#else // __PIC__
asm (
        // HEADER
        ".LFECFA1:\n"
        "       .globl  __gcfa_personality_v0\n"
        "       .section        .gcc_except_table,\"a\",@progbits\n"
        // TABLE HEADER (important field is the BODY length at the end)
        ".LLSDACFA2:\n"
        "       .byte   0xff\n"
        "       .byte   0xff\n"
        "       .byte   0x1\n"
        "       .uleb128 .LLSDACSECFA2-.LLSDACSBCFA2\n"
        // BODY (language specific data)
        ".LLSDACSBCFA2:\n"
        //      Handled area start (relative to start of function)
        "       .uleb128 .TRYSTART-__cfaehm_try_terminate\n"
        //      Handled area length
        "       .uleb128 .TRYEND-.TRYSTART\n"
        //      Handler landing pad address (relative to start of function)
        "       .uleb128 .CATCH-__cfaehm_try_terminate\n"
        //      Action code, gcc seems to always use 0.
        "       .uleb128 1\n"
        // TABLE FOOTER
        ".LLSDACSECFA2:\n"
        "       .text\n"
        "       .size   __cfaehm_try_terminate, .-__cfaehm_try_terminate\n"
        "       .ident  \"GCC: (Ubuntu 6.2.0-3ubuntu11~16.04) 6.2.0 20160901\"\n"
        "       .section        .note.GNU-stack,\"x\",@progbits\n"
);
#endif // __PIC__

#pragma GCC pop_options