source: doc/working/exception/impl/exception.c@ 637568b

#include "exception.h"

// Implementation of the secret header.

#include <stdlib.h>
#include <stdio.h>
#include <unwind.h>

#include "lsda.h"

struct shared_stack_t shared_stack;


// This macro should be the only thing that needs to change across machines.
// Used in the personality function, way down in termination.
// struct _Unwind_Context * -> _Unwind_Reason_Code(*)()
#define MATCHER_FROM_CONTEXT(ptr_to_context) \
        (*(_Unwind_Reason_Code(**)())(_Unwind_GetCFA(ptr_to_context) + 8))


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

void __throw_resume(exception except) {

        // DEBUG
        printf("Throwing resumption exception %d\n", except);

        struct __try_resume_node * original_head = shared_stack.current_resume;
        struct __try_resume_node * current =
                (original_head) ? original_head->next : shared_stack.top_resume;

        for ( ; current ; current = current->next) {
                shared_stack.current_resume = current;
                if (current->try_to_handle(except)) {
                        shared_stack.current_resume = original_head;
                        return;
                }
        }

        printf("Unhandled exception %d\n", except);
        shared_stack.current_resume = original_head;

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


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

// Requires -fexceptions to work.

// Global which defines the current exception
// Currently an int just to make matching easier
//int this_exception; (became shared_stack.current_exception)

// 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 exceptionClass,
                struct _Unwind_Exception * unwind_exception,
                struct _Unwind_Context * context,
                void * some_param) {
        if( actions & _UA_END_OF_STACK  ) exit(1);
        if( actions & _UA_CLEANUP_PHASE ) return _URC_NO_REASON;

        return _URC_FATAL_PHASE2_ERROR;
}

// Example throw routine
void __throw_terminate( int val ) {
        // Store the current exception
        shared_stack.current_exception = val;

        // DEBUG
        printf("Throwing termination exception %d\n", val);

        // 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 several way
                // 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,
        // Something wen't wrong
        printf("UNWIND ERROR %d after raise exception\n", ret);
        abort();
}

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

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

        // DEBUG
        if (actions & _UA_SEARCH_PHASE) {
                printf(" lookup phase");
        }
        // DEBUG
        else if (actions & _UA_CLEANUP_PHASE) {
                printf(" 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 = (const unsigned char*) _Unwind_GetLanguageSpecificData( 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( context, lsd, &lsd_info);
        _Unwind_Ptr instruction_ptr = _Unwind_GetIP( 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 ) {
                        //DEBUG BEGIN
                        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;
                        printf("\nfound %p - %p (%p, %p, %p), looking for %p\n", bp, ep, ls, cs, cl, ip);
                        //DEBUG END
                        continue;
                }

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

                // Something to do?
                if( callsite_landing_pad ) {
                        // Which phase are we in
                        if (actions & _UA_SEARCH_PHASE) {
                                // Search phase, this means we probably found a potential handler and must check if it is a match

                                // If 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);

                                        // Get a function pointer from the relative offset and call it
                                        // _Unwind_Reason_Code (*matcher)() = (_Unwind_Reason_Code (*)())lsd_info.LPStart + imatcher;                                   

                                        _Unwind_Reason_Code (*matcher)() =
                                                MATCHER_FROM_CONTEXT(context);
                                        int index = matcher(shared_stack.current_exception);
                                        _Unwind_Reason_Code ret = (0 == index)
                                                ? _URC_CONTINUE_UNWIND : _URC_HANDLER_FOUND;
                                        shared_stack.current_handler_index = index;

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

                                // This is only a cleanup handler, ignore it
                                printf(" no action");
                        }
                        else if (actions & _UA_CLEANUP_PHASE) {

                                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( context, __builtin_eh_return_data_regno(0), (_Unwind_Ptr) unwind_exception );
                                _Unwind_SetGR( 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( context, lsd_info.LPStart + callsite_landing_pad );

                                // DEBUG
                                printf(" action\n");

                                // Return have some action to run
                                return _URC_INSTALL_CONTEXT;
                        }
                }

                // Nothing to do, move along
                printf(" no landing pad");
        }
        // No handling found
        printf(" table end reached\n");

        // DEBUG
        UNWIND:
        printf(" unwind\n");

        // Keep unwinding the stack
        return _URC_CONTINUE_UNWIND;
}

// Try statements are hoisted out see comments for details
// With 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 __try_terminate(void (*try_block)(),
                void (*catch_block)(int index, exception except),
                __attribute__((unused)) int (*match_block)(exception except)) {
        //! volatile int xy = 0;
        //! printf("%p %p %p %p\n", &try_block, &catch_block, &match_block, &xy);

        // Setup statments
        // These 2 statments won't actually result in any code,
        // they only setup global tables.
        // However, they clobber gcc cancellation support from gcc.
        // We can replace the personality routine but replacing the exception
        // table gcc generates is not really doable, it generates labels based
        // on how the assembly works.
        // Setup the personality routine
        asm volatile (".cfi_personality 0x3,__gcfa_personality_v0");
        // Setup the exception table
        asm volatile (".cfi_lsda 0x3, .LLSDACFA2");

        // 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
        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 will be 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
        catch_block(shared_stack.current_handler_index,
                    shared_stack.current_exception);
}

// Exception table data we need to generate
// While this is almost generic, the custom data refers to
// foo_try_match try match, which is no way generic
// Some more works need to be done if we want to have a single
// call to the try routine
asm (
        //HEADER
        ".LFECFA1:\n"
        "       .globl  __gcfa_personality_v0\n"
        "       .section        .gcc_except_table,\"a\",@progbits\n"
        ".LLSDACFA2:\n"                                                 //TABLE header
        "       .byte   0xff\n"
        "       .byte   0xff\n"
        "       .byte   0x1\n"
        "       .uleb128 .LLSDACSECFA2-.LLSDACSBCFA2\n"         // BODY length
        // Body uses language specific data and therefore could be modified arbitrarily
        ".LLSDACSBCFA2:\n"                                              // BODY start
        "       .uleb128 .TRYSTART-__try_terminate\n"           // Handled area start  (relative to start of function)
        "       .uleb128 .TRYEND-.TRYSTART\n"                           // Handled area length
        "       .uleb128 .CATCH-__try_terminate\n"                              // Hanlder landing pad adress  (relative to start of function)
        "       .uleb128 1\n"                                           // Action code, gcc seems to use always 0
        ".LLSDACSECFA2:\n"                                              // BODY end
        "       .text\n"                                                        // TABLE footer
        "       .size   __try_terminate, .-__try_terminate\n"
        "       .ident  \"GCC: (Ubuntu 6.2.0-3ubuntu11~16.04) 6.2.0 20160901\"\n"
//      "       .section        .note.GNU-stack,\"x\",@progbits\n"
);