biosint.c

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#include <string.h>
#include <errno.h>
#include <realmode.h>
#include <biosint.h>

/**
 * @file BIOS interrupts
 *
 */

FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );

/**
 * Hook INT vector
 *
 * @v interrupt         INT number
 * @v handler           Offset within .text16 to interrupt handler
 * @v chain_vector      Vector for chaining to previous handler
 *
 * Hooks in an i386 INT handler.  The handler itself must reside
 * within the .text16 segment.  @c chain_vector will be filled in with
 * the address of the previously-installed handler for this interrupt;
 * the handler should probably exit by ljmping via this vector.
 */
void hook_bios_interrupt ( unsigned int interrupt, unsigned int handler,
                           struct segoff *chain_vector ) {
        struct segoff vector = {
                .segment = rm_cs,
                .offset = handler,
        };

        DBG ( "Hooking INT %#02x to %04x:%04x\n",
              interrupt, rm_cs, handler );

        if ( ( chain_vector->segment != 0 ) ||
             ( chain_vector->offset != 0 ) ) {
                /* Already hooked; do nothing */
                DBG ( "...already hooked\n" );
                return;
        }

        copy_from_real ( chain_vector, 0, ( interrupt * 4 ),
                         sizeof ( *chain_vector ) );
        DBG ( "...chaining to %04x:%04x\n",
              chain_vector->segment, chain_vector->offset );
        if ( DBG_LOG ) {
                char code[64];
                copy_from_real ( code, chain_vector->segment,
                                 chain_vector->offset, sizeof ( code ) );
                DBG_HDA ( *chain_vector, code, sizeof ( code ) );
        }

        copy_to_real ( 0, ( interrupt * 4 ), &vector, sizeof ( vector ) );
        hooked_bios_interrupts++;
}

/**
 * Unhook INT vector
 *
 * @v interrupt         INT number
 * @v handler           Offset within .text16 to interrupt handler
 * @v chain_vector      Vector containing address of previous handler
 *
 * Unhooks an i386 interrupt handler hooked by hook_i386_vector().
 * Note that this operation may fail, if some external code has hooked
 * the vector since we hooked in our handler.  If it fails, it means
 * that it is not possible to unhook our handler, and we must leave it
 * (and its chaining vector) resident in memory.
 */
int unhook_bios_interrupt ( unsigned int interrupt, unsigned int handler,
                            struct segoff *chain_vector ) {
        struct segoff vector;

        DBG ( "Unhooking INT %#02x from %04x:%04x\n",
              interrupt, rm_cs, handler );

        copy_from_real ( &vector, 0, ( interrupt * 4 ), sizeof ( vector ) );
        if ( ( vector.segment != rm_cs ) || ( vector.offset != handler ) ) {
                DBG ( "...cannot unhook; vector points to %04x:%04x\n",
                      vector.segment, vector.offset );
                return -EBUSY;
        }

        DBG ( "...restoring to %04x:%04x\n",
              chain_vector->segment, chain_vector->offset );
        copy_to_real ( 0, ( interrupt * 4 ), chain_vector,
                       sizeof ( *chain_vector ) );

        chain_vector->segment = 0;
        chain_vector->offset = 0;
        hooked_bios_interrupts--;
        return 0;
}

/**
 * Dump changes to interrupt vector table (for debugging)
 *
 */
void check_bios_interrupts ( void ) {
        static struct segoff vectors[256];
        static uint8_t initialised;
        struct segoff vector;
        unsigned int i;

        /* Print any changed interrupt vectors */
        for ( i = 0; i < ( sizeof ( vectors ) / sizeof ( vectors[0] ) ); i++ ) {
                copy_from_real ( &vector, 0, ( i * sizeof ( vector ) ),
                                 sizeof ( vector ) );
                if ( memcmp ( &vector, &vectors[i], sizeof ( vector ) ) == 0 )
                        continue;
                if ( initialised ) {
                        dbg_printf ( "INT %02x changed %04x:%04x => "
                                     "%04x:%04x\n", i, vectors[i].segment,
                                     vectors[i].offset, vector.segment,
                                     vector.offset );
                }
                memcpy ( &vectors[i], &vector, sizeof ( vectors[i] ) );
        }
        initialised = 1;
}