Unix/Darwin/lowmem.c

/*
 *  lowmem.c - enable access to low memory globals on Darwin
 *
 *  Copyright (c) 2003 Michael Z. Sliczniak
 *
 *  Basilisk II (C) 1997-2004 Christian Bauer
 *
 *  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 2 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, write to the Free Software
 *  Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
 */

#include <sys/types.h>
#include <sys/mman.h>
#include <fcntl.h>
#include <errno.h>
#include <stdio.h>
#include <string.h>
#include <mach/vm_prot.h>
#include <mach-o/loader.h>

static const char progname[] = "lowmem";

/*
 * Under Mach there is very little assumed about the memory map of object
 * files. It is the job of the loader to create the initial memory map of an
 * executable. In a Mach-O executable there will be numerous loader commands
 * that the loader must process. Some of these will create the initial memory
 * map used by the executable. Under Darwin the static object file linker,
 * ld, automatically adds the __PAGEZERO segment to all executables. The
 * default size of this segment is the page size of the target system and
 * the initial and maximum permissions are set to allow no access. This is so
 * that all programs fault on a NULL pointer dereference. Arguably this is
 * incorrect and the maximum permissions shoould be rwx so that programs can
 * change this default behavior. Then programs could be written that assume
 * a null string at the null address, which was the convention on some
 * systems. In our case we need to have 8K mapped at zero for the low memory
 * globals and this program modifies the segment load command in the
 * basiliskII executable so that it can be used for data.
 */

int
main(int argc, const char *argv[])
{
        int fd;
        char *addr;
        struct mach_header *machhead;
        struct segment_command *sc_cmd;

        if (argc != 2) {
                (void)fprintf(stderr, "Usage: %s executable\n", progname);
                exit(1);
        }

        fd = open(argv[1], O_RDWR, 0);
        if (fd == -1) {
                (void)fprintf(stderr, "%s: could not open %s: %s\n",
                        progname, argv[1], strerror(errno));
                exit(1);
        }

        /*
         * Size does not really matter, it will be rounded-up to a multiple
         * of the page size automatically.
         */
        addr = mmap(NULL, 0x1000, PROT_READ | PROT_WRITE,
            MAP_FILE | MAP_SHARED, fd, 0);
        if (addr == NULL) {
                (void)fprintf(stderr, "%s: could not mmap %s: %s\n",
                                progname, argv[1], strerror(errno));
                exit(1);
        }

        /*
         * Check to see if the Mach-O magic bytes are in the header.
         * If we cared about cross compiling we would also check against
         * MH_CIGAM and then change the endianness with every access, but
         * we do not care about that.
         */
        machhead = (void *)addr;
        if (machhead->magic != MH_MAGIC) {
                (void)fprintf(stderr, "%s: %s does not appear to be a Mach-O object file\n",
                                progname, argv[1]);
                exit(1);
        }

        if (machhead->filetype != MH_EXECUTE) {
                (void)fprintf(stderr, "%s: %s does not appear to be an executable file\n",
                                progname, argv[1]);
                exit(1);
        }

        if (machhead->ncmds == 0) {
                (void)fprintf(stderr, "%s: %s does not contain any load commands\n",
                                progname, argv[1]);
                exit(1);
        }

        sc_cmd = (void *)&machhead[1];
        if (sc_cmd->cmd != LC_SEGMENT){
                (void)fprintf(stderr, "%s: load segment not first command in %s\n",
                                progname, argv[1]);
                exit(1);
        }

        if (strncmp(sc_cmd->segname, "__PAGEZERO",
                         sizeof (*sc_cmd->segname))) {
                (void)fprintf(stderr, "%s: zero page not first segment in %s\n",
                                progname, argv[1]);
                exit(1);
        }

        /* change the permissions */
        sc_cmd->maxprot = VM_PROT_ALL;
        sc_cmd->initprot = VM_PROT_ALL;

        /*
         * We do not make __PAGEZERO 8K in this program because then
         * all of the offsets would be wrong in the object file after
         * this segment. Instead we use the -pagezero_size option
         * to link the executable.
         */
        if (msync(addr, 0x1000, MS_SYNC) == -1) {
                (void)fprintf(stderr, "%s: could not sync %s: %s\n",
                                progname, argv[1], strerror(errno));
                exit(1);
        }

        if (munmap(addr, 0x1000) == -1) {
                (void)fprintf(stderr, "%s: could not unmap %s: %s\n",
                                progname, argv[1], strerror(errno));
                exit(1);
        }

        (void)close(fd);

        exit(0);
}
Revision:	1.2
Committed:	2004-01-12T15:54:42Z (20 years, 8 months ago) by cebix
Content type:	text/plain
Branch:	MAIN
Changes since 1.1:	+1 -1 lines
Log Message:	Happy New Year!
#	User	Rev	Content
1	gbeauche	1.1	/*
2			* lowmem.c - enable access to low memory globals on Darwin
3			*
4			* Copyright (c) 2003 Michael Z. Sliczniak
5			*
6	cebix	1.2	* Basilisk II (C) 1997-2004 Christian Bauer
7	gbeauche	1.1	*
8			* This program is free software; you can redistribute it and/or modify
9			* it under the terms of the GNU General Public License as published by
10			* the Free Software Foundation; either version 2 of the License, or
11			* (at your option) any later version.
12			*
13			* This program is distributed in the hope that it will be useful,
14			* but WITHOUT ANY WARRANTY; without even the implied warranty of
15			* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
16			* GNU General Public License for more details.
17			*
18			* You should have received a copy of the GNU General Public License
19			* along with this program; if not, write to the Free Software
20			* Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
21			*/
22
23			#include <sys/types.h>
24			#include <sys/mman.h>
25			#include <fcntl.h>
26			#include <errno.h>
27			#include <stdio.h>
28			#include <string.h>
29			#include <mach/vm_prot.h>
30			#include <mach-o/loader.h>
31
32			static const char progname[] = "lowmem";
33
34			/*
35			* Under Mach there is very little assumed about the memory map of object
36			* files. It is the job of the loader to create the initial memory map of an
37			* executable. In a Mach-O executable there will be numerous loader commands
38			* that the loader must process. Some of these will create the initial memory
39			* map used by the executable. Under Darwin the static object file linker,
40			* ld, automatically adds the __PAGEZERO segment to all executables. The
41			* default size of this segment is the page size of the target system and
42			* the initial and maximum permissions are set to allow no access. This is so
43			* that all programs fault on a NULL pointer dereference. Arguably this is
44			* incorrect and the maximum permissions shoould be rwx so that programs can
45			* change this default behavior. Then programs could be written that assume
46			* a null string at the null address, which was the convention on some
47			* systems. In our case we need to have 8K mapped at zero for the low memory
48			* globals and this program modifies the segment load command in the
49			* basiliskII executable so that it can be used for data.
50			*/
51
52			int
53			main(int argc, const char *argv[])
54			{
55			int fd;
56			char *addr;
57			struct mach_header *machhead;
58			struct segment_command *sc_cmd;
59
60			if (argc != 2) {
61			(void)fprintf(stderr, "Usage: %s executable\n", progname);
62			exit(1);
63			}
64
65			fd = open(argv[1], O_RDWR, 0);
66			if (fd == -1) {
67			(void)fprintf(stderr, "%s: could not open %s: %s\n",
68			progname, argv[1], strerror(errno));
69			exit(1);
70			}
71
72			/*
73			* Size does not really matter, it will be rounded-up to a multiple
74			* of the page size automatically.
75			*/
76			addr = mmap(NULL, 0x1000, PROT_READ \| PROT_WRITE,
77			MAP_FILE \| MAP_SHARED, fd, 0);
78			if (addr == NULL) {
79			(void)fprintf(stderr, "%s: could not mmap %s: %s\n",
80			progname, argv[1], strerror(errno));
81			exit(1);
82			}
83
84			/*
85			* Check to see if the Mach-O magic bytes are in the header.
86			* If we cared about cross compiling we would also check against
87			* MH_CIGAM and then change the endianness with every access, but
88			* we do not care about that.
89			*/
90			machhead = (void *)addr;
91			if (machhead->magic != MH_MAGIC) {
92			(void)fprintf(stderr, "%s: %s does not appear to be a Mach-O object file\n",
93			progname, argv[1]);
94			exit(1);
95			}
96
97			if (machhead->filetype != MH_EXECUTE) {
98			(void)fprintf(stderr, "%s: %s does not appear to be an executable file\n",
99			progname, argv[1]);
100			exit(1);
101			}
102
103			if (machhead->ncmds == 0) {
104			(void)fprintf(stderr, "%s: %s does not contain any load commands\n",
105			progname, argv[1]);
106			exit(1);
107			}
108
109			sc_cmd = (void *)&machhead[1];
110			if (sc_cmd->cmd != LC_SEGMENT){
111			(void)fprintf(stderr, "%s: load segment not first command in %s\n",
112			progname, argv[1]);
113			exit(1);
114			}
115
116			if (strncmp(sc_cmd->segname, "__PAGEZERO",
117			sizeof (*sc_cmd->segname))) {
118			(void)fprintf(stderr, "%s: zero page not first segment in %s\n",
119			progname, argv[1]);
120			exit(1);
121			}
122
123			/* change the permissions */
124			sc_cmd->maxprot = VM_PROT_ALL;
125			sc_cmd->initprot = VM_PROT_ALL;
126
127			/*
128			* We do not make __PAGEZERO 8K in this program because then
129			* all of the offsets would be wrong in the object file after
130			* this segment. Instead we use the -pagezero_size option
131			* to link the executable.
132			*/
133			if (msync(addr, 0x1000, MS_SYNC) == -1) {
134			(void)fprintf(stderr, "%s: could not sync %s: %s\n",
135			progname, argv[1], strerror(errno));
136			exit(1);
137			}
138
139			if (munmap(addr, 0x1000) == -1) {
140			(void)fprintf(stderr, "%s: could not unmap %s: %s\n",
141			progname, argv[1], strerror(errno));
142			exit(1);
143			}
144
145			(void)close(fd);
146
147			exit(0);
148			}