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#include <stdio.h>
#include <stdlib.h>
#include <stdarg.h>
#include <sys/types.h>
#include <fcntl.h>
#include <unistd.h>
#include <string.h>
typedef unsigned char uchar;
typedef unsigned int uint;
typedef unsigned long long vlong;
enum {
Sectsz = 0x200,
Psectsz = 11,
Pclustsc = 13,
Presvd = 14,
Pnumfat = 16,
Pfatsz = 22,
Pfatsz32 = 36,
Pvolid = 67,
};
int
readn(int f, void *av, int n)
{
char *a;
int m, t;
a = av;
t = 0;
while(t < n){
m = read(f, a+t, n-t);
if(m <= 0){
if(t == 0)
return m;
break;
}
t += m;
}
return t;
}
void
sysfatal(char *fmt, ...)
{
va_list va;
va_start(va, fmt);
vfprintf(stderr, fmt, va);
va_end(va);
fprintf(stderr, "\n");
exit(1);
}
void
readsect(int fd, uint n, void *data)
{
loff_t off;
off = (loff_t) n * Sectsz;
if(llseek(fd, off, SEEK_SET) != off)
sysfatal("seek to sector 0x%x failed", n);
if(readn(fd, data, Sectsz) != Sectsz)
sysfatal("short read: %m");
}
void
writesect(int fd, uint n, void *data)
{
loff_t off;
off = (loff_t) n * Sectsz;
if(llseek(fd, off, SEEK_SET) != off)
sysfatal("seek to sector 0x%x failed", n);
if(write(fd, data, Sectsz) != Sectsz)
sysfatal("short write: %m");
}
uint
getulong(uchar *s)
{
return s[0] | (s[1] << 8) | (s[2] << 16) | (s[3] << 24);
}
void
putulong(uchar *s, uint n)
{
*s++ = n & 0xff;
*s++ = (n >> 8) & 0xff;
*s++ = (n >> 16) & 0xff;
*s++ = (n >> 24) & 0xff;
}
uint
getushort(uchar *s)
{
return s[0] | (s[1] << 8);
}
int
checksig(uchar *s)
{
return s[0x1fe] == 0x55 && s[0x1ff] == 0xaa;
}
void
fixpbs(uchar *pbs, uchar *pbs9, uint lba)
{
uint a;
uint fatsz, resvd, numfat;
numfat = pbs[Pnumfat];
fatsz = getushort(&pbs[Pfatsz]);
if(fatsz == 0)
fatsz = getulong(&pbs[Pfatsz32]);
resvd = getushort(&pbs[Presvd]);
a = pbs9[1] + 2;
memcpy(pbs, pbs9, 3);
memcpy(pbs+a, pbs9+a, Sectsz-a-2);
a = lba + numfat * fatsz + resvd;
printf("Xroot=%x\n", a);
putulong(&pbs[Pvolid], a);
}
int
main(int argc, char *argv[])
{
int dev, fd, i;
uchar mbr9[Sectsz], pbs9[Sectsz];
uchar mbr[Sectsz], pbs[Sectsz];
uint lba;
int part, want;
char *mbrfn, *pbsfn, *devfn;
if(argc < 4)
sysfatal("usage: <device> <mbrfile> <pbsfile> [part]");
devfn = argv[1];
mbrfn = argv[2];
pbsfn = argv[3];
want = argc >= 5 ? atoi(argv[4]) : -1;
part = -1;
dev = open(devfn, O_RDWR);
if(dev < 0)
sysfatal("%s: %m", devfn);
if((fd = open(mbrfn, O_RDONLY)) < 0)
sysfatal("%s: %m", mbrfn);
if(readn(fd, mbr9, Sectsz) < 3)
sysfatal("%s: too short", mbrfn);
close(fd);
fd = open(pbsfn, O_RDONLY);
if(fd < 0)
sysfatal("%s: %m", pbsfn);
if(readn(fd, pbs9, Sectsz) < 3)
sysfatal("%s: too short", pbsfn);
if(pbs9[0] != 0xeb)
sysfatal("first byte of pbs not a short jump");
close(fd);
readsect(dev, 0, mbr);
if(!checksig(mbr))
sysfatal("sector 0 is missing signature");
for(i=0; i<4; i++){
if(mbr[0x1be + i*16] == 0x80 && (part == -1 || i == want))
part = i;
}
if(part == -1)
sysfatal("no bootable partitions found");
if(want != -1 && part != want)
sysfatal("partition %d is not bootable", want);
lba = getulong(&mbr[0x1be + part*16 + 8]);
if(lba == 0)
sysfatal("partition %d has zero LBA", part);
readsect(dev, lba, pbs);
if(!checksig(pbs))
sysfatal("partition %d (LBA=0x%x) is missing signaure", part, lba);
if(getushort(&pbs[Psectsz]) != 512)
sysfatal("sector size not 512");
printf("using partition %d, LBA=0x%x\n", part, lba);
memcpy(mbr, mbr9, 446);
fixpbs(pbs, pbs9, lba);
writesect(dev, 0, mbr);
writesect(dev, lba, pbs);
close(dev);
return 0;
}
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