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extr_initrd.c
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extr_initrd.c
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#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/mman.h>
#include <sys/stat.h>
#define CPIO_PATTERN "070701........................................................................................................"
#define CPIO_END CPIO_PATTERN "TRAILER!!!"
int find_cpio(void *buf, int len, int e);
int find_initrd(void *kern, int len, int *initrd_pos, int *initrd_len);
void extract_swap_initrd(int swap, const char *kernel, const char *initrd, const char *output);
void usage(const char *pname);
int main(int argc, char **argv) {
if(argc==1) {
usage(argv[0]);
return 0;
}
if(strcmp(argv[1], "extract")==0) {
if(argc<=3) {
fprintf(stderr, "extract: missing arguments\n");
usage(argv[0]);
return 1;
}
extract_swap_initrd(0, argv[2], argv[3], NULL);
} else if(strcmp(argv[1], "swap")==0) {
if(argc<=4) {
fprintf(stderr, "swap: missing arguments\n");
usage(argv[0]);
return 1;
}
extract_swap_initrd(1, argv[2], argv[3], argv[4]);
} else {
fprintf(stderr, "Unknown command: %s\n", argv[1]);
usage(argv[0]);
return 1;
}
return 0;
}
void usage(const char *pname) {
printf("Usage: %s command [arguments...]\n", pname);
printf("Initrd manipulation tool for uncompressed kernel binary\n");
printf("Available commands:\n");
printf("\textract <kernel> <output> - tries to extract initrd from a kernel binary\n");
printf("\tswap <kernel> <initrd> <output> - swaps kernel initrd (if possible)\n");
}
int find_cpio(void *buf, int len, int e) {
static const char cpio_pattern[] = CPIO_PATTERN;
static const char cpio_end[] = CPIO_END;
char *p = buf;
int pattern_l;
const char *pattern;
char a;
int i;
int j;
int f;
if(e) {
pattern = cpio_end;
pattern_l = sizeof(cpio_end)-1;
} else {
pattern = cpio_pattern;
pattern_l = sizeof(cpio_pattern)-1;
}
f = 0;
for(i=0; i+pattern_l<len; i++) {
f = 1;
for(j=0; j<pattern_l; j++) {
a = p[i+j];
if(pattern[j]=='.') {
if(a>='A' && a<='F') a = a-'A'+'a';
if(!((a>='0' && a<='9') || (a>='a' && a<='f'))) {
f = 0;
break;
}
} else if(a!=pattern[j]) {
f = 0;
break;
}
}
if(f) break;
}
if(f) return i;
return -1;
}
int find_initrd(void *kern, int len, int *initrd_pos, int *initrd_len) {
char *pkern;
char *initrd;
int i_pos;
int i_len;
pkern = kern;
i_pos = find_cpio(pkern, len, 0);
if(i_pos<0) return 1;
initrd = &pkern[i_pos];
i_len = find_cpio(initrd, len-i_pos, 1);
if(i_len>=0) i_len = (i_len + 0x1ff) & ~0x1ff;
if(i_len<0 || i_len+i_pos>len) return 1;
*initrd_pos = i_pos;
*initrd_len = i_len;
printf("Found CPIO archive at 0x%06x\n", i_pos);
return 0;
}
void extract_swap_initrd(int swap, const char *kernel, const char *initrd, const char *output) {
char buf[4096];
struct stat kern_stat;
struct stat initrd_stat;
int kfd;
int k_len;
int i_pos;
int i_len;
int ifd;
int ofd;
int i;
int j;
int n;
char *k_mmap;
char *initrd_mmap;
kfd = open(kernel, O_RDONLY);
if(kfd<0) exit(1);
fstat(kfd, &kern_stat);
k_len = kern_stat.st_size;
k_mmap = mmap(NULL, k_len, PROT_READ, MAP_PRIVATE, kfd, 0);
if(find_initrd(k_mmap, k_len, &i_pos, &i_len)!=0) {
fprintf(stderr, "Unable to find initrd in %s\n", kernel);
munmap(k_mmap, k_len);
close(kfd);
exit(1);
}
initrd_mmap = &k_mmap[i_pos];
if(swap) {
ifd = open(initrd, O_RDONLY);
if(ifd<0) exit(1);
fstat(ifd, &initrd_stat);
n = initrd_stat.st_size;
if(n>i_len) {
fprintf(stderr, "Unable to fit the initrd into %d bytes of space\n", i_len);
close(ifd);
munmap(k_mmap, k_len);
close(kfd);
exit(1);
}
ofd = open(output, O_RDWR | O_CREAT, 0644);
write(ofd, k_mmap, i_pos);
while((i=read(ifd, buf, sizeof(buf)))>0) {
write(ofd, buf, i);
}
for(i=0; i<sizeof(buf); i++) buf[i] = 0;
for(i=i_len-n; i>0; i-=j) {
if(i>sizeof(buf)) j = write(ofd, buf, sizeof(buf));
else j = write(ofd, buf, i);
}
write(ofd, &initrd_mmap[i_len], k_len-i_pos-i_len);
close(ofd);
close(ifd);
printf("Replaced initrd in %s to %s (%d bytes)\n", output, initrd, n);
} else {
ifd = open(initrd, O_RDWR | O_CREAT, 0644);
n = write(ifd, initrd_mmap, i_len);
printf("Extracted %d bytes of (possibly) initrd to %s\n", n, initrd);
close(ifd);
}
munmap(k_mmap, k_len);
close(kfd);
}