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grub-kernel.c
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grub-kernel.c
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#define _GNU_SOURCE
#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <string.h>
#include <errno.h>
int main(int argc, char** argv) {
if (argc != 2) {
fprintf(stderr, "Usage: grub-kernel /path/to/grub/kernel.img\n");
fprintf(stderr, "\nYou need to provide one path to kernel.img, generally\n");
fprintf(stderr, "/boot/grub/kernel.img\n");
return 1;
}
int fd = open(argv[1], O_RDONLY);
if (fd < 0) {
fprintf(stderr, "Could not open for read %s: %s\n", argv[1], strerror(errno));
return 1;
}
printf("opened: %s\n", argv[1]);
char buffer[1024 * 1024];
int data = read(fd, buffer, sizeof(buffer));
if (data < 0) {
fprintf(stderr, "Could not read %s: %s\n", argv[1], strerror(errno));
return 2;
}
close(fd);
printf("read: %d bytes from %s\n", data, argv[1]);
# define DECLARE_MATCH(str) { str, sizeof(str) - 1 }
struct {
const char* match;
const int length;
} matches[] = {
DECLARE_MATCH("Welcome to GRUB!\n\n\0"),
};
int found = 0;
for (int i = 0; i < sizeof(matches) / sizeof(matches[0]); ++i) {
const char* match = matches[i].match;
const int length = matches[i].length;
void* ptr = memmem(buffer, data, match, length);
if (!ptr) {
printf("match[%d]: NOT FOUND\n", i);
continue;
}
size_t offset = (char*)ptr - buffer;
printf("match[%d]: found at %ld, \"%s\", '%c'\n",
i, offset, buffer + offset, buffer[offset]);
++found;
buffer[offset] = '\0';
}
if (found != sizeof(matches) / sizeof(matches[0])) {
fprintf(stderr, "Not all required matches were found, giving up.\n");
fprintf(stderr, "(if you run this command more than once, it's good!\n");
fprintf(stderr, "it means the first run succeeded)\n");
return 10;
}
fd = open(argv[1], O_RDWR);
if (fd < 0) {
fprintf(stderr, "Could not open for write %s: %s\n", argv[1], strerror(errno));
return 3;
}
data = write(fd, buffer, sizeof(buffer));
if (data < 0) {
fprintf(stderr, "Could not write %s, good luck: %s\n", argv[1], strerror(errno));
return 4;
}
if (close(fd) < 0) {
fprintf(stderr, "Close failed! Good luck: %s - %s\n", argv[1], strerror(errno));
return 5;
}
printf("SUCCESS! (maybe! you won't know until you reboot!)\n");
return 0;
}