ubase/switch_root.c

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2.8 KiB
C
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/* See LICENSE file for copyright and license details. */
#include <dirent.h>
#include <fcntl.h>
#include <limits.h>
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#include <stdio.h>
#include <stdlib.h>
#include <string.h>
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#include <sys/mount.h>
#include <sys/stat.h>
#include <sys/vfs.h>
#include <unistd.h>
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#include "util.h"
#define RAMFS_MAGIC 0x858458f6 /* some random number */
#define TMPFS_MAGIC 0x01021994
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static void
delete_content(const char *dir, dev_t curdevice)
{
char path[PATH_MAX];
DIR *d;
struct stat st;
struct dirent *dent;
/* don't dive into other filesystems */
if (lstat(dir, &st) || st.st_dev != curdevice)
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return;
/* delete contents recursively */
if (S_ISDIR(st.st_mode)) {
d = opendir(dir);
if (d) {
for(; (dent = readdir(d)) ;) {
/* skip ".." and "." */
if (strcmp(dent->d_name, ".") == 0 ||
strcmp(dent->d_name, "..") == 0)
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continue;
/* build path and dive deeper */
strlcat(path, dir, sizeof(path));
strlcat(path, dent->d_name, sizeof(path));
delete_content(path, curdevice);
path[0] = 0;
}
closedir(d);
/* remove now empty dir */
rmdir(dir);
}
} else {
/* unlink non-directory */
unlink(dir);
}
}
static void
usage(void)
{
eprintf("usage: %s [-c console] [newroot] [init] (PID 1)\n", argv0);
}
int
main(int argc, char **argv)
{
char *console = NULL;
dev_t curdev;
struct stat st;
struct statfs stfs;
ARGBEGIN {
case 'c':
console = EARGF(usage());
break;
default:
usage();
} ARGEND;
/* check number of args and if we are PID 1 */
if (argc != 2 || getpid() != 1)
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usage();
/* chdir to newroot and make sure it's a different fs */
if (chdir(argv[0]))
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eprintf("chdir %s:", argv[0]);
if (stat("/", &st))
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eprintf("stat %s:", "/");
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curdev = st.st_dev;
if (stat(".", &st))
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eprintf("stat %s:", ".");
if (st.st_dev == curdev)
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usage();
/* avoids trouble with real filesystems */
if (stat("/init", &st) || !S_ISREG(st.st_mode))
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eprintf("/init is not a regular file\n");
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statfs("/", &stfs);
if ((unsigned)stfs.f_type != RAMFS_MAGIC && (unsigned)stfs.f_type != TMPFS_MAGIC)
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eprintf("current filesystem is not a RAMFS or TMPFS\n");
/* wipe / */
delete_content("/", curdev);
/* overmount / with newroot and chroot into it */
if (mount(".", "/", NULL, MS_MOVE, NULL))
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eprintf("mount %s:", ".");
if (chroot("."))
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eprintf("chroot failed\n");
/* if -c is set, redirect stdin/stdout/stderr to console */
if (console) {
close(STDIN_FILENO);
if(open(console, O_RDWR) == -1)
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eprintf("open %s:", console);
if (dup2(STDIN_FILENO, STDOUT_FILENO) != STDOUT_FILENO)
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eprintf("dup2 %s:", "stdin,stdout");
if (dup2(STDIN_FILENO, STDERR_FILENO) != STDERR_FILENO)
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eprintf("dup2 %s:", "stdin,stderr");
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}
/* execute init */
execv(argv[1], argv);
eprintf("can't execute '%s'\n", argv[1]);
return EXIT_FAILURE;
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}