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/* chsh.c - Change login shell.
*
* Copyright 2021 Michael Christensen
*
* See http://refspecs.linuxfoundation.org/LSB_4.1.0/LSB-Core-generic/LSB-Core-generic/chsh.html
USE_CHSH(NEWTOY(chsh, "s:", TOYFLAG_USR|TOYFLAG_BIN|TOYFLAG_STAYROOT))
config CHSH
bool "chsh"
default y
help
usage: chsh [-s login_shell] [user]
The "change shell" command changes the user login shell.
-s login_shell Use specified shell rather than interactive prompt
The shell must be an absolute path to an executable file. An unpriviliged
user can only change his/her own shell to one listed in /etc/shells, and only if
he/she is not restricted (implementation-defined).
*/
#define FOR_chsh
#include "toys.h"
GLOBALS(
char *s;
)
void chsh_main()
{
int i;
FILE *file;
size_t size, buf_size;
char *user, *line, *shell, *password, *encrypted;
struct passwd *passwd_info;
struct spwd *shadow_info;
// Use max login name size for buffer size
if (-1 == (buf_size = sysconf(_SC_LOGIN_NAME_MAX))) buf_size = 256;
if (!(user = malloc(buf_size * sizeof(user)))) perror_exit("Failed to allocate memory");
if (!(shell = malloc(buf_size * sizeof(shell)))) perror_exit("Failed to allocate memory");
if (!(line = malloc(buf_size * sizeof(line)))) perror_exit("Failed to allocate memory");
if (MAP_FAILED == (password = mmap(NULL, buf_size, PROT_WRITE | PROT_READ, MAP_PRIVATE | MAP_ANONYMOUS | MAP_LOCKED | MAP_NORESERVE, -1, 0))) perror_exit("Failed to get memory map");
// Get uid user information, may be discarded later
if (!(passwd_info = getpwuid(getuid()))) perror_exit("Failed to get passwd record");
if ((user = *toys.optargs)) {
errno = 0;
if (!(passwd_info = getpwnam(user)) && !errno) error_exit("Failed to get user info");
// Are we either root or changing our own shell?
if (getuid() && strcmp(passwd_info->pw_name, user)) error_exit("Permission denied\n");
} else user = passwd_info->pw_name;
// Get a password, encrypt it, wipe it, and check it
if (!(shadow_info = getspnam(passwd_info->pw_name))) perror_exit("Failed to get shadow record");
if (!read_password(password, buf_size, "Password: ")) error_exit("Failed to read password\n");
if (!(encrypted = crypt(password, shadow_info->sp_pwdp))) perror_exit("Failed to encrypt password");
memset(password, 0, buf_size);
if (!munmap(password, buf_size)) perror_exit("Failed to unmap memory");
if (strcmp(encrypted, shadow_info->sp_pwdp)) perror_exit("Incorrect password");
// Get new shell (either -s or interactive)
if (!(file = fopen("/etc/shells", "r"))) perror_exit("Failed to open /etc/shells");
if (toys.optflags) shell = TT.s;
else {
xprintf("Changing the login shell for %s\nEnter the new value, or press ENTER for default\n Login shell [%s]: ", user, passwd_info->pw_shell);
errno = 0; size = 0;
while (EOF != (i = fgetc(stdin))) {
if (errno) perror_exit("Failed to read character from stdin");
if ('\n' != i) *(shell + size++) = i;
else {
*(shell + size) = '\0'; break;
}
}
}
// Is shell in /etc/shells?
if (strlen(shell)) {
line = NULL; size = 0; i = 0; errno = 0;
while (EOF != getline(&line, &size, file)) {
if (errno) perror_exit("Failed to read from /etc/shells");
size = strlen(line) - 1;
if ('\n' == *(line + size)) *(line + size) = '\0';
if (!strcmp(shell, line)) {
i = 1; break;
}
}
if (!i) error_exit("Shell not found in '/etc/shells'");
} else {
// Get default shell, ignoring comments and blank lines
do {
shell = NULL;
if (-1 == getline(&shell, &size, file)) perror_exit("Failed to read from /etc/shells");
} while (*shell != '/');
size = strlen(shell) - 1;
if ('\n' == *(shell + size)) *(shell + size) = '\0';
}
// Update shell and write passwd entry to tempfile
strncpy(passwd_info->pw_shell, shell, buf_size);
if (!(file = tmpfile())) perror_exit("Failed to create tempfile");
if (!putpwent(passwd_info, file)) perror_exit("Failed to write to passwd entry");
// Move passwd entry from file to string
if (-1 == (i = ftell(file))) perror_exit("Failed to get tempfile offset");
if (buf_size < i && !realloc(line, i)) perror_exit("Failed to reallocate memory");
rewind(file);
if (fread(line, 1, i, file) < i) perror_exit("Failed to read from tempfile");
// Update /etc/passwd using that string
if (-1 == update_password("/etc/passwd", user, NULL)) perror_exit("Failed to remove passwd entry");
if (-1 == update_password("/etc/passwd", user, line)) perror_exit("Failed to add passwd entry");
}
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