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Diffstat (limited to 'lib/chdir-long.c')
| -rw-r--r-- | lib/chdir-long.c | 264 |
1 files changed, 264 insertions, 0 deletions
diff --git a/lib/chdir-long.c b/lib/chdir-long.c new file mode 100644 index 0000000..f4efb20 --- /dev/null +++ b/lib/chdir-long.c @@ -0,0 +1,264 @@ +/* provide a chdir function that tries not to fail due to ENAMETOOLONG + Copyright (C) 2004-2022 Free Software Foundation, Inc. + + This program is free software: you can redistribute it and/or modify + it under the terms of the GNU General Public License as published by + the Free Software Foundation, either version 3 of the License, or + (at your option) any later version. + + This program is distributed in the hope that it will be useful, + but WITHOUT ANY WARRANTY; without even the implied warranty of + MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the + GNU General Public License for more details. + + You should have received a copy of the GNU General Public License + along with this program. If not, see <https://www.gnu.org/licenses/>. */ + +/* written by Jim Meyering */ + +#include <config.h> + +#include "chdir-long.h" + +#include <errno.h> +#include <fcntl.h> +#include <stdlib.h> +#include <stdbool.h> +#include <string.h> +#include <stdio.h> + +#include "assure.h" + +#ifndef PATH_MAX +# error "compile this file only if your system defines PATH_MAX" +#endif + +/* The results of openat() in this file are not leaked to any + single-threaded code that could use stdio. + FIXME - if the kernel ever adds support for multi-thread safety for + avoiding standard fds, then we should use openat_safer. */ + +struct cd_buf +{ + int fd; +}; + +static void +cdb_init (struct cd_buf *cdb) +{ + cdb->fd = AT_FDCWD; +} + +static int +cdb_fchdir (struct cd_buf const *cdb) +{ + return fchdir (cdb->fd); +} + +static void +cdb_free (struct cd_buf const *cdb) +{ + if (0 <= cdb->fd) + { + bool close_fail = close (cdb->fd); + assure (! close_fail); + } +} + +/* Given a file descriptor of an open directory (or AT_FDCWD), CDB->fd, + try to open the CDB->fd-relative directory, DIR. If the open succeeds, + update CDB->fd with the resulting descriptor, close the incoming file + descriptor, and return zero. Upon failure, return -1 and set errno. */ +static int +cdb_advance_fd (struct cd_buf *cdb, char const *dir) +{ + int new_fd = openat (cdb->fd, dir, + O_SEARCH | O_DIRECTORY | O_NOCTTY | O_NONBLOCK); + if (new_fd < 0) + return -1; + + cdb_free (cdb); + cdb->fd = new_fd; + + return 0; +} + +/* Return a pointer to the first non-slash in S. */ +static char * _GL_ATTRIBUTE_PURE +find_non_slash (char const *s) +{ + size_t n_slash = strspn (s, "/"); + return (char *) s + n_slash; +} + +/* This is a function much like chdir, but without the PATH_MAX limitation + on the length of the directory name. A significant difference is that + it must be able to modify (albeit only temporarily) the directory + name. It handles an arbitrarily long directory name by operating + on manageable portions of the name. On systems without the openat + syscall, this means changing the working directory to more and more + "distant" points along the long directory name and then restoring + the working directory. If any of those attempts to save or restore + the working directory fails, this function exits nonzero. + + Note that this function may still fail with errno == ENAMETOOLONG, but + only if the specified directory name contains a component that is long + enough to provoke such a failure all by itself (e.g. if the component + has length PATH_MAX or greater on systems that define PATH_MAX). */ + +int +chdir_long (char *dir) +{ + int e = chdir (dir); + if (e == 0 || errno != ENAMETOOLONG) + return e; + + { + size_t len = strlen (dir); + char *dir_end = dir + len; + struct cd_buf cdb; + size_t n_leading_slash; + + cdb_init (&cdb); + + /* If DIR is the empty string, then the chdir above + must have failed and set errno to ENOENT. */ + assure (0 < len); + assure (PATH_MAX <= len); + + /* Count leading slashes. */ + n_leading_slash = strspn (dir, "/"); + + /* Handle any leading slashes as well as any name that matches + the regular expression, m!^//hostname[/]*! . Handling this + prefix separately usually results in a single additional + cdb_advance_fd call, but it's worthwhile, since it makes the + code in the following loop cleaner. */ + if (n_leading_slash == 2) + { + int err; + /* Find next slash. + We already know that dir[2] is neither a slash nor '\0'. */ + char *slash = memchr (dir + 3, '/', dir_end - (dir + 3)); + if (slash == NULL) + { + errno = ENAMETOOLONG; + return -1; + } + *slash = '\0'; + err = cdb_advance_fd (&cdb, dir); + *slash = '/'; + if (err != 0) + goto Fail; + dir = find_non_slash (slash + 1); + } + else if (n_leading_slash) + { + if (cdb_advance_fd (&cdb, "/") != 0) + goto Fail; + dir += n_leading_slash; + } + + assure (*dir != '/'); + assure (dir <= dir_end); + + while (PATH_MAX <= dir_end - dir) + { + int err; + /* Find a slash that is PATH_MAX or fewer bytes away from dir. + I.e. see if there is a slash that will give us a name of + length PATH_MAX-1 or less. */ + char *slash = memrchr (dir, '/', PATH_MAX); + if (slash == NULL) + { + errno = ENAMETOOLONG; + return -1; + } + + *slash = '\0'; + assure (slash - dir < PATH_MAX); + err = cdb_advance_fd (&cdb, dir); + *slash = '/'; + if (err != 0) + goto Fail; + + dir = find_non_slash (slash + 1); + } + + if (dir < dir_end) + { + if (cdb_advance_fd (&cdb, dir) != 0) + goto Fail; + } + + if (cdb_fchdir (&cdb) != 0) + goto Fail; + + cdb_free (&cdb); + return 0; + + Fail: + { + int saved_errno = errno; + cdb_free (&cdb); + errno = saved_errno; + return -1; + } + } +} + +#if TEST_CHDIR + +# include "closeout.h" +# include "error.h" + +int +main (int argc, char *argv[]) +{ + char *line = NULL; + size_t n = 0; + int len; + + atexit (close_stdout); + + len = getline (&line, &n, stdin); + if (len < 0) + { + int saved_errno = errno; + if (feof (stdin)) + exit (0); + + error (EXIT_FAILURE, saved_errno, + "reading standard input"); + } + else if (len == 0) + exit (0); + + if (line[len-1] == '\n') + line[len-1] = '\0'; + + if (chdir_long (line) != 0) + error (EXIT_FAILURE, errno, + "chdir_long failed: %s", line); + + if (argc <= 1) + { + /* Using 'pwd' here makes sense only if it is a robust implementation, + like the one in coreutils after the 2004-04-19 changes. */ + char const *cmd = "pwd"; + execlp (cmd, (char *) NULL); + error (EXIT_FAILURE, errno, "%s", cmd); + } + + fclose (stdin); + fclose (stderr); + + exit (EXIT_SUCCESS); +} +#endif + +/* +Local Variables: +compile-command: "gcc -DTEST_CHDIR=1 -g -O -W -Wall chdir-long.c libcoreutils.a" +End: +*/ |