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| -rw-r--r-- | lib/chdir-long.c | 342 |
1 files changed, 342 insertions, 0 deletions
diff --git a/lib/chdir-long.c b/lib/chdir-long.c new file mode 100644 index 000000000..cb9b7d725 --- /dev/null +++ b/lib/chdir-long.c @@ -0,0 +1,342 @@ +/* provide a chdir function that tries not to fail due to ENAMETOOLONG + Copyright (C) 2004 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 2, 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, write to the Free Software Foundation, + Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA. */ + +/* written by Jim Meyering */ + +#include <config.h> + +#include "chdir-long.h" + +#include <stdlib.h> +#include <stdbool.h> +#include <string.h> +#include <unistd.h> +#include <errno.h> +#include <stdio.h> +#include <assert.h> +#include <limits.h> + +#include "mempcpy.h" +#include "openat.h" + +#ifndef O_DIRECTORY +# define O_DIRECTORY 0 +#endif + +#ifndef MIN +# define MIN(a, b) ((a) < (b) ? (a) : (b)) +#endif + +#ifndef PATH_MAX +# ifdef MAXPATHLEN +# define PATH_MAX MAXPATHLEN +# else +# error "use this module only if your system defines PATH_MAX" +# endif +#endif + +/* FIXME: this use of `MIN' is our sole concession to arbitrary limitations. + If, for some system, PATH_MAX is larger than 8191 and you call + chdir_long with a directory name that is longer than PATH_MAX, + yet that contains a single component that is more than 8191 bytes + long, then this function will fail. */ +#define MAX_COMPONENT_LENGTH MIN (PATH_MAX - 1, 8 * 1024) + +struct cd_buf +{ + /* FIXME maybe allocate this via malloc, rather than using the stack. + But that would be the sole use of malloc. Is it worth it to + let chdir_long fail due to a low-memory condition? + But when using malloc, and assuming we remove the `concession' + above, we'll still have to avoid allocating 2^31 bytes on + systems that define PATH_MAX to very large number. + Ideally, we'd allocate enough to deal with most names, and + dynamically increase the buffer size only necessary. */ + char buffer[MAX_COMPONENT_LENGTH + 1]; + char *avail; + int fd; +}; + +/* Like memchr, but return the number of bytes from MEM + to the first occurrence of C thereafter. Search only + LEN bytes. Return LEN if C is not found. */ +static inline size_t +memchrcspn (char const *mem, int c, size_t len) +{ + char const *found = memchr (mem, c, len); + if (!found) + return len; + + len = found - mem; + return len; +} + +static void +cdb_init (struct cd_buf *cdb) +{ + cdb->avail = cdb->buffer; + cdb->fd = AT_FDCWD; +} + +static inline bool +cdb_empty (struct cd_buf const *cdb) +{ + return cdb->avail == cdb->buffer; +} + +static inline int +cdb_fchdir (struct cd_buf const *cdb) +{ + return fchdir (cdb->fd); +} + +static int +cdb_advance_fd (struct cd_buf *cdb, char const *dir) +{ + int new_fd = openat (cdb->fd, dir, O_RDONLY | O_DIRECTORY); + if (new_fd < 0) + { + new_fd = openat (cdb->fd, dir, O_WRONLY | O_DIRECTORY); + if (new_fd < 0) + return -1; + } + + if (cdb->fd != AT_FDCWD) + close (cdb->fd); + cdb->fd = new_fd; + + return 0; +} + +static int +cdb_flush (struct cd_buf *cdb) +{ + if (cdb_empty (cdb)) + return 0; + + cdb->avail[0] = '\0'; + if (cdb_advance_fd (cdb, cdb->buffer) != 0) + return -1; + + cdb->avail = cdb->buffer; + + return 0; +} + +static void +cdb_free (struct cd_buf *cdb) +{ + if (0 <= cdb->fd && close (cdb->fd) != 0) + abort (); +} + +static int +cdb_append (struct cd_buf *cdb, char const *s, size_t len) +{ + char const *end = cdb->buffer + sizeof cdb->buffer; + + /* Insert a slash separator if there is a preceding byte + and it's not a slash. */ + bool need_slash = (cdb->buffer < cdb->avail && cdb->avail[-1] != '/'); + size_t n_free; + + if (sizeof cdb->buffer < len + 1) + { + /* This single component is too long. */ + errno = ENAMETOOLONG; + return -1; + } + + /* See if there's enough room for the `/', the new component and + a trailing NUL. */ + n_free = end - cdb->avail; + if (n_free < need_slash + len + 1) + { + if (cdb_flush (cdb) != 0) + return -1; + need_slash = false; + } + + if (need_slash) + *(cdb->avail)++ = '/'; + + cdb->avail = mempcpy (cdb->avail, s, len); + return 0; +} + +/* This is a wrapper around chdir that works even on PATH_MAX-limited + systems. It handles an arbitrarily long directory name by extracting + and processing 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 change 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 is longer than PATH_MAX on systems that define PATH_MAX). */ + +int +chdir_long (char const *dir) +{ + int e = chdir (dir); + if (e == 0 || errno != ENAMETOOLONG) + return e; + + { + size_t len = strlen (dir); + char const *dir_end = dir + len; + char const *d; + struct cd_buf cdb; + + cdb_init (&cdb); + + /* If DIR is the empty string, then the chdir above + must have failed and set errno to ENOENT. */ + assert (0 < len); + + if (*dir == '/') + { + /* Names starting with exactly two slashes followed by at least + one non-slash are special -- + for example, in some environments //Hostname/file may + denote a file on a different host. + Preserve those two leading slashes. Treat all other + sequences of slashes like a single one. */ + if (3 <= len && dir[1] == '/' && dir[2] != '/') + { + size_t name_len = 1 + strcspn (dir + 3, "/"); + if (cdb_append (&cdb, dir, 2 + name_len) != 0) + goto Fail; + /* Advance D to next slash or to end of string. */ + d = dir + 2 + name_len; + assert (*d == '/' || *d == '\0'); + } + else + { + if (cdb_append (&cdb, "/", 1) != 0) + goto Fail; + d = dir + 1; + } + } + else + { + d = dir; + } + + while (1) + { + /* Skip any slashes to find start of next component -- + or the end of DIR. */ + char const *start = d + strspn (d, "/"); + if (*start == '\0') + { + if (cdb_flush (&cdb) != 0) + goto Fail; + break; + } + /* If the remaining portion is no longer than PATH_MAX, then + flush anything that is buffered and do the rest in one chunk. */ + if (dir_end - start <= PATH_MAX) + { + if (cdb_flush (&cdb) != 0 + || cdb_advance_fd (&cdb, start) != 0) + goto Fail; + break; + } + + len = memchrcspn (start, '/', dir_end - start); + assert (len == strcspn (start, "/")); + d = start + len; + if (cdb_append (&cdb, start, len) != 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 <stdio.h> +# include "closeout.h" +# include "error.h" + +char *program_name; + +int +main (int argc, char *argv[]) +{ + char *line = NULL; + size_t n = 0; + int len; + + program_name = argv[0]; + 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); + + { + /* 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); + } + + /* not reached */ + abort (); +} +#endif + +/* +Local Variables: +compile-command: "gcc -DTEST_CHDIR=1 -DHAVE_CONFIG_H -I.. -g -O -W -Wall chdir-long.c libfetish.a" +End: +*/ |