| 1 | /* Read symbolic links into a buffer without size limitation, relative to fd. |
| 2 | |
| 3 | Copyright (C) 2001, 2003-2004, 2007, 2009-2012 Free Software Foundation, |
| 4 | Inc. |
| 5 | |
| 6 | This program is free software: you can redistribute it and/or modify |
| 7 | it under the terms of the GNU General Public License as published by |
| 8 | the Free Software Foundation; either version 3 of the License, or |
| 9 | (at your option) any later version. |
| 10 | |
| 11 | This program is distributed in the hope that it will be useful, |
| 12 | but WITHOUT ANY WARRANTY; without even the implied warranty of |
| 13 | MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the |
| 14 | GNU General Public License for more details. |
| 15 | |
| 16 | You should have received a copy of the GNU General Public License |
| 17 | along with this program. If not, see <http://www.gnu.org/licenses/>. */ |
| 18 | |
| 19 | /* Written by Paul Eggert, Bruno Haible, and Jim Meyering. */ |
| 20 | |
| 21 | #include <config.h> |
| 22 | |
| 23 | #include "careadlinkat.h" |
| 24 | |
| 25 | #include <errno.h> |
| 26 | #include <limits.h> |
| 27 | #include <stdlib.h> |
| 28 | #include <string.h> |
| 29 | #include <unistd.h> |
| 30 | |
| 31 | /* Define this independently so that stdint.h is not a prerequisite. */ |
| 32 | #ifndef SIZE_MAX |
| 33 | # define SIZE_MAX ((size_t) -1) |
| 34 | #endif |
| 35 | |
| 36 | #ifndef SSIZE_MAX |
| 37 | # define SSIZE_MAX ((ssize_t) (SIZE_MAX / 2)) |
| 38 | #endif |
| 39 | |
| 40 | #include "allocator.h" |
| 41 | |
| 42 | /* Get the symbolic link value of FILENAME and put it into BUFFER, with |
| 43 | size BUFFER_SIZE. This function acts like readlink but has |
| 44 | readlinkat's signature. */ |
| 45 | ssize_t |
| 46 | careadlinkatcwd (int fd, char const *filename, char *buffer, |
| 47 | size_t buffer_size) |
| 48 | { |
| 49 | /* FD must be AT_FDCWD here, otherwise the caller is using this |
| 50 | function in contexts for which it was not meant for. */ |
| 51 | if (fd != AT_FDCWD) |
| 52 | abort (); |
| 53 | return readlink (filename, buffer, buffer_size); |
| 54 | } |
| 55 | |
| 56 | /* Assuming the current directory is FD, get the symbolic link value |
| 57 | of FILENAME as a null-terminated string and put it into a buffer. |
| 58 | If FD is AT_FDCWD, FILENAME is interpreted relative to the current |
| 59 | working directory, as in openat. |
| 60 | |
| 61 | If the link is small enough to fit into BUFFER put it there. |
| 62 | BUFFER's size is BUFFER_SIZE, and BUFFER can be null |
| 63 | if BUFFER_SIZE is zero. |
| 64 | |
| 65 | If the link is not small, put it into a dynamically allocated |
| 66 | buffer managed by ALLOC. It is the caller's responsibility to free |
| 67 | the returned value if it is nonnull and is not BUFFER. A null |
| 68 | ALLOC stands for the standard allocator. |
| 69 | |
| 70 | The PREADLINKAT function specifies how to read links. It operates |
| 71 | like POSIX readlinkat() |
| 72 | <http://pubs.opengroup.org/onlinepubs/9699919799/functions/readlink.html> |
| 73 | but can assume that its first argument is the same as FD. |
| 74 | |
| 75 | If successful, return the buffer address; otherwise return NULL and |
| 76 | set errno. */ |
| 77 | |
| 78 | char * |
| 79 | careadlinkat (int fd, char const *filename, |
| 80 | char *buffer, size_t buffer_size, |
| 81 | struct allocator const *alloc, |
| 82 | ssize_t (*preadlinkat) (int, char const *, char *, size_t)) |
| 83 | { |
| 84 | char *buf; |
| 85 | size_t buf_size; |
| 86 | size_t buf_size_max = |
| 87 | SSIZE_MAX < SIZE_MAX ? (size_t) SSIZE_MAX + 1 : SIZE_MAX; |
| 88 | char stack_buf[1024]; |
| 89 | |
| 90 | if (! alloc) |
| 91 | alloc = &stdlib_allocator; |
| 92 | |
| 93 | if (! buffer_size) |
| 94 | { |
| 95 | /* Allocate the initial buffer on the stack. This way, in the |
| 96 | common case of a symlink of small size, we get away with a |
| 97 | single small malloc() instead of a big malloc() followed by a |
| 98 | shrinking realloc(). */ |
| 99 | buffer = stack_buf; |
| 100 | buffer_size = sizeof stack_buf; |
| 101 | } |
| 102 | |
| 103 | buf = buffer; |
| 104 | buf_size = buffer_size; |
| 105 | |
| 106 | do |
| 107 | { |
| 108 | /* Attempt to read the link into the current buffer. */ |
| 109 | ssize_t link_length = preadlinkat (fd, filename, buf, buf_size); |
| 110 | size_t link_size; |
| 111 | if (link_length < 0) |
| 112 | { |
| 113 | /* On AIX 5L v5.3 and HP-UX 11i v2 04/09, readlink returns -1 |
| 114 | with errno == ERANGE if the buffer is too small. */ |
| 115 | int readlinkat_errno = errno; |
| 116 | if (readlinkat_errno != ERANGE) |
| 117 | { |
| 118 | if (buf != buffer) |
| 119 | { |
| 120 | alloc->free (buf); |
| 121 | errno = readlinkat_errno; |
| 122 | } |
| 123 | return NULL; |
| 124 | } |
| 125 | } |
| 126 | |
| 127 | link_size = link_length; |
| 128 | |
| 129 | if (link_size < buf_size) |
| 130 | { |
| 131 | buf[link_size++] = '\0'; |
| 132 | |
| 133 | if (buf == stack_buf) |
| 134 | { |
| 135 | char *b = (char *) alloc->allocate (link_size); |
| 136 | buf_size = link_size; |
| 137 | if (! b) |
| 138 | break; |
| 139 | memcpy (b, buf, link_size); |
| 140 | buf = b; |
| 141 | } |
| 142 | else if (link_size < buf_size && buf != buffer && alloc->reallocate) |
| 143 | { |
| 144 | /* Shrink BUF before returning it. */ |
| 145 | char *b = (char *) alloc->reallocate (buf, link_size); |
| 146 | if (b) |
| 147 | buf = b; |
| 148 | } |
| 149 | |
| 150 | return buf; |
| 151 | } |
| 152 | |
| 153 | if (buf != buffer) |
| 154 | alloc->free (buf); |
| 155 | |
| 156 | if (buf_size <= buf_size_max / 2) |
| 157 | buf_size *= 2; |
| 158 | else if (buf_size < buf_size_max) |
| 159 | buf_size = buf_size_max; |
| 160 | else if (buf_size_max < SIZE_MAX) |
| 161 | { |
| 162 | errno = ENAMETOOLONG; |
| 163 | return NULL; |
| 164 | } |
| 165 | else |
| 166 | break; |
| 167 | buf = (char *) alloc->allocate (buf_size); |
| 168 | } |
| 169 | while (buf); |
| 170 | |
| 171 | if (alloc->die) |
| 172 | alloc->die (buf_size); |
| 173 | errno = ENOMEM; |
| 174 | return NULL; |
| 175 | } |