1 | /* vi: set sw=4 ts=4: */
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2 | /*
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3 | * xreadlink.c - safe implementation of readlink.
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4 | * Returns a NULL on failure.
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5 | *
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6 | * Licensed under GPLv2, see file LICENSE in this source tree.
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7 | */
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8 |
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9 | #include "libbb.h"
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10 |
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11 | /* Some systems (eg Hurd) do not have MAXSYMLINKS definition,
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12 | * set it to some reasonable value if it isn't defined */
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13 | #ifndef MAXSYMLINKS
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14 | # define MAXSYMLINKS 20
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15 | #endif
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16 |
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17 | /*
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18 | * NOTE: This function returns a malloced char* that you will have to free
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19 | * yourself.
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20 | */
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21 | char* FAST_FUNC xmalloc_readlink(const char *path)
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22 | {
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23 | enum { GROWBY = 80 }; /* how large we will grow strings by */
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24 |
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25 | char *buf = NULL;
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26 | int bufsize = 0, readsize = 0;
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27 |
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28 | do {
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29 | bufsize += GROWBY;
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30 | buf = xrealloc(buf, bufsize);
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31 | readsize = readlink(path, buf, bufsize);
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32 | if (readsize == -1) {
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33 | free(buf);
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34 | return NULL;
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35 | }
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36 | } while (bufsize < readsize + 1);
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37 |
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38 | buf[readsize] = '\0';
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39 |
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40 | return buf;
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41 | }
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42 |
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43 | /*
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44 | * This routine is not the same as realpath(), which
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45 | * canonicalizes the given path completely. This routine only
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46 | * follows trailing symlinks until a real file is reached and
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47 | * returns its name. If the path ends in a dangling link or if
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48 | * the target doesn't exist, the path is returned in any case.
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49 | * Intermediate symlinks in the path are not expanded -- only
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50 | * those at the tail.
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51 | * A malloced char* is returned, which must be freed by the caller.
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52 | */
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53 | char* FAST_FUNC xmalloc_follow_symlinks(const char *path)
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54 | {
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55 | char *buf;
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56 | char *lpc;
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57 | char *linkpath;
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58 | int bufsize;
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59 | int looping = MAXSYMLINKS + 1;
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60 |
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61 | buf = xstrdup(path);
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62 | goto jump_in;
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63 |
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64 | while (1) {
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65 | linkpath = xmalloc_readlink(buf);
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66 | if (!linkpath) {
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67 | /* not a symlink, or doesn't exist */
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68 | if (errno == EINVAL || errno == ENOENT)
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69 | return buf;
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70 | goto free_buf_ret_null;
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71 | }
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72 |
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73 | if (!--looping) {
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74 | free(linkpath);
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75 | free_buf_ret_null:
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76 | free(buf);
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77 | return NULL;
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78 | }
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79 |
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80 | if (*linkpath != '/') {
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81 | bufsize += strlen(linkpath);
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82 | buf = xrealloc(buf, bufsize);
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83 | lpc = bb_get_last_path_component_strip(buf);
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84 | strcpy(lpc, linkpath);
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85 | free(linkpath);
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86 | } else {
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87 | free(buf);
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88 | buf = linkpath;
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89 | jump_in:
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90 | bufsize = strlen(buf) + 1;
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91 | }
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92 | }
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93 | }
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94 |
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95 | char* FAST_FUNC xmalloc_readlink_or_warn(const char *path)
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96 | {
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97 | char *buf = xmalloc_readlink(path);
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98 | if (!buf) {
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99 | /* EINVAL => "file: Invalid argument" => puzzled user */
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100 | const char *errmsg = "not a symlink";
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101 | int err = errno;
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102 | if (err != EINVAL)
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103 | errmsg = strerror(err);
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104 | bb_error_msg("%s: cannot read link: %s", path, errmsg);
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105 | }
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106 | return buf;
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107 | }
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108 |
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109 | char* FAST_FUNC xmalloc_realpath(const char *path)
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110 | {
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111 | /* NB: uclibc also defines __GLIBC__
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112 | * Therefore the test "if glibc, or uclibc >= 0.9.31" looks a bit weird:
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113 | */
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114 | #if defined(__GLIBC__) && \
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115 | (!defined(__UCLIBC__) || UCLIBC_VERSION >= KERNEL_VERSION(0, 9, 31))
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116 | /* glibc provides a non-standard extension */
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117 | /* new: POSIX.1-2008 specifies this behavior as well */
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118 | return realpath(path, NULL);
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119 | #else
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120 | char buf[PATH_MAX+1];
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121 |
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122 | /* on error returns NULL (xstrdup(NULL) == NULL) */
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123 | return xstrdup(realpath(path, buf));
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124 | #endif
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125 | }
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