symlinks.c

   1 #include "cache.h"
   2
   3 /*
   4  * Returns the length (on a path component basis) of the longest
   5  * common prefix match of 'name_a' and 'name_b'.
   6  */
   7 static int longest_path_match(const char *name_a, int len_a,
   8                               const char *name_b, int len_b,
   9                               int *previous_slash)
  10 {
  11         int max_len, match_len = 0, match_len_prev = 0, i = 0;
  12
  13         max_len = len_a < len_b ? len_a : len_b;
  14         while (i < max_len && name_a[i] == name_b[i]) {
  15                 if (name_a[i] == '/') {
  16                         match_len_prev = match_len;
  17                         match_len = i;
  18                 }
  19                 i++;
  20         }
  21         /*
  22          * Is 'name_b' a substring of 'name_a', the other way around,
  23          * or is 'name_a' and 'name_b' the exact same string?
  24          */
  25         if (i >= max_len && ((len_a > len_b && name_a[len_b] == '/') ||
  26                              (len_a < len_b && name_b[len_a] == '/') ||
  27                              (len_a == len_b))) {
  28                 match_len_prev = match_len;
  29                 match_len = i;
  30         }
  31         *previous_slash = match_len_prev;
  32         return match_len;
  33 }
  34
  35 static struct cache_def {
  36         char path[PATH_MAX + 1];
  37         int len;
  38         int flags;
  39         int track_flags;
  40         int prefix_len_stat_func;
  41 } cache;
  42
  43 static inline void reset_lstat_cache(void)
  44 {
  45         cache.path[0] = '\0';
  46         cache.len = 0;
  47         cache.flags = 0;
  48         /*
  49          * The track_flags and prefix_len_stat_func members is only
  50          * set by the safeguard rule inside lstat_cache()
  51          */
  52 }
  53
  54 #define FL_DIR      (1 << 0)
  55 #define FL_NOENT    (1 << 1)
  56 #define FL_SYMLINK  (1 << 2)
  57 #define FL_LSTATERR (1 << 3)
  58 #define FL_ERR      (1 << 4)
  59 #define FL_FULLPATH (1 << 5)
  60
  61 /*
  62  * Check if name 'name' of length 'len' has a symlink leading
  63  * component, or if the directory exists and is real, or not.
  64  *
  65  * To speed up the check, some information is allowed to be cached.
  66  * This can be indicated by the 'track_flags' argument, which also can
  67  * be used to indicate that we should check the full path.
  68  *
  69  * The 'prefix_len_stat_func' parameter can be used to set the length
  70  * of the prefix, where the cache should use the stat() function
  71  * instead of the lstat() function to test each path component.
  72  */
  73 static int lstat_cache(int len, const char *name,
  74                        int track_flags, int prefix_len_stat_func)
  75 {
  76         int match_len, last_slash, last_slash_dir, previous_slash;
  77         int match_flags, ret_flags, save_flags, max_len, ret;
  78         struct stat st;
  79
  80         if (cache.track_flags != track_flags ||
  81             cache.prefix_len_stat_func != prefix_len_stat_func) {
  82                 /*
  83                  * As a safeguard rule we clear the cache if the
  84                  * values of track_flags and/or prefix_len_stat_func
  85                  * does not match with the last supplied values.
  86                  */
  87                 reset_lstat_cache();
  88                 cache.track_flags = track_flags;
  89                 cache.prefix_len_stat_func = prefix_len_stat_func;
  90                 match_len = last_slash = 0;
  91         } else {
  92                 /*
  93                  * Check to see if we have a match from the cache for
  94                  * the 2 "excluding" path types.
  95                  */
  96                 match_len = last_slash =
  97                         longest_path_match(name, len, cache.path, cache.len,
  98                                            &previous_slash);
  99                 match_flags = cache.flags & track_flags & (FL_NOENT|FL_SYMLINK);
 100                 if (match_flags && match_len == cache.len)
 101                         return match_flags;
 102                 /*
 103                  * If we now have match_len > 0, we would know that
 104                  * the matched part will always be a directory.
 105                  *
 106                  * Also, if we are tracking directories and 'name' is
 107                  * a substring of the cache on a path component basis,
 108                  * we can return immediately.
 109                  */
 110                 match_flags = track_flags & FL_DIR;
 111                 if (match_flags && len == match_len)
 112                         return match_flags;
 113         }
 114
 115         /*
 116          * Okay, no match from the cache so far, so now we have to
 117          * check the rest of the path components.
 118          */
 119         ret_flags = FL_DIR;
 120         last_slash_dir = last_slash;
 121         max_len = len < PATH_MAX ? len : PATH_MAX;
 122         while (match_len < max_len) {
 123                 do {
 124                         cache.path[match_len] = name[match_len];
 125                         match_len++;
 126                 } while (match_len < max_len && name[match_len] != '/');
 127                 if (match_len >= max_len && !(track_flags & FL_FULLPATH))
 128                         break;
 129                 last_slash = match_len;
 130                 cache.path[last_slash] = '\0';
 131
 132                 if (last_slash <= prefix_len_stat_func)
 133                         ret = stat(cache.path, &st);
 134                 else
 135                         ret = lstat(cache.path, &st);
 136
 137                 if (ret) {
 138                         ret_flags = FL_LSTATERR;
 139                         if (errno == ENOENT)
 140                                 ret_flags |= FL_NOENT;
 141                 } else if (S_ISDIR(st.st_mode)) {
 142                         last_slash_dir = last_slash;
 143                         continue;
 144                 } else if (S_ISLNK(st.st_mode)) {
 145                         ret_flags = FL_SYMLINK;
 146                 } else {
 147                         ret_flags = FL_ERR;
 148                 }
 149                 break;
 150         }
 151
 152         /*
 153          * At the end update the cache.  Note that max 3 different
 154          * path types, FL_NOENT, FL_SYMLINK and FL_DIR, can be cached
 155          * for the moment!
 156          */
 157         save_flags = ret_flags & track_flags & (FL_NOENT|FL_SYMLINK);
 158         if (save_flags && last_slash > 0 && last_slash <= PATH_MAX) {
 159                 cache.path[last_slash] = '\0';
 160                 cache.len = last_slash;
 161                 cache.flags = save_flags;
 162         } else if ((track_flags & FL_DIR) &&
 163                    last_slash_dir > 0 && last_slash_dir <= PATH_MAX) {
 164                 /*
 165                  * We have a separate test for the directory case,
 166                  * since it could be that we have found a symlink or a
 167                  * non-existing directory and the track_flags says
 168                  * that we cannot cache this fact, so the cache would
 169                  * then have been left empty in this case.
 170                  *
 171                  * But if we are allowed to track real directories, we
 172                  * can still cache the path components before the last
 173                  * one (the found symlink or non-existing component).
 174                  */
 175                 cache.path[last_slash_dir] = '\0';
 176                 cache.len = last_slash_dir;
 177                 cache.flags = FL_DIR;
 178         } else {
 179                 reset_lstat_cache();
 180         }
 181         return ret_flags;
 182 }
 183
 184 /*
 185  * Invalidate the given 'name' from the cache, if 'name' matches
 186  * completely with the cache.
 187  */
 188 void invalidate_lstat_cache(int len, const char *name)
 189 {
 190         int match_len, previous_slash;
 191
 192         match_len = longest_path_match(name, len, cache.path, cache.len,
 193                                        &previous_slash);
 194         if (len == match_len) {
 195                 if ((cache.track_flags & FL_DIR) && previous_slash > 0) {
 196                         cache.path[previous_slash] = '\0';
 197                         cache.len = previous_slash;
 198                         cache.flags = FL_DIR;
 199                 } else
 200                         reset_lstat_cache();
 201         }
 202 }
 203
 204 /*
 205  * Completely clear the contents of the cache
 206  */
 207 void clear_lstat_cache(void)
 208 {
 209         reset_lstat_cache();
 210 }
 211
 212 #define USE_ONLY_LSTAT  0
 213
 214 /*
 215  * Return non-zero if path 'name' has a leading symlink component
 216  */
 217 int has_symlink_leading_path(int len, const char *name)
 218 {
 219         return lstat_cache(len, name,
 220                            FL_SYMLINK|FL_DIR, USE_ONLY_LSTAT) &
 221                 FL_SYMLINK;
 222 }
 223
 224 /*
 225  * Return non-zero if path 'name' has a leading symlink component or
 226  * if some leading path component does not exists.
 227  */
 228 int has_symlink_or_noent_leading_path(int len, const char *name)
 229 {
 230         return lstat_cache(len, name,
 231                            FL_SYMLINK|FL_NOENT|FL_DIR, USE_ONLY_LSTAT) &
 232                 (FL_SYMLINK|FL_NOENT);
 233 }
 234
 235 /*
 236  * Return non-zero if all path components of 'name' exists as a
 237  * directory.  If prefix_len > 0, we will test with the stat()
 238  * function instead of the lstat() function for a prefix length of
 239  * 'prefix_len', thus we then allow for symlinks in the prefix part as
 240  * long as those points to real existing directories.
 241  */
 242 int has_dirs_only_path(int len, const char *name, int prefix_len)
 243 {
 244         return lstat_cache(len, name,
 245                            FL_DIR|FL_FULLPATH, prefix_len) &
 246                 FL_DIR;
 247 }