symlinks.c

   1 #include "cache.h"
   2
   3 static struct cache_def {
   4         char path[PATH_MAX + 1];
   5         int len;
   6         int flags;
   7         int track_flags;
   8         int prefix_len_stat_func;
   9 } cache;
  10
  11 /*
  12  * Returns the length (on a path component basis) of the longest
  13  * common prefix match of 'name' and the cached path string.
  14  */
  15 static inline int longest_match_lstat_cache(int len, const char *name,
  16                                             int *previous_slash)
  17 {
  18         int max_len, match_len = 0, match_len_prev = 0, i = 0;
  19
  20         max_len = len < cache.len ? len : cache.len;
  21         while (i < max_len && name[i] == cache.path[i]) {
  22                 if (name[i] == '/') {
  23                         match_len_prev = match_len;
  24                         match_len = i;
  25                 }
  26                 i++;
  27         }
  28         /*
  29          * Is the cached path string a substring of 'name', is 'name'
  30          * a substring of the cached path string, or is 'name' and the
  31          * cached path string the exact same string?
  32          */
  33         if (i >= max_len && ((len > cache.len && name[cache.len] == '/') ||
  34                              (len < cache.len && cache.path[len] == '/') ||
  35                              (len == cache.len))) {
  36                 match_len_prev = match_len;
  37                 match_len = i;
  38         }
  39         *previous_slash = match_len_prev;
  40         return match_len;
  41 }
  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_match_lstat_cache(len, name, &previous_slash);
  98                 match_flags = cache.flags & track_flags & (FL_NOENT|FL_SYMLINK);
  99                 if (match_flags && match_len == cache.len)
 100                         return match_flags;
 101                 /*
 102                  * If we now have match_len > 0, we would know that
 103                  * the matched part will always be a directory.
 104                  *
 105                  * Also, if we are tracking directories and 'name' is
 106                  * a substring of the cache on a path component basis,
 107                  * we can return immediately.
 108                  */
 109                 match_flags = track_flags & FL_DIR;
 110                 if (match_flags && len == match_len)
 111                         return match_flags;
 112         }
 113
 114         /*
 115          * Okay, no match from the cache so far, so now we have to
 116          * check the rest of the path components.
 117          */
 118         ret_flags = FL_DIR;
 119         last_slash_dir = last_slash;
 120         max_len = len < PATH_MAX ? len : PATH_MAX;
 121         while (match_len < max_len) {
 122                 do {
 123                         cache.path[match_len] = name[match_len];
 124                         match_len++;
 125                 } while (match_len < max_len && name[match_len] != '/');
 126                 if (match_len >= max_len && !(track_flags & FL_FULLPATH))
 127                         break;
 128                 last_slash = match_len;
 129                 cache.path[last_slash] = '\0';
 130
 131                 if (last_slash <= prefix_len_stat_func)
 132                         ret = stat(cache.path, &st);
 133                 else
 134                         ret = lstat(cache.path, &st);
 135
 136                 if (ret) {
 137                         ret_flags = FL_LSTATERR;
 138                         if (errno == ENOENT)
 139                                 ret_flags |= FL_NOENT;
 140                 } else if (S_ISDIR(st.st_mode)) {
 141                         last_slash_dir = last_slash;
 142                         continue;
 143                 } else if (S_ISLNK(st.st_mode)) {
 144                         ret_flags = FL_SYMLINK;
 145                 } else {
 146                         ret_flags = FL_ERR;
 147                 }
 148                 break;
 149         }
 150
 151         /*
 152          * At the end update the cache.  Note that max 3 different
 153          * path types, FL_NOENT, FL_SYMLINK and FL_DIR, can be cached
 154          * for the moment!
 155          */
 156         save_flags = ret_flags & track_flags & (FL_NOENT|FL_SYMLINK);
 157         if (save_flags && last_slash > 0 && last_slash <= PATH_MAX) {
 158                 cache.path[last_slash] = '\0';
 159                 cache.len = last_slash;
 160                 cache.flags = save_flags;
 161         } else if ((track_flags & FL_DIR) &&
 162                    last_slash_dir > 0 && last_slash_dir <= PATH_MAX) {
 163                 /*
 164                  * We have a separate test for the directory case,
 165                  * since it could be that we have found a symlink or a
 166                  * non-existing directory and the track_flags says
 167                  * that we cannot cache this fact, so the cache would
 168                  * then have been left empty in this case.
 169                  *
 170                  * But if we are allowed to track real directories, we
 171                  * can still cache the path components before the last
 172                  * one (the found symlink or non-existing component).
 173                  */
 174                 cache.path[last_slash_dir] = '\0';
 175                 cache.len = last_slash_dir;
 176                 cache.flags = FL_DIR;
 177         } else {
 178                 reset_lstat_cache();
 179         }
 180         return ret_flags;
 181 }
 182
 183 /*
 184  * Invalidate the given 'name' from the cache, if 'name' matches
 185  * completely with the cache.
 186  */
 187 void invalidate_lstat_cache(int len, const char *name)
 188 {
 189         int match_len, previous_slash;
 190
 191         match_len = longest_match_lstat_cache(len, name, &previous_slash);
 192         if (len == match_len) {
 193                 if ((cache.track_flags & FL_DIR) && previous_slash > 0) {
 194                         cache.path[previous_slash] = '\0';
 195                         cache.len = previous_slash;
 196                         cache.flags = FL_DIR;
 197                 } else
 198                         reset_lstat_cache();
 199         }
 200 }
 201
 202 /*
 203  * Completely clear the contents of the cache
 204  */
 205 void clear_lstat_cache(void)
 206 {
 207         reset_lstat_cache();
 208 }
 209
 210 #define USE_ONLY_LSTAT  0
 211
 212 /*
 213  * Return non-zero if path 'name' has a leading symlink component
 214  */
 215 int has_symlink_leading_path(int len, const char *name)
 216 {
 217         return lstat_cache(len, name,
 218                            FL_SYMLINK|FL_DIR, USE_ONLY_LSTAT) &
 219                 FL_SYMLINK;
 220 }
 221
 222 /*
 223  * Return non-zero if path 'name' has a leading symlink component or
 224  * if some leading path component does not exists.
 225  */
 226 int has_symlink_or_noent_leading_path(int len, const char *name)
 227 {
 228         return lstat_cache(len, name,
 229                            FL_SYMLINK|FL_NOENT|FL_DIR, USE_ONLY_LSTAT) &
 230                 (FL_SYMLINK|FL_NOENT);
 231 }
 232
 233 /*
 234  * Return non-zero if all path components of 'name' exists as a
 235  * directory.  If prefix_len > 0, we will test with the stat()
 236  * function instead of the lstat() function for a prefix length of
 237  * 'prefix_len', thus we then allow for symlinks in the prefix part as
 238  * long as those points to real existing directories.
 239  */
 240 int has_dirs_only_path(int len, const char *name, int prefix_len)
 241 {
 242         return lstat_cache(len, name,
 243                            FL_DIR|FL_FULLPATH, prefix_len) &
 244                 FL_DIR;
 245 }