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