symlinks.c

   1 #include "cache.h"
   2
   3 static struct cache_def {
   4         char path[PATH_MAX];
   5         int len;
   6         int flags;
   7         int track_flags;
   8 } cache;
   9
  10 /*
  11  * Returns the length (on a path component basis) of the longest
  12  * common prefix match of 'name' and the cached path string.
  13  */
  14 static inline int longest_match_lstat_cache(int len, const char *name)
  15 {
  16         int max_len, match_len = 0, i = 0;
  17
  18         max_len = len < cache.len ? len : cache.len;
  19         while (i < max_len && name[i] == cache.path[i]) {
  20                 if (name[i] == '/')
  21                         match_len = i;
  22                 i++;
  23         }
  24         /* Is the cached path string a substring of 'name'? */
  25         if (i == cache.len && cache.len < len && name[cache.len] == '/')
  26                 match_len = cache.len;
  27         /* Is 'name' a substring of the cached path string? */
  28         else if ((i == len && len < cache.len && cache.path[len] == '/') ||
  29                  (i == len && len == cache.len))
  30                 match_len = len;
  31         return match_len;
  32 }
  33
  34 static inline void reset_lstat_cache(int track_flags)
  35 {
  36         cache.path[0] = '\0';
  37         cache.len = 0;
  38         cache.flags = 0;
  39         cache.track_flags = track_flags;
  40 }
  41
  42 #define FL_DIR      (1 << 0)
  43 #define FL_NOENT    (1 << 1)
  44 #define FL_SYMLINK  (1 << 2)
  45 #define FL_LSTATERR (1 << 3)
  46 #define FL_ERR      (1 << 4)
  47
  48 /*
  49  * Check if name 'name' of length 'len' has a symlink leading
  50  * component, or if the directory exists and is real, or not.
  51  *
  52  * To speed up the check, some information is allowed to be cached.
  53  * This can be indicated by the 'track_flags' argument.
  54  */
  55 static int lstat_cache(int len, const char *name,
  56                        int track_flags)
  57 {
  58         int match_len, last_slash, last_slash_dir;
  59         int match_flags, ret_flags, save_flags, max_len;
  60         struct stat st;
  61
  62         if (cache.track_flags != track_flags) {
  63                 /*
  64                  * As a safeguard we clear the cache if the value of
  65                  * track_flags does not match with the last supplied
  66                  * value.
  67                  */
  68                 reset_lstat_cache(track_flags);
  69                 match_len = last_slash = 0;
  70         } else {
  71                 /*
  72                  * Check to see if we have a match from the cache for
  73                  * the 2 "excluding" path types.
  74                  */
  75                 match_len = last_slash = longest_match_lstat_cache(len, name);
  76                 match_flags = cache.flags & track_flags & (FL_NOENT|FL_SYMLINK);
  77                 if (match_flags && match_len == cache.len)
  78                         return match_flags;
  79                 /*
  80                  * If we now have match_len > 0, we would know that
  81                  * the matched part will always be a directory.
  82                  *
  83                  * Also, if we are tracking directories and 'name' is
  84                  * a substring of the cache on a path component basis,
  85                  * we can return immediately.
  86                  */
  87                 match_flags = track_flags & FL_DIR;
  88                 if (match_flags && len == match_len)
  89                         return match_flags;
  90         }
  91
  92         /*
  93          * Okay, no match from the cache so far, so now we have to
  94          * check the rest of the path components.
  95          */
  96         ret_flags = FL_DIR;
  97         last_slash_dir = last_slash;
  98         max_len = len < PATH_MAX ? len : PATH_MAX;
  99         while (match_len < max_len) {
 100                 do {
 101                         cache.path[match_len] = name[match_len];
 102                         match_len++;
 103                 } while (match_len < max_len && name[match_len] != '/');
 104                 if (match_len >= max_len)
 105                         break;
 106                 last_slash = match_len;
 107                 cache.path[last_slash] = '\0';
 108
 109                 if (lstat(cache.path, &st)) {
 110                         ret_flags = FL_LSTATERR;
 111                         if (errno == ENOENT)
 112                                 ret_flags |= FL_NOENT;
 113                 } else if (S_ISDIR(st.st_mode)) {
 114                         last_slash_dir = last_slash;
 115                         continue;
 116                 } else if (S_ISLNK(st.st_mode)) {
 117                         ret_flags = FL_SYMLINK;
 118                 } else {
 119                         ret_flags = FL_ERR;
 120                 }
 121                 break;
 122         }
 123
 124         /*
 125          * At the end update the cache.  Note that max 3 different
 126          * path types, FL_NOENT, FL_SYMLINK and FL_DIR, can be cached
 127          * for the moment!
 128          */
 129         save_flags = ret_flags & track_flags & (FL_NOENT|FL_SYMLINK);
 130         if (save_flags && last_slash > 0 && last_slash < PATH_MAX) {
 131                 cache.path[last_slash] = '\0';
 132                 cache.len = last_slash;
 133                 cache.flags = save_flags;
 134         } else if (track_flags & FL_DIR &&
 135                    last_slash_dir > 0 && last_slash_dir < PATH_MAX) {
 136                 /*
 137                  * We have a separate test for the directory case,
 138                  * since it could be that we have found a symlink or a
 139                  * non-existing directory and the track_flags says
 140                  * that we cannot cache this fact, so the cache would
 141                  * then have been left empty in this case.
 142                  *
 143                  * But if we are allowed to track real directories, we
 144                  * can still cache the path components before the last
 145                  * one (the found symlink or non-existing component).
 146                  */
 147                 cache.path[last_slash_dir] = '\0';
 148                 cache.len = last_slash_dir;
 149                 cache.flags = FL_DIR;
 150         } else {
 151                 reset_lstat_cache(track_flags);
 152         }
 153         return ret_flags;
 154 }
 155
 156 /*
 157  * Return non-zero if path 'name' has a leading symlink component
 158  */
 159 int has_symlink_leading_path(int len, const char *name)
 160 {
 161         return lstat_cache(len, name,
 162                            FL_SYMLINK|FL_DIR) &
 163                 FL_SYMLINK;
 164 }
 165
 166 /*
 167  * Return non-zero if path 'name' has a leading symlink component or
 168  * if some leading path component does not exists.
 169  */
 170 int has_symlink_or_noent_leading_path(int len, const char *name)
 171 {
 172         return lstat_cache(len, name,
 173                            FL_SYMLINK|FL_NOENT|FL_DIR) &
 174                 (FL_SYMLINK|FL_NOENT);
 175 }