From 189e4364741e927216a254b8a51d96af2d954df8 Mon Sep 17 00:00:00 2001 From: Paul Eggert Date: Tue, 7 Jun 2011 15:16:55 -0700 Subject: [PATCH] tar: use birthtime rather than ctime when checking identity Without this fix, tar would sometimes mishandle the extraction of hard links to symbolic links. Problem reported in by Andreas Stolcke. * src/extract.c (struct delayed_link): Change ctime member to birthtime. (same_birthtime): New function. (create_placeholder_file, extract_link, apply_delayed_links): Use it to compare birthtimes (when available) instead of ctime. --- src/extract.c | 26 +++++++++++++++++++------- 1 file changed, 19 insertions(+), 7 deletions(-) diff --git a/src/extract.c b/src/extract.c index aaea56e..ecbde1a 100644 --- a/src/extract.c +++ b/src/extract.c @@ -110,12 +110,15 @@ struct delayed_link /* The next delayed link in the list. */ struct delayed_link *next; - /* The device, inode number and ctime of the placeholder. Use - ctime, not mtime, to make false matches less likely if some - other process removes the placeholder. */ + /* The device, inode number and birthtime of the placeholder. + birthtime.tv_nsec is negative if the birthtime is not available. + Don't use mtime as this would allow for false matches if some + other process removes the placeholder. Don't use ctime as + this would cause race conditions and other screwups, e.g., + when restoring hard-linked symlinks. */ dev_t dev; ino_t ino; - struct timespec ctime; + struct timespec birthtime; /* True if the link is symbolic. */ bool is_symlink; @@ -268,6 +271,15 @@ set_mode (char const *file_name, } } +/* Return true if A and B are the same birthtimes. + Unavailable birthtimes, which have negative tv_nsec members, + all compare equal to each other. */ +static bool +same_birthtime (struct timespec a, struct timespec b) +{ + return (a.tv_nsec == b.tv_nsec && (a.tv_nsec < 0 || a.tv_sec == b.tv_sec)); +} + /* Check time after successfully setting FILE_NAME's time stamp to T. */ static void check_time (char const *file_name, struct timespec t) @@ -1076,7 +1088,7 @@ create_placeholder_file (char *file_name, bool is_symlink, bool *interdir_made) delayed_link_head = p; p->dev = st.st_dev; p->ino = st.st_ino; - p->ctime = get_stat_ctime (&st); + p->birthtime = get_stat_birthtime (&st); p->is_symlink = is_symlink; if (is_symlink) { @@ -1134,7 +1146,7 @@ extract_link (char *file_name, int typeflag) if (ds->change_dir == chdir_current && ds->dev == st1.st_dev && ds->ino == st1.st_ino - && timespec_cmp (ds->ctime, get_stat_ctime (&st1)) == 0) + && same_birthtime (ds->birthtime, get_stat_birthtime (&st1))) { struct string_list *p = xmalloc (offsetof (struct string_list, string) + strlen (file_name) + 1); @@ -1500,7 +1512,7 @@ apply_delayed_links (void) if (fstatat (chdir_fd, source, &st, AT_SYMLINK_NOFOLLOW) == 0 && st.st_dev == ds->dev && st.st_ino == ds->ino - && timespec_cmp (get_stat_ctime (&st), ds->ctime) == 0) + && same_birthtime (get_stat_birthtime (&st), ds->birthtime)) { /* Unlink the placeholder, then create a hard link if possible, a symbolic link otherwise. */ -- 2.45.2