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chase: fix corner case when using CHASE_PARENT with a path ending in ".."

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If we use CHASE_PARENT on a path ending in ".." then things are a bit
weird, because we the last path we look at is actually the *parent* and not
the *child* of the preceeding path. Hence we cannot just return the 2nd
to last fd we look at. We have to correct it, by going *two* levels up,
to get to the actual parent, and make sure CHASE_PARENT does what it
should.

Example: for the path /a/b/c chase() with CHASE_PARENT will return
/a/b/c as path, and the fd returned points to /a/b. All good.  But now,
for the path /a/b/c/.. chase() with CHASE_PARENT would previously return
/a/b as path (which is OK) but the fd would point to /a/b/c, which is
*not* the parent of /a/b, after all! To get to the actual parent of
/a/b we have to go *two* levels up to get to /a.

Very confusing. But that's what we here for, no?

@mrc0mmand ran into this in https://github.com/systemd/systemd/pull/28891#issuecomment-1782833722
This commit is contained in:
Lennart Poettering
2023-11-01 14:43:24 +00:00
committed by Luca Boccassi
parent 7c2fd96dba
commit 9c21cfdd7d
2 changed files with 48 additions and 1 deletions
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src/basic/chase.c
+21 -1
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@@ -335,8 +335,28 @@ int chaseat(int dir_fd, const char *path, ChaseFlags flags, char **ret_path, int
unsafe_transition(&st, &st_parent))
return log_unsafe_transition(fd, fd_parent, path, flags);
if (FLAGS_SET(flags, CHASE_PARENT) && isempty(todo))
/* If the path ends on a "..", and CHASE_PARENT is specified then our current 'fd' is
* the child of the returned normalized path, not the parent as requested. To correct
* this we have to go *two* levels up. */
if (FLAGS_SET(flags, CHASE_PARENT) && isempty(todo)) {
_cleanup_close_ int fd_grandparent = -EBADF;
struct stat st_grandparent;
fd_grandparent = openat(fd_parent, "..", O_CLOEXEC|O_NOFOLLOW|O_PATH|O_DIRECTORY);
if (fd_grandparent < 0)
return -errno;
if (fstat(fd_grandparent, &st_grandparent) < 0)
return -errno;
if (FLAGS_SET(flags, CHASE_SAFE) &&
unsafe_transition(&st_parent, &st_grandparent))
return log_unsafe_transition(fd_parent, fd_grandparent, path, flags);
st = st_grandparent;
close_and_replace(fd, fd_grandparent);
break;
}
/* update fd and stat */
st = st_parent;