flink() at last?
The problem with this reasoning, as noted by Andy Lutomirski in a patch merged for 3.11-rc5, is that this functionality is already available by way of the linkat() system call. All it takes is having the /proc filesystem mounted — and a system without /proc is quite rare. But the incantation needed to make a link in this way is a bit arduous:
linkat(AT_FDCWD, "/proc/self/fd/N", destdirfd, newname, AT_SYMLINK_FOLLOW);
Where "N" is the number of the relevant file descriptor. It would be a lot nicer, he said, to just allow the use of the AT_EMPTY_PATH option, which causes the link to be made to the file behind the original file descriptor:
linkat(fd, "", destdirfd, newname, AT_EMPTY_PATH);
In current kernels, though, that option is restricted to processes with the
CAP_DAC_READ_SEARCH capability out of the same security concerns
as described above. But, as Andy pointed out, the restriction makes no
sense given that the desired functionality is available anyway. So his
patch removes the check, making the second variant available to all users.
This functionality is expected to be useful with files opened with the
O_TMPFILE option, but other uses can be imagined as well. It will
be generally available in the 3.11 kernel.
| Index entries for this article | |
|---|---|
| Kernel | flink() |