fs: Allow unprivileged linkat(..., AT_EMPTY_PATH) aka flink
Every now and then someone proposes a new flink syscall, and this spawns a long discussion of whether it would be a security problem. I think that this is missing the point: flink is *already* allowed without privilege as long as /proc is mounted -- it's called AT_SYMLINK_FOLLOW. Now that O_TMPFILE is here, the ability to create a file with O_TMPFILE, write it, and link it in is very convenient. The only problem is that it requires that /proc be mounted so that you can do: linkat(AT_FDCWD, "/proc/self/fd/<tmpfd>", dfd, path, AT_SYMLINK_NOFOLLOW) This sucks -- it's much nicer to do: linkat(tmpfd, "", dfd, path, AT_EMPTY_PATH) Let's allow it. If this turns out to be excessively scary, it we could instead require that the inode in question be I_LINKABLE, but this seems pointless given the /proc situation Signed-off-by: Andy Lutomirski <luto@amacapital.net> Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
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