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    SubjectShould unprivileged linkat(..., AT_EMPTY_PATH) work on O_TMPFILE files?
    The change:

    commit f4e0c30c191f87851c4a53454abb55ee276f4a7e
    Author: Al Viro <viro@zeniv.linux.org.uk>
    Date: Tue Jun 11 08:34:36 2013 +0400

    allow the temp files created by open() to be linked to

    O_TMPFILE | O_CREAT => linkat() with AT_SYMLINK_FOLLOW and /proc/self/fd/<n>
    as oldpath (i.e. flink()) will create a link
    O_TMPFILE | O_CREAT | O_EXCL => ENOENT on attempt to link those guys

    Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

    makes it possible to hardlink an O_TMPFILE file using procfs. Should
    linkat(fd, "", newdirfd, newpath, AT_EMPTY_PATH) also work?

    AFAICS it currently requires CAP_DAC_READ_SEARCH, but I'm a bit
    confused as to why linkat(..., AT_EMPTY_PATH) should have a stricter
    check than linkat(AT_FDCWD, "/proc/self/fd/n", ...,
    AT_SYMLINK_FOLLOW), (The relevant change is
    11a7b371b64ef39fc5fb1b6f2218eef7c4d035e3.)

    FWIW, this program works on Linux 3.9, which makes me doubt that the
    security restriction on linkat is doing any good:

    #include <stdio.h>
    #include <err.h>
    #include <fcntl.h>
    #include <unistd.h>

    int main(int argc, char **argv)
    {
    char buf[128];

    if (argc != 3)
    errx(1, "Usage: flink FD PATH");

    sprintf(buf, "/proc/self/fd/%d", atoi(argv[1]));
    if (linkat(AT_FDCWD, buf, AT_FDCWD, argv[2], AT_SYMLINK_FOLLOW) != 0)
    err(1, "linkat");
    return 0;
    }


    Removing the check from the AT_EMPTY_PATH case would simplify code
    that wants to write a file, fsync it, and then flink it in.


    --Andy

    P.S. For even more fun, I'd *love* a linkat flag that would allow the
    destination to be overwritten, but that's a different can of worms.


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