Re: [PATCH] fcntl-h: default O_SEARCH, O_EXEC to O_PATH if available

[Jim Meyering]

Paul Eggert wrote:

> On 11/05/2012 02:02 PM, Eric Blake wrote:
>
>> Do we really want the ability to open a symlink as an fd
>> when using O_EXEC permissions, instead of following the
>> symlink into the file that it normally points to?
>
> Opening the symlink with open ("symlink", O_EXEC) should be
> OK, since it will follow the symlink.
>
> So I assume you're talking about opening a symlink with
> open ("symlink", O_EXEC|O_NOFOLLOW).  This now succeeds,
> where it formerly failed as POSIX requires.  So in this
> sense, defining O_EXEC to O_PATH has a problem -- it's not a
> perfect emulation of O_EXEC.  But the same is true for
> defining O_EXEC to O_RDONLY.  So the question is whether
> falling back to O_PATH is better, overall, than falling back
> to O_RDONLY is.  My impression is that O_PATH is a better
> fallback for most GNU applications, but it's quite possible
> that I'm missing something.
>
> Another possibility would be to redefine 'open' and/or
> 'openat' so that they reject any attempt to open a symlink
> as an fd, when the user specified O_EXEC and/or O_SEARCH.
> I.e., Gnulib defines O_EXEC to some value other than O_PATH,
> and implements O_EXEC as O_PATH-with-some-extra-checks.
> That should do an even better job of emulation, but it'd
> take more work.

Hi Paul,

I haven't had time to make sure, but partial bisection suggests a very
recent change (like this one?) induces 4 new coreutils test failures
when coreutils uses the latest from gnulib.  Each failure included the
EBADF diagnostic.  Here are examples:

    FAIL: tests/mkdir/special-1
    ===========================
    ...
    mkdir (GNU coreutils) 8.20.19-78f6d
    ...
    + set_mode_string=u=rwx,g=rx,o=w,-s,+t
    + output_mode_string=drwxr-x-wT
    + tmp=t
    + mkdir -mu=rwx,g=rx,o=w,-s,+t t
    mkdir: cannot change permissions of 't': Bad file descriptor
    + fail=1
    + test -d t
    ++ ls -ld t
    ++ cut -b-10
    + mode=drwxr-x--T
    + case "$mode" in
    + fail=1
    + rmdir t
    + tmp2=t/sub
    + mkdir -mu=rwx,g=rx,o=w,-s,+t t/sub
    + mkdir --parents -mu=rwx,g=rx,o=w,-s,+t t/sub
    mkdir: cannot change permissions of 't/sub': Bad file descriptor
    + fail=1