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    SubjectRe: [PATCH] utimensat() non-conformances and fixes [v3]
    On Tue, Jun 3, 2008 at 1:22 PM, Al Viro <viro@zeniv.linux.org.uk> wrote:
    > On Tue, Jun 03, 2008 at 01:13:00PM +0200, Miklos Szeredi wrote:
    >> > > I'm not sure of the correct way to get the required nameidata (to do a
    >> > > vfs_permission() call) from the file descriptor. Can you give me a
    >> > > tip there?
    >> >
    >> > Could you point me at the right way of doing this?
    >>
    >> You don't need nameidata for this at all. Just call permission() with
    >> a NULL nameidata.
    >>
    >> Ugly API? Yes, will be cleaned up if we manage to find some common
    >> ground with the VFS maintainers.
    >
    > As soon as I'm done with sysctls...
    >
    > FWIW, I very much doubt that you are right wrt required permissions, though.
    > AFAICS, intent here is "if you can write to file, you can touch the timestamps
    > anyway" and having descriptor opened for write gives that, current permissions
    > be damned.

    The standard is pretty clear on this point:

    [[
    Only a process with the effective user ID equal to the user ID of the
    file, or with write access to the file, or with appropriate privileges
    may use futimens( ) or utimensat( ) with a null pointer as the times
    argument or with both tv_nsec fields set to the special value
    UTIME_NOW.
    ]]

    The crucial words here are "a process ... with write access to the
    file" -- in other words, the permissions are determined by the
    process's credentials, not by the access mode of the file descriptor.
    I was not 100% sure on that to start with, so I did check it out with
    one of the folk at The Open Group, to make sure of my understanding.

    --
    Michael Kerrisk
    Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
    Found a bug? http://www.kernel.org/doc/man-pages/reporting_bugs.html


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