Re: [musl] Re: [RFC] Possible new execveat(2) Linux syscall

[Rich Felker <dalias-/miJ2pyFWUyWIDz0JBNUog@public.gmane.org>]

On Fri, Nov 21, 2014 at 01:49:35PM +0000, David Drysdale wrote:
> On Fri, Nov 21, 2014 at 10:13 AM, Christoph Hellwig <hch-wEGCiKHe2LqWVfeAwA7xHQ@public.gmane.org> wrote:
> > On Sun, Nov 16, 2014 at 02:52:46PM -0500, Rich Felker wrote:
> >> I've been following the discussions so far and everything looks mostly
> >> okay. There are still issues to be resolved with the different
> >> semantics between Linux O_PATH and what POSIX requires for O_EXEC (and
> >> O_SEARCH) but as long as the intent is that, once O_EXEC is defined to
> >> save the permissions at the time of open and cause them to be used in
> >> place of the current file permissions at the time of execveat
> >
> > As far as I can tell we only need the little patch below to make Linux
> > O_PATH a valid O_SEARCH implementation.  Rich, you said you wanted to
> > look over it?
> >
> > For O_EXEC my interpretation is that we basically just need this new
> > execveat syscall + a patch to add FMODE_EXEC and enforce it.  So we
> > wouldn't even need the O_PATH|3 hack.  But unless someone more familar
> > with the arcane details of the Posix language verifies it I'm tempted to
> > give up trying to help to implent these flags :(
> 
> I'm not particularly familiar with POSIX details either, but I thought the
> O_PATH|3 hack would be needed for the interaction with O_ACCMODE -- just
> using FMODE_EXEC as O_EXEC would confuse existing code that examines
> (flags & O_ACCMODE).

To conform to POSIX, O_ACCMODE needs to contain all the bits of
O_RDONLY|O_WRONLY|O_RDWR|O_SEARCH|O_EXEC. Certainly it's possible that
code compiled with an old definition of O_ACCMODE as 3 could inherit
(or otherwise obtain) a file descriptor in O_SEARCH/O_EXEC mode, so
it's preferable to have the low 2 bits be distinct from the existing
access modes, but O_ACCMODE's definition (at least in userspace)
really does need to be updated to equal O_PATH|3.

> >From [1]:
>   "Applications shall specify exactly one of the ...five ... file access
>   modes ... O_EXEC / O_RDONLY / O_RDWR / O_SEARCH / O_WRONLY"
> (and O_EXEC and O_SEARCH are allowed to be the same value,
> as one only applies to files and the other only applies to directories).
> 
> As O_ACCMODE is 3, there are only 4 possible access modes that work
> with any existing code that checks (flags & O_ACCMODE), and 3 of the
> values are taken (0=O_RDONLY, 1=O_WRONLY, 2=O_RDWR).  So I
> guess that's where the idea for the |3 hack comes from.

3 is also "taken" too, but it's a mostly-undocumented hack.

Rich