Re: [PATCH man-pages] Document encoded I/O

[Aleksa Sarai <cyphar@cyphar.com>]

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On 2019-10-23, Amir Goldstein <amir73il@gmail.com> wrote:
> > >
> > > No, I see why you choose to add the flag to open(2).
> > > I have no objection.
> > >
> > > I once had a crazy thought how to add new open flags
> > > in a non racy manner without adding a new syscall,
> > > but as you wrote, this is not relevant for O_ALLOW_ENCODED.
> > >
> > > Something like:
> > >
> > > /*
> > >  * Old kernels silently ignore unsupported open flags.
> > >  * New kernels that gets __O_CHECK_NEWFLAGS do
> > >  * the proper checking for unsupported flags AND set the
> > >  * flag __O_HAVE_NEWFLAGS.
> > >  */
> > > #define O_FLAG1 __O_CHECK_NEWFLAGS|__O_FLAG1
> > > #define O_HAVE_FLAG1 __O_HAVE_NEWFLAGS|__O_FLAG1
> > >
> > > fd = open(path, O_FLAG1);
> > > if (fd < 0)
> > >     return -errno;
> > > flags = fcntl(fd, F_GETFL, 0);
> > > if (flags < 0)
> > >     return flags;
> > > if ((flags & O_HAVE_FLAG1) != O_HAVE_FLAG1) {
> > >     close(fd);
> > >     return -EINVAL;
> > > }
> >
> > You don't need to add __O_HAVE_NEWFLAGS to do this -- this already works
> > today for userspace to check whether a flag works properly
> > (specifically, __O_FLAG1 will only be set if __O_FLAG1 is supported --
> > otherwise it gets cleared during build_open_flags).
> 
> That's a behavior of quite recent kernels since
> 629e014bb834 fs: completely ignore unknown open flags
> and maybe some stable kernels. Real old kernels don't have that luxury.

Ah okay -- so the key feature is that __O_CHECK_NEWFLAGS gets
transformed into __O_HAVE_NEWFLAGS (making it so that both the older and
current behaviours are detected). Apologies, I missed that on my first
read-through.

While it is a little bit ugly, it probably wouldn't be a bad idea to
have something like that.

> > The problem with adding new flags is that an *old* program running on a
> > *new* kernel could pass a garbage flag (__O_CHECK_NEWFLAGS for instance)
> > that causes an error only on the new kernel.
> 
> That's a theoretic problem. Same as O_PATH|O_TMPFILE.
> Show me a real life program that passes garbage files to open.

Has "that's a theoretical problem" helped when we faced this issue in
the past? I don't disagree that this is mostly theoretical, but I have a
feeling that this is an argument that won't hold water.

As for an example of semi-garbage flag passing -- systemd passes
O_PATH|O_NOCTTY in several places. Yes, they're known flags (so not
entirely applicable to this discussion) but it's also not a meaningful
combination of flags and yet is permitted.

> > The only real solution to this (and several other problems) is
> > openat2().
> 
> No argue about that. Come on, let's get it merged ;-)

Believe me, I'm trying. ;)

> > As for O_ALLOW_ENCODED -- the current semantics (-EPERM if it
> > is set without CAP_SYS_ADMIN) *will* cause backwards compatibility
> > issues for programs that have garbage flags set...
> >
> 
> Again, that's theoretical. In practice, O_ALLOW_ENCODED can work with
> open()/openat(). In fact, even if O_ALLOW_ENCODED gets merged after
> openat2(), I don't think it should be forbidden by open()/openat(),
> right? Do in that sense, O_ALLOW_ENCODED does not depend on openat2().

If it's a valid open() flag it'll also be a valid openat2(2) flag. The
only question is whether the garbage-flag problem justifies making it a
no-op for open(2).

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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