Re: [RFC PATCH v1 0/2] Add O_DENY_WRITE (complement AT_EXECVE_CHECK)

On Thu, Aug 28, 2025 at 11:01=E2=80=AFPM Serge E. Hallyn <[email protected]>=
 wrote:
> On Wed, Aug 27, 2025 at 05:32:02PM -0700, Andy Lutomirski wrote:
> > On Wed, Aug 27, 2025 at 5:14=E2=80=AFPM Aleksa Sarai <[email protected]=
> wrote:
> > >
> > > On 2025-08-26, Micka=C3=ABl Sala=C3=BCn <[email protected]> wrote:
> > > > On Tue, Aug 26, 2025 at 11:07:03AM +0200, Christian Brauner wrote:
> > > > > Nothing has changed in that regard and I'm not interested in stuf=
fing
> > > > > the VFS APIs full of special-purpose behavior to work around the =
fact
> > > > > that this is work that needs to be done in userspace. Change the =
apps,
> > > > > stop pushing more and more cruft into the VFS that has no busines=
s
> > > > > there.
> > > >
> > > > It would be interesting to know how to patch user space to get the =
same
> > > > guarantees...  Do you think I would propose a kernel patch otherwis=
e?
> > >
> > > You could mmap the script file with MAP_PRIVATE. This is the *actual*
> > > protection the kernel uses against overwriting binaries (yes, ETXTBSY=
 is
> > > nice but IIRC there are ways to get around it anyway).
> >
> > Wait, really?  MAP_PRIVATE prevents writes to the mapping from
> > affecting the file, but I don't think that writes to the file will
> > break the MAP_PRIVATE CoW if it's not already broken.
> >
> > IPython says:
> >
> > In [1]: import mmap, tempfile
> >
> > In [2]: f =3D tempfile.TemporaryFile()
> >
> > In [3]: f.write(b'initial contents')
> > Out[3]: 16
> >
> > In [4]: f.flush()
> >
> > In [5]: map =3D mmap.mmap(f.fileno(), f.tell(), flags=3Dmmap.MAP_PRIVAT=
E,
> > prot=3Dmmap.PROT_READ)
> >
> > In [6]: map[:]
> > Out[6]: b'initial contents'
> >
> > In [7]: f.seek(0)
> > Out[7]: 0
> >
> > In [8]: f.write(b'changed')
> > Out[8]: 7
> >
> > In [9]: f.flush()
> >
> > In [10]: map[:]
> > Out[10]: b'changed contents'
>
> That was surprising to me, however, if I split the reader
> and writer into different processes, so

Testing this in python is a terrible idea because it obfuscates the
actual syscalls from you.

> P1:
> f =3D open("/tmp/3", "w")
> f.write('initial contents')
> f.flush()
>
> P2:
> import mmap
> f =3D open("/tmp/3", "r")
> map =3D mmap.mmap(f.fileno(), f.tell(), flags=3Dmmap.MAP_PRIVATE, prot=3D=
mmap.PROT_READ)
>
> Back to P1:
> f.seek(0)
> f.write('changed')
>
> Back to P2:
> map[:]
>
> Then P2 gives me:
>
> b'initial contents'

Because when you executed `f.write('changed')`, Python internally
buffered the write. "changed" is never actually written into the file
in your example. If you add a `f.flush()` in P1 after this, running
`map[:]` in P2 again will show you the new data.