On Thu, Aug 28, 2025 at 11:01 PM Serge E. Hallyn <se...@hallyn.com> wrote:
> On Wed, Aug 27, 2025 at 05:32:02PM -0700, Andy Lutomirski wrote:
> > On Wed, Aug 27, 2025 at 5:14 PM Aleksa Sarai <cyp...@cyphar.com> wrote:
> > >
> > > On 2025-08-26, Mickaël Salaün <m...@digikod.net> 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 stuffing
> > > > > 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 business
> > > > > 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 otherwise?
> > >
> > > 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 = tempfile.TemporaryFile()
> >
> > In [3]: f.write(b'initial contents')
> > Out[3]: 16
> >
> > In [4]: f.flush()
> >
> > In [5]: map = mmap.mmap(f.fileno(), f.tell(), flags=mmap.MAP_PRIVATE,
> > prot=mmap.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 = open("/tmp/3", "w")
> f.write('initial contents')
> f.flush()
>
> P2:
> import mmap
> f = open("/tmp/3", "r")
> map = mmap.mmap(f.fileno(), f.tell(), flags=mmap.MAP_PRIVATE,
> prot=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.
