On Wed, Aug 27, 2025 at 5:14 PM Aleksa Sarai <cyphar@xxxxxxxxxx> wrote: > > On 2025-08-26, Mickaël Salaün <mic@xxxxxxxxxxx> 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'
