On Thu, Nov 28, 2024 at 3:34 PM Mateusz Guzik <mjguzik@xxxxxxxxx> wrote:
>
> On Thu, Nov 28, 2024 at 03:25:32PM +0100, Amir Goldstein wrote:
> > Commit 2a010c412853 ("fs: don't block i_writecount during exec") removed
> > the legacy behavior of getting ETXTBSY on attempt to open and executable
> > file for write while it is being executed.
> >
> > This commit was reverted because an application that depends on this
> > legacy behavior was broken by the change.
> >
> > We need to allow HSM writing into executable files while executed to
> > fill their content on-the-fly.
> >
> > To that end, disable the ETXTBSY legacy behavior for files that are
> > watched by pre-content events.
> >
> > This change is not expected to cause regressions with existing systems
> > which do not have any pre-content event listeners.
> >
> > +
> > +/*
> > + * Do not prevent write to executable file when watched by pre-content events.
> > + */
> > +static inline int exe_file_deny_write_access(struct file *exe_file)
> > +{
> > + if (unlikely(FMODE_FSNOTIFY_HSM(exe_file->f_mode)))
> > + return 0;
> > + return deny_write_access(exe_file);
> > +}
> > +static inline void exe_file_allow_write_access(struct file *exe_file)
> > +{
> > + if (unlikely(FMODE_FSNOTIFY_HSM(exe_file->f_mode)))
> > + return;
> > + allow_write_access(exe_file);
> > +}
> > +
>
> so this depends on FMODE_FSNOTIFY_HSM showing up on the file before any
> of the above calls and staying there for its lifetime -- does that hold?
Yes!
>
> I think it would be less error prone down the road to maintain the
> counters, except not return the error if HSM is on.
Cannot.
The "deny write counter" and "writers counter" are implemented on the
same counter, so open cannot get_write_access() if we maintain the
negative deny counters from exec.
Also, note that there are still calls to {allow,deny}_write_access() that
are not bypassed even with HSM, so for example, mixing fsverity and
HSM is probably not a good idea...
Thanks,
Amir.
