On Fri, May 03, 2024 at 12:36:14PM +0200, Peter Zijlstra wrote: > On Fri, May 03, 2024 at 11:37:25AM +0200, Christian Brauner wrote: > > On Thu, May 02, 2024 at 05:41:23PM -0700, Kees Cook wrote: > > > On Fri, May 03, 2024 at 01:14:45AM +0100, Al Viro wrote: > > > > On Thu, May 02, 2024 at 05:10:18PM -0700, Kees Cook wrote: > > > > > > > > > But anyway, there needs to be a general "oops I hit 0"-aware form of > > > > > get_file(), and it seems like it should just be get_file() itself... > > > > > > > > ... which brings back the question of what's the sane damage mitigation > > > > for that. Adding arseloads of never-exercised failure exits is generally > > > > a bad idea - it's asking for bitrot and making the thing harder to review > > > > in future. > > > > > > Linus seems to prefer best-effort error recovery to sprinkling BUG()s > > > around. But if that's really the solution, then how about get_file() > > > switching to to use inc_not_zero and BUG on 0? > > > > Making get_file() return an error is not an option. For all current > > callers that's pointless churn for a condition that's not supposed to > > happen at all. > > > > Additionally, iirc *_inc_not_zero() variants are implemented with > > try_cmpxchg() which scales poorly under contention for a condition > > that's not supposed to happen. > > unsigned long old = atomic_long_fetch_inc_relaxed(&f->f_count); > WARN_ON(!old); > > Or somesuch might be an option? Yeah, I'd be fine with that. WARN_ON() (or WARN_ON_ONCE() even?) and then people can do their panic_on_warn stuff to get the BUG_ON() behavior if they want to.
