On Fri, May 03, 2024 at 12:12:28AM +0100, Al Viro wrote: > On Thu, May 02, 2024 at 03:52:21PM -0700, Kees Cook wrote: > > > As for semantics, what do you mean? Detecting dec-below-zero means we > > catch underflow, and detected inc-from-zero means we catch resurrection > > attempts. In both cases we avoid double-free, but we have already lost > > to a potential dangling reference to a freed struct file. But just > > letting f_count go bad seems dangerous. > > Detected inc-from-zero can also mean an RCU lookup detecting a descriptor > in the middle of getting closed. And it's more subtle than that, actually, > thanks to SLAB_TYPESAFE_BY_RCU for struct file. But isn't that already handled by __get_file_rcu()? i.e. shouldn't it be impossible for a simple get_file() to ever see a 0 f_count under normal conditions? -- Kees Cook
