On Tue, Jan 30, 2024 at 02:09:34PM +0000, Mark Brown wrote: > On Tue, Jan 30, 2024 at 11:51:07AM +0000, Will Deacon wrote: > > On Fri, Jan 19, 2024 at 12:29:13PM +0000, Mark Brown wrote: > > > > - if (test_thread_flag(TIF_SVE)) > > > + if (current->thread.fp_type == FP_STATE_SVE) > > > sve_to_fpsimd(current); > > > } > > > I don't think this hunk applies on -rc2 ^^. > > Hrm, git seemed to figure out a rebase with no intervention - I've > thrown it at my CI and will resend assuming no changes from the rest of > the discussion. > > > > - if (add_all || test_thread_flag(TIF_SVE) || > > > + if (add_all || current->thread.fp_type == FP_STATE_SVE || > > > thread_sm_enabled(¤t->thread)) { > > > int vl = max(sve_max_vl(), sme_max_vl()); > > > I think this code is preemptible, so I'm struggling to understand what > > happens if the fp_type changes under our feet as a result of a context > > switch. > > We are relying here on having forced a flush of the floating point > register state prior to this code running, simple preemption won't > change the state from what was already saved. The same consideration > also applies to the check for streaming mode here. > > That said if this is preempted ptrace *could* come in and rewrite the > data or at worst change the vector length (which could leave us with > sve_state deallocated or a different size, possibly while we're in the > middle of accessing it). This could also happen with the existing check > for TIF_SVE so I don't think there's anything new here - AFAICT this has > always been an issue with the vector code, unless I'm missing some > bigger thing which excludes ptrace. I think any change that's needed > there won't overlap with this one, I'm looking. I'm pretty sure that terrible things will happen treewide if ptrace can ever access or manipulate the internal state of a _running_ task. I think the logic is that any ptrace call that can access or manipulate the state of a task is gated on the task being ptrace-stopped. Once we have committed to deliveing a signal, we have obviously run past the opportunity to stop (and hence be ptraced) on that signal. Cases where a multiple signals are delivered before acutally reaching userspace might want some thought. I haven't tracked down the smokeproof gun in the code yet, though. >From memory, I think that the above forced flush was there to protect against the context switch code rather than ptrace, and guarantees that any change that ctxsw _might_ spontaneously make to the task state has already been done and dusted before we do the actual signal delivery. This may be a red herring so far as ptrace hazards are concerned. Cheers ---Dave