On 3/19/21 8:22 AM, Mark Brown wrote: > On Thu, Mar 18, 2021 at 05:22:49PM -0500, Madhavan T. Venkataraman wrote: >> On 3/18/21 12:40 PM, Mark Brown wrote: > >>> Unless I'm misreading what's going on here this is more trying to set a >>> type for the stack as a whole than for a specific stack frame. I'm also >>> finding this a bit confusing as the unwinder already tracks things it >>> calls frame types and it handles types that aren't covered here like >>> SDEI. At the very least there's a naming issue here. > >> Both these frames are on the task stack. So, it is not a stack type. > > OTOH it's also not something that applies to every frame but only to the > base frame from each stack which I think was more where I was coming > from there. In any case, the issue is also that there's already another > thing that the unwinder calls a frame type so there's at least that > collision which needs to be resolved if nothing else. > The base frame from each stack as well as intermediate marker frames such as the EL1 frame and the Ftrace frame. As for the frame type, I will try to come up with a better name. >>> Taking a step back though do we want to be tracking this via pt_regs? >>> It's reliant on us robustly finding the correct pt_regs and on having >>> the things that make the stack unreliable explicitly go in and set the >>> appropriate type. That seems like it will be error prone, I'd been >>> expecting to do something more like using sections to filter code for >>> unreliable features based on the addresses of the functions we find on >>> the stack or similar. This could still go wrong of course but there's >>> fewer moving pieces, and especially fewer moving pieces specific to >>> reliable stack trace. > >> In that case, I suggest doing both. That is, check the type as well >> as specific functions. For instance, in the EL1 pt_regs, in addition >> to the above checks, check the PC against el1_sync(), el1_irq() and >> el1_error(). I have suggested this in the cover letter. > >> If this is OK with you, we could do that. We want to make really sure that >> nothing goes wrong with detecting the exception frame. > > ... > >> If you dislike the frame type, I could remove it and just do the >> following checks: > >> FP == pt_regs->regs[29] >> PC == pt_regs->pc >> and the address check against el1_*() functions > >> and similar changes for EL0 as well. > >> I still think that the frame type check makes it more robust. > > Yeah, we know the entry points so they can serve the same role as > checking an explicitly written value. It does mean one less operation > on exception entry, though I'm not sure that's that a big enough > overhead to actually worry about. I don't have *super* strong opinons > against adding the explicitly written value other than it being one more > thing we don't otherwise use which we have to get right for reliable > stack trace, there's a greater risk of bitrot if it's not something that > we ever look at outside of the reliable stack trace code. > So, I will add the address checks for robustness. I will think some more about the frame type. >>>> EL1_FRAME >>>> EL1 exception frame. > >>> We do trap into EL2 as well, the patch will track EL2 frames as EL1 >>> frames. Even if we can treat them the same the naming ought to be >>> clear. > >> Are you referring to ARMv8.1 VHE extension where the kernel can run >> at EL2? Could you elaborate? I thought that EL2 was basically for >> Hypervisors. > > KVM is the main case, yes - IIRC otherwise it's mainly error handlers > but I might be missing something. We do recommend that the kernel is > started at EL2 where possible. > > Actually now I look again it's just not adding anything on EL2 entries > at all, they use a separate set of macros which aren't updated - this > will only update things for EL0 and EL1 entries so my comment above > about this tracking EL2 as EL1 isn't accurate. > OK. Madhavan
