On 3/23/21 9:33 AM, Mark Rutland wrote: > On Tue, Mar 23, 2021 at 08:31:50AM -0500, Madhavan T. Venkataraman wrote: >> On 3/23/21 8:04 AM, Mark Rutland wrote: >>> On Tue, Mar 23, 2021 at 07:46:10AM -0500, Madhavan T. Venkataraman wrote: >>>> On 3/23/21 5:42 AM, Mark Rutland wrote: >>>>> On Mon, Mar 15, 2021 at 11:57:56AM -0500, madvenka@xxxxxxxxxxxxxxxxxxx wrote: >>>>>> From: "Madhavan T. Venkataraman" <madvenka@xxxxxxxxxxxxxxxxxxx> >>>>>> >>>>>> EL1 exceptions can happen on any instruction including instructions in >>>>>> the frame pointer prolog or epilog. Depending on where exactly they happen, >>>>>> they could render the stack trace unreliable. >>>>>> >>>>>> If an EL1 exception frame is found on the stack, mark the stack trace as >>>>>> unreliable. >>>>>> >>>>>> Now, the EL1 exception frame is not at any well-known offset on the stack. >>>>>> It can be anywhere on the stack. In order to properly detect an EL1 >>>>>> exception frame the following checks must be done: >>>>>> >>>>>> - The frame type must be EL1_FRAME. >>>>>> >>>>>> - When the register state is saved in the EL1 pt_regs, the frame >>>>>> pointer x29 is saved in pt_regs->regs[29] and the return PC >>>>>> is saved in pt_regs->pc. These must match with the current >>>>>> frame. >>>>> >>>>> Before you can do this, you need to reliably identify that you have a >>>>> pt_regs on the stack, but this patch uses a heuristic, which is not >>>>> reliable. >>>>> >>>>> However, instead you can identify whether you're trying to unwind >>>>> through one of the EL1 entry functions, which tells you the same thing >>>>> without even having to look at the pt_regs. >>>>> >>>>> We can do that based on the entry functions all being in .entry.text, >>>>> which we could further sub-divide to split the EL0 and EL1 entry >>>>> functions. >>>> >>>> Yes. I will check the entry functions. But I still think that we should >>>> not rely on just one check. The additional checks will make it robust. >>>> So, I suggest that the return address be checked first. If that passes, >>>> then we can be reasonably sure that there are pt_regs. Then, check >>>> the other things in pt_regs. >>> >>> What do you think this will catch? >> >> I am not sure that I have an exact example to mention here. But I will attempt >> one. Let us say that a task has called arch_stack_walk() in the recent past. >> The unwinder may have copied a stack frame onto some location in the stack >> with one of the return addresses we check. Let us assume that there is some >> stack corruption that makes a frame pointer point to that exact record. Now, >> we will get a match on the return address on the next unwind. > > I don't see how this is material to the pt_regs case, as either: > > * When the unwinder considers this frame, it appears to be in the middle > of an EL1 entry function, and the unwinder must mark the unwinding as > unreliable regardless of the contents of any regs (so there's no need > to look at the regs). > > * When the unwinder considers this frame, it does not appear to be in > the middle of an EL1 entry function, so the unwinder does not think > there are any regs to consider, and so we cannot detect this case. > > ... unless I've misunderstood the example? > > There's a general problem that it's possible to corrupt any portion of > the chain to skip records, e.g. > > A -> B -> C -> D -> E -> F -> G -> H -> [final] > > ... could get corrupted to: > > A -> B -> D -> H -> [final] > > ... regardless of whether C/E/F/G had associated pt_regs. AFAICT there's > no good way to catch this generally unless we have additional metadata > to check the unwinding against. > > The likelihood of this happening without triggering other checks is > vanishingly low, and as we don't have a reliable mechanism for detecting > this, I don't think it's worthwhile attempting to do so. > > If and when we try to unwind across EL1 exception boundaries, the > potential mismatch between the frame record and regs will be more > significant, and I agree at that point thisd will need more thought. > >> Pardon me if the example is somewhat crude. My point is that it is >> highly unlikely but not impossible for the return address to be on the >> stack and for the unwinder to get an unfortunate match. > > I agree that this is possible in theory, but as above I don't think this > is a practical concern. > OK. What you say makes sense. Thanks. Madhavan
