Messages in this thread | | | Date | Thu, 14 Nov 2019 10:20:43 -0800 | From | Sean Christopherson <> | Subject | Re: [PATCH 2/3] x86/traps: Print non-canonical address on #GP |
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On Thu, Nov 14, 2019 at 10:00:35AM -0800, Andy Lutomirski wrote: > On Thu, Nov 14, 2019 at 9:46 AM Sean Christopherson > <sean.j.christopherson@intel.com> wrote: > > > + /* > > > + * For the user half, check against TASK_SIZE_MAX; this way, if the > > > + * access crosses the canonical address boundary, we don't miss it. > > > + */ > > > + if (addr_ref <= TASK_SIZE_MAX) > > > > Any objection to open coding the upper bound instead of using > > TASK_SIZE_MASK to make the threshold more obvious? > > > > > + return; > > > + > > > + pr_alert("dereferencing non-canonical address 0x%016lx\n", addr_ref); > > > > Printing the raw address will confuse users in the case where the access > > straddles the lower canonical boundary. Maybe combine this with open > > coding the straddle case? With a rough heuristic to hedge a bit for > > instructions whose operand size isn't accurately reflected in opnd_bytes. > > > > if (addr_ref > __VIRTUAL_MASK) > > pr_alert("dereferencing non-canonical address 0x%016lx\n", addr_ref); > > else if ((addr_ref + insn->opnd_bytes - 1) > __VIRTUAL_MASK) > > pr_alert("straddling non-canonical boundary 0x%016lx - 0x%016lx\n", > > addr_ref, addr_ref + insn->opnd_bytes - 1); > > else if ((addr_ref + PAGE_SIZE - 1) > __VIRTUAL_MASK) > > pr_alert("potentially straddling non-canonical boundary 0x%016lx - 0x%016lx\n", > > addr_ref, addr_ref + PAGE_SIZE - 1); > > This is unnecessarily complicated, and I suspect that Jann had the > right idea but just didn't quite explain it enough. The secret here > is that TASK_SIZE_MAX is a full page below the canonical boundary > (thanks, Intel, for screwing up SYSRET), so, if we get #GP for an > address above TASK_SIZE_MAX,
Ya, I followed all that. My point is that if "addr_ref + insn->opnd_bytes" straddles the boundary then it's extremely likely the #GP is due to a non-canonical access, i.e. the pr_alert() doesn't have to hedge (as much).
> then it's either a #GP for a different > reason or it's a genuine non-canonical access.
Heh, "canonical || !canonical" would be the options :-D
> > So I think that just a comment about this would be enough. > > *However*, the printout should at least hedge a bit and say something > like "probably dereferencing non-canonical address", since there are > plenty of ways to get #GP with an operand that is nominally > non-canonical but where the actual cause of #GP is different. And I > think this code should be skipped entirely if error_code != 0. > > --Andy
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