Messages in this thread | | | Date | Thu, 5 Oct 2017 08:57:34 -0500 | From | Josh Poimboeuf <> | Subject | Re: [lockdep] b09be676e0 BUG: unable to handle kernel NULL pointer dereference at 000001f2 |
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On Thu, Oct 05, 2017 at 08:02:33PM +0900, Tetsuo Handa wrote: > Josh Poimboeuf wrote: > > On Wed, Oct 04, 2017 at 06:44:50AM +0900, Tetsuo Handa wrote: > > > Josh Poimboeuf wrote: > > > > On Tue, Oct 03, 2017 at 11:28:15AM -0500, Josh Poimboeuf wrote: > > > > > There are two bugs: > > > > > > > > > > 1) Somebody -- presumably lockdep -- is corrupting the stack. Need the > > > > > lockdep people to look at that. > > > > > > > > > > 2) The 32-bit FP unwinder isn't handling the corrupt stack very well, > > > > > It's blindly dereferencing untrusted data: > > > > > > > > > > /* Is the next frame pointer an encoded pointer to pt_regs? */ > > > > > regs = decode_frame_pointer(next_bp); > > > > > if (regs) { > > > > > frame = (unsigned long *)regs; > > > > > len = regs_size(regs); > > > > > state->got_irq = true; > > > > > > > > > > On 32-bit, regs_size() dereferences the regs pointer before we know it > > > > > points to a valid stack. I'll fix that, along with the other unwinder > > > > > improvements I discussed with Linus. > > > > > > > > Tetsuo and/or Fengguang, > > > > > > > > Would you mind testing with this patch? It should at least prevent the > > > > unwinder panic and should hopefully print a useful unwinder dump > > > > instead. > > > > > > > Here are two outputs. > > > > Tetsuo, would you mind trying the following patch? > > > Here are two outputs. Same kernel with different host hardware.
Thanks, these dumps are more "normal":
- The first shows a missing frame pointer setup in atomic64_add_unless_cx8().
- The second shows some frame pointer related issue in the kthread creation path.
I don't plan on fixing those, because we don't yet have objtool support for 32-bit and we don't have anything which needs reliable stack traces there. I'll probably just disable those unwinder dumps on 32-bit.
-- Josh
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