On Fri, Jan 11, 2019 at 01:00:38AM +0100, Nicolai Stange wrote: > Hi Joe, > > Joe Lawrence <joe.lawrence@xxxxxxxxxx> writes: > > > tl;dr: On ppc64le, what is top-most stack frame for scheduled tasks > > about? > > If I'm reading the code in _switch() correctly, the first frame is > completely uninitialized except for the pointer back to the caller's > stack frame. > > For completeness: _switch() saves the return address, i.e. the link > register into its parent's stack frame, as is mandated by the ABI and > consistent with your findings below: it's always the second stack frame > where the return address into __switch_to() is kept. > Hi Nicolai, Good, that makes a lot of sense. I couldn't find any reference explaining the contents of frame 0, only unwinding code here and there (as in crash-utility) that stepped over it. > <snip> > > > > > > > Example 1 (RHEL-7) > > ================== > > > > crash> struct task_struct.thread c00000022fd015c0 | grep ksp > > ksp = 0xc0000000288af9c0 > > > > crash> rd 0xc0000000288af9c0 -e 0xc0000000288b0000 > > > > - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - > > > > sp[0]: > > > > c0000000288af9c0: c0000000288afb90 0000000000dd0000 ...(............ > > c0000000288af9d0: c000000000002a94 c000000001c60a00 .*.............. > > > > crash> sym c000000000002a94 > > c000000000002a94 (T) hardware_interrupt_common+0x114 > > So that c000000000002a94 certainly wasn't stored by _switch(). I think > what might have happened is that the switching frame aliased with some > prior interrupt frame as setup by hardware_interrupt_common(). > > The interrupt and switching frames seem to share a common layout as far > as the lower STACK_FRAME_OVERHEAD + sizeof(struct pt_regs) bytes are > concerned. > > That address into hardware_interrupt_common() could have been written by > the do_IRQ() called from there. > That was my initial theory, but then when I saw an ordinary scheduled task with a similarly strange frame 0, I thought that _switch() might have been doing something clever (or not). But according your earlier explanation, it would line up that these values may be inherited from do_IRQ() or the like. > > > c0000000288af9e0: c000000001c60a80 0000000000000000 ................ > > c0000000288af9f0: c0000000288afbc0 0000000000dd0000 ...(............ > > c0000000288afa00: c0000000014322e0 c000000001c60a00 ."C............. > > c0000000288afa10: c0000002303ae380 c0000002303ae380 ..:0......:0.... > > c0000000288afa20: 7265677368657265 0000000000002200 erehsger."...... > > > > Uh-oh... > > > > /* Mark stacktraces with exception frames as unreliable. */ > > stack[STACK_FRAME_MARKER] == STACK_FRAME_REGS_MARKER > > > Aliasing of the switching stack frame with some prior interrupt stack > frame would explain why that STACK_FRAME_REGS_MARKER is still found on > the stack, i.e. it's a leftover. > > For testing, could you try whether clearing the word at STACK_FRAME_MARKER > from _switch() helps? > > I.e. something like (completely untested): I'll kick off some builds tonight and try to get tests lined up tomorrow. Unfortunately these take a bit of time to run setup, schedule and complete, so perhaps by next week. > > diff --git a/arch/powerpc/kernel/entry_64.S b/arch/powerpc/kernel/entry_64.S > index 435927f549c4..b747d0647ec4 100644 > --- a/arch/powerpc/kernel/entry_64.S > +++ b/arch/powerpc/kernel/entry_64.S > @@ -596,6 +596,10 @@ _GLOBAL(_switch) > SAVE_8GPRS(14, r1) > SAVE_10GPRS(22, r1) > std r0,_NIP(r1) /* Return to switch caller */ > + > + li r23,0 > + std r23,96(r1) /* 96 == STACK_FRAME_MARKER * sizeof(long) */ > + > mfcr r23 > std r23,_CCR(r1) > std r1,KSP(r3) /* Set old stack pointer */ > > This may be sufficient to avoid the condition, but if the contents of frame 0 are truely uninitialized (not to be trusted), should the unwinder be even looking at that frame (for STACK_FRAMES_REGS_MARKER), aside from the LR and other stack size geometry sanity checks? > <snap> > > > > > save_stack_trace_tsk_reliable > > ============================= > > > > arch/powerpc/kernel/stacktrace.c :: save_stack_trace_tsk_reliable() does > > take into account the first stackframe, but only to verify that the link > > register is indeed pointing at kernel code address. > > It's actually the other way around: > > if (!firstframe && !__kernel_text_address(ip)) > return 1; > > > So the address gets sanitized only if it's _not_ coming from the first > frame. Yup, that's right, I had it backwards. Thanks! -- Joe
