On Wed, Feb 14, 2018 at 7:17 PM, Andy Lutomirski <l...@kernel.org> wrote: > On Wed, Feb 14, 2018 at 6:21 PM, Dominik Brodowski > <li...@dominikbrodowski.net> wrote: >> Moving the switch to IRQ stack from the interrupt macro to the helper >> function requires some trickery: All ENTER_IRQ_STACK really cares about >> is where the "original" stack -- meaning the GP registers etc. -- is >> stored. Therefore, we need to offset the stored RSP value by 8 whenever >> ENTER_IRQ_STACK is called from within a function. In such cases, and >> after switching to the IRQ stack, we need to push the "original" return >> address (i.e. the return address from the call to the interrupt entry >> function) to the IRQ stack. >> >> This trickery allows us to carve another 1k from the text size: >> >> text data bss dec hex filename >> 17905 0 0 17905 45f1 entry_64.o-orig >> 16897 0 0 16897 4201 entry_64.o >> >> Signed-off-by: Dominik Brodowski <li...@dominikbrodowski.net> >> --- >> arch/x86/entry/entry_64.S | 53 >> +++++++++++++++++++++++++++++++---------------- >> 1 file changed, 35 insertions(+), 18 deletions(-) >> >> diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S >> index de8a0da0d347..3046b12a1acb 100644 >> --- a/arch/x86/entry/entry_64.S >> +++ b/arch/x86/entry/entry_64.S >> @@ -449,10 +449,18 @@ END(irq_entries_start) >> * >> * The invariant is that, if irq_count != -1, then the IRQ stack is in use. >> */ >> -.macro ENTER_IRQ_STACK regs=1 old_rsp >> +.macro ENTER_IRQ_STACK regs=1 old_rsp save_ret=0 >> DEBUG_ENTRY_ASSERT_IRQS_OFF >> movq %rsp, \old_rsp >> >> + .if \save_ret >> + /* >> + * If save_ret is set, the original stack contains one additional >> + * entry -- the return address. >> + */ >> + addq $8, \old_rsp >> + .endif >> + > > This is a bit alarming in that you now have live data below RSP. For > x86_32, this would be a big no-no due to NMI. For x86_64, it might > still be bad if there are code paths where NMI is switched to non-IST > temporarily, which was the case at some point and might still be the > case. (I think it is.) Remember that the x86_64 *kernel* ABI has no > red zone. > > It also means that, if you manage to hit vmalloc_fault() in here when > you touch the IRQ stack, you're dead. IOW you hit: > > movq \old_rsp, PER_CPU_VAR(irq_stack_union + IRQ_STACK_SIZE - 8) > > which gets #PF and eats your return pointer. Debugging this will be > quite nasty because you'll only hit it on really huge systems after a > thread gets migrated, and even then only if you get unlucky on your > stack alignment. > > So can you find another way to do this?
It's adding 8 to the temp register, not %rsp. -- Brian Gerst