Messages in this thread | | | From | Andy Lutomirski <> | Date | Thu, 9 Mar 2017 18:43:59 -0800 | Subject | Re: [PATCH 2/2] x86/nmi: Fix and optimize the NMI stack check code |
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On Thu, Mar 9, 2017 at 2:42 PM, Steven Rostedt <rostedt@goodmis.org> wrote: > From: "Steven Rostedt (Red Hat)" <rostedt@goodmis.org> > > Andy Lutomirski reported an off by one in the NMI stack check > for the nested NMI code, where if the stack pointer was one above > the actual stack (stack start + STACK_SIZE) it would trigger a false > positive. This is not that big of a deal because the stack pointer > should never be that. Even if a stack was using the pages just > above the NMI stack, it would require the stack about to overflow > for this to trigger, which is a much bigger bug than this is fixing. > > Also, Linus Torvalds pointed out that doing two compares can be > accomplish with a single compare. That is: > > ("reg" is top of stack we are comparing "stack" to) > > cmpq reg, stack > jae label // note, code had one off "ja" instead of "jae" > subq size, reg > cmpq reg, stack > jb label > > Is the same as: > > subq $1, reg > subq stack, reg > cmpq size, reg > jae label > > The subq $1 was added into the leaq by doing: > > leaq 5*8+7(%rsp), %rdx > > Added more comments as well.
Nice.
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