On Fri, 3 May 2019 09:20:55 -0700 Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote: > So here’s a somewhat nutty suggestion: how about we tweak the 32-bit entry code to emulate the sane 64-bit frame, not just for int3 but always? Basically, the entry asm for entries from kernel mode would do, roughly: > > push $0 ;dummy for call emulation > push %ss > push $0 ;a dummy for ESP > push 3*4(%esp) ;EFLAGS > push 3*4(%esp) ;CS > push 3*4(%esp) ;EIP > push %rax > lea 7*4(%esp), %rax > mov %rax, 4*4(%esp) ;ESP > > And the exit asm would do a little dance to write EFLAGS, CS, and EIP to the right spot, then load ESP-3*4 into %esp and do IRET. > > Now the annoying kernel_stack_pointer() hack can just go away, since regs->sp is always correct! > > I probably screwed up some arithmetic there, but it’s the idea that counts :) Yeah, as it will end up with: $0 ; dummy for call emulation %ss $0 ; dummy for ESP EIP $0 %ss $0 As 3 only gets you over what you already pushed. I think 5*4 is what you want. I guess the real question is, what's the performance impact of doing that? Although, this is only needed for kernel -> kernel exceptions, which are hopefully a rarity. -- Steve
