On Fri, 3 May 2019 09:20:55 -0700 Andy Lutomirski luto@amacapital.net 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