Re: use of setjmp/longjmp in x86 emulator.

From: Gleb Natapov
Date: Tue Mar 02 2010 - 03:49:23 EST


On Mon, Mar 01, 2010 at 02:56:59PM -0800, H. Peter Anvin wrote:
> On 03/01/2010 02:31 PM, H. Peter Anvin wrote:
> > On 03/01/2010 11:18 AM, Zachary Amsden wrote:
> >>
> >> It's going to be ugly to emulate segmentation, NX and write protect
> >> support without hardware to do this checking for you, but it's just what
> >> you have to do in this slow path - tedious, fully specified emulation.
> >>
> >> Just because it's tedious doesn't mean we need to use setjmp / longjmp.
> >> Throw / catch might be effective, but it's still pretty bizarre to do
> >> tricks like that in C.
> >>
> >
> > Well, setjmp/longjmp really is not much more than exception handling in C.
> >
>
> For what it's worth, I think that setjmp/longjmp is not anywhere near as
> dangerous as people want to make it out to be. gcc will warn for
> dangerous uses (and a lot of non-dangerous uses), but generally the
> difficult problems can be dealt with by moving the setjmp-protected code
> into a separate function.
>
Can I consider this as ACK for something like the patch blow? :) (with
proper x86 version of setjmp/longjmp of course).

diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index cfcb6f0..089a405 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -35,6 +35,45 @@
#include "x86.h"
#include "tss.h"

+typedef unsigned long jmp_buf[8];
+int setjmp(jmp_buf);
+void longjmp(jmp_buf, int);
+
+asm (
+" .align 4\n"
+" .type setjmp, @function\n"
+"setjmp:\n"
+" pop %rsi # Return address, and adjust the stack\n"
+" xorl %eax,%eax # Return value\n"
+" movq %rbx,(%rdi)\n"
+" movq %rsp,8(%rdi) # Post-return %rsp!\n"
+" push %rsi # Make the call/return stack happy\n"
+" movq %rbp,16(%rdi)\n"
+" movq %r12,24(%rdi)\n"
+" movq %r13,32(%rdi)\n"
+" movq %r14,40(%rdi)\n"
+" movq %r15,48(%rdi)\n"
+" movq %rsi,56(%rdi) # Return address\n"
+" ret\n"
+" .size setjmp,.-setjmp\n"
+
+" .align 4\n"
+" .type longjmp, @function\n"
+"longjmp:\n"
+" movl %esi,%eax # Return value (int)\n"
+" movq (%rdi),%rbx\n"
+" movq 8(%rdi),%rsp\n"
+" movq 16(%rdi),%rbp\n"
+" movq 24(%rdi),%r12\n"
+" movq 32(%rdi),%r13\n"
+" movq 40(%rdi),%r14\n"
+" movq 48(%rdi),%r15\n"
+" jmp *56(%rdi)\n"
+" .size longjmp,.-longjmp\n"
+ );
+
+static jmp_buf jb;
+
/*
* Opcode effective-address decode tables.
* Note that we only emulate instructions that have at least one memory
@@ -1729,7 +1768,7 @@ static inline int writeback(struct x86_emulate_ctxt *ctxt,
c->dst.bytes,
ctxt->vcpu);
if (rc != X86EMUL_CONTINUE)
- return rc;
+ longjmp(jb, 1);
break;
case OP_NONE:
/* no writeback */
@@ -2391,6 +2430,11 @@ x86_emulate_insn(struct x86_emulate_ctxt *ctxt, struct x86_emulate_ops *ops)
memcpy(c->regs, ctxt->vcpu->arch.regs, sizeof c->regs);
saved_eip = c->eip;

+ if (setjmp(jb)) {
+ printk(KERN_ERR"setjump() == 1\n");
+ return 0;
+ }
+
if (ctxt->mode == X86EMUL_MODE_PROT64 && (c->d & No64)) {
kvm_queue_exception(ctxt->vcpu, UD_VECTOR);
goto done;
--
Gleb.
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