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libaml: allow amlmapio() to re-enter the interpreter (can happen by pciadd() -> amleval())

- make frame base pointer variable
- in rwreg(), save/restore the interpreter state and allocate a Frame* on the stack
- add overflow checks for frame base pointer to xec() and amleval()
- gc() scans the whole stack from FP to the *real* bottom F0

--- a/sys/src/libaml/aml.c

+++ b/sys/src/libaml/aml.c

@@ -123,14 +123,17 @@

 struct Interp {

 	uchar	*pc;

 	Frame	*fp;

+	Frame	*fb;

 };

 static Interp	interp;

 static Frame	stack[32];

-#define PC	interp.pc

+#define	PC	interp.pc

 #define	FP	interp.fp

-#define FB	stack

+#define	FB	interp.fb

+

+#define F0	stack

 #define FT	&stack[nelem(stack)]

 static Heap *hp;

@@ -234,7 +237,7 @@

 		if(h->mark & 2)

 			gcmark(H2D(h));

-	for(f = FP; f >= FB; f--){

+	for(f = FP; f >= F0; f--){

 		for(i=0; i<f->narg; i++)

 			gcmark(f->arg[i]);

 		gcmark(f->env);

@@ -485,9 +488,13 @@

 static uvlong

 rwreg(void *reg, int off, int len, uvlong v, int write)

 {

+	Interp save;

 	uchar buf[8], *p;

 	Region *r;

+	save = interp;	/* save, in case we reenter the interpreter */

+	FB = FP+1;	/* allocate new base */

+

 	switch(TAG(reg)){

 	case 'b':

 		p = reg;

@@ -498,7 +505,7 @@

 			putle(p, len, v);

 		else

 			v = getle(p, len);

-		return v;

+		goto Out;

 	case 'r':

 		r = reg;

@@ -547,10 +554,13 @@

 					(Name*)r->name, spacename[r->space],

 					r->off, off, len, v);

 		}

-		return v;

+		goto Out;

 	}

-	return ~0;

+	v = -1;

+Out:

+	interp = save;	/* restore */

+	return v;

 }

 static uvlong

@@ -937,6 +947,8 @@

 	void *r;

 	PC = pc;

+	if(FB < F0 || FB >= FT)

+		goto Out;

 	FP = FB;

 	FP->tag = 0;

@@ -946,7 +958,7 @@

 	FP->start = PC;

 	FP->end = end;

 	FP->aux = end;

-	FB->ref = nil;

+	FP->ref = nil;

 	FP->dot = dot;

 	FP->env = env;

 	FP->op = nil;

@@ -2153,6 +2165,9 @@

 {

 	Name *n;

+	FB = F0;

+	FP = F0-1;

+

 	fmtinstall('V', Vfmt);

 	fmtinstall('N', Nfmt);

@@ -2190,7 +2205,8 @@

 amlexit(void)

 {

 	amlroot = nil;

-	FP = FB-1;

+	FB = F0;

+	FP = F0-1;

 	gc();

 }

@@ -2269,6 +2285,8 @@

 			return -1;

 		if(m->eval == nil)

 			return xec(m->start, m->end, forkname(m->name), e, r);

+		if(FB < F0 || FB >= FT)

+			return -1;

 		FP = FB;

 		FP->op = nil;

 		FP->env = e;