- exact magic values for entered/nonentered states in recursive enter check

- read_*() functions to read segment selector values
 - decode loaded segments on panic

kernel/arch/i386/klib386.s

@@ -41,6 +41,9 @@
 .define	_thecr3
 .define	_write_cr4
 .define	_catch_pagefaults
+.define	_read_ds
+.define	_read_cs
+.define	_read_ss
 
 ! The routines only guarantee to preserve the registers the C compiler
 ! expects to be preserved (ebx, esi, edi, ebp, esp, segment registers, and
@@ -573,6 +576,20 @@ _write_cr4:
 	pop	ebp
 	ret
 
+_read_ds:
+	mov	eax, 0
+	mov	ax, ds
+	ret
+
+_read_cs:
+	mov	eax, 0
+	mov	ax, cs
+	ret
+
+_read_ss:
+	mov	eax, 0
+	mov	ax, ss
+	ret
 
 !*===========================================================================*
 !*				getcr3val				*

kernel/arch/i386/protect.c

@@ -403,6 +403,10 @@ PUBLIC void printseg(char *banner, int iscs, struct proc *pr, u32_t selector)
 			kprintf("invalid index in ldt\n");
 			return;
 		}
+		if(!pr) {
+			kprintf("local selector but unknown process\n");
+			return;
+		}
 		desc = &pr->p_seg.p_ldt[index];
 	} else {
 		kprintf("GDT");

kernel/debug.h

@@ -41,24 +41,38 @@
 #define TRACE(code, statement)
 #endif
 
+#define ENTERED		0xBA5E1514
+#define NOTENTERED	0x1415BEE1
+
 #define NOREC_ENTER(varname) \
-	static int varname = 0;	\
+	static int varname = NOTENTERED;	\
 	int mustunlock = 0; \
 	if(!intr_disabled()) { lock; mustunlock = 1; } \
+	if(varname != ENTERED && varname != NOTENTERED) { \
+		printf("magictest: 0x%lx local: 0x%lx\n", magictest, varname);\
+		minix_panic(#varname " bogus value on enter", varname); \
+	}	\
 	if(magictest != MAGICTEST) {	\
+		printf("magictest: 0x%lx local: 0x%lx\n", magictest, varname);\
 		minix_panic(#varname " magictest failed", __LINE__); \
 	}	\
-	if(varname) {	\
+	if(varname == ENTERED) {	\
 		minix_panic(#varname " recursive enter", __LINE__); \
 	} \
-	varname = 1;
+	varname = ENTERED;
 
 #define NOREC_RETURN(varname, v) do {	\
-	if(!varname)		\
-		minix_panic(#varname " flag off", __LINE__); \
 	if(!intr_disabled())	\
 		minix_panic(#varname " interrupts on", __LINE__); \
-	varname = 0;	\
+	if(magictest != MAGICTEST) {	\
+		printf("magictest: 0x%lx local: 0x%lx\n", magictest, varname);\
+		minix_panic(#varname " magictest failed", __LINE__); \
+	}	\
+	if(varname != ENTERED && varname != NOTENTERED) { \
+		printf("magictest: 0x%lx local: 0x%lx\n", magictest, varname);\
+		minix_panic(#varname " bogus value on return", varname); \
+	}	\
+	varname = NOTENTERED;	\
 	if(mustunlock)	{ unlock;	} \
 	return v;	\
 	} while(0)

kernel/utility.c

@@ -48,6 +48,12 @@ if (minix_panicing++) {
 	kprintf("\n");
   }
 
+  printseg("cs: ", 1, NULL, read_cs());
+  printseg("ds: ", 0, NULL, read_ds());
+  if(read_ds() != read_ss()) {
+	  printseg("ss: ", 0, NULL, read_ss());
+  }
+
   kprintf("kernel: ");
   util_stacktrace();