Re: Using Linux+dosemu to install Windows

[Stas Sergeev <stssppnn@yahoo.com>]

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Hello.

Patrick J. LoPresti wrote:
> By instrumenting the code a bit, I learned
> that winnt.exe is attempting to invoke the pushfd and popfd
> instructions (opcodes 0x9c and 0x9d, respectively), which
> dosemu does not support.
Good investigation but actually
you are hacking the wrong place.
These instructions are handled in
handle_vm86_fault() which is in
/usr/src/linux/arch/i386/kernel/vm86.c
The attached patch must be a good
start for fixing the problem. It is
against the latest 2.4.19-pre-ac
kernels and probably can't be applied
to 2.4.18 because -ac tree have a lot
of changes in vm86.

> All winnt.exe is really doing is trying to determine whether the
> processor is a 486 or higher.  It does this by using pushfd and popfd
> to attempt to alter the next-to-highest bit of the EFLAGS register.
No, it tries to alter the AC flag which
is a bit 2 of the higher word of eflags.
But dosemu explicitly clears this flag
because due to some unknown bugs it
sometimes sets spuriously.
grep the do_vm86.c for the string "BUG:"
and comment the relevant code, then
winnt.exe will work.

> So, my question boils down to this: How can I modify the EFLAGS
> register from within vm86_GP_fault()?  If someone will tell me this, I
> will submit a patch to add support for emulating the pushf/pushfd and
> popf/popfd opcodes.
Well, but this won't solve the AC
problem...


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--- linux/arch/i386/kernel/vm86.c	Sat Jun 15 19:46:46 2002
+++ linux/arch/i386/kernel/vm86.c	Fri Jul 26 14:25:35 2002
@@ -502,8 +502,9 @@
 
 void handle_vm86_fault(struct kernel_vm86_regs * regs, long error_code)
 {
-	unsigned char *csp, *ssp;
+	unsigned char *csp, *ssp, opcode;
 	unsigned short ip, sp;
+	int prefix66, pref_done;
 
 #define CHECK_IF_IN_TRAP \
 	if (VMPI.vm86dbg_active && VMPI.vm86dbg_TFpendig) \
@@ -518,63 +519,61 @@
 	sp = SP(regs);
 	ip = IP(regs);
 
-	switch (popb(csp, ip, simulate_sigsegv)) {
-
-	/* operand size override */
-	case 0x66:
-		switch (popb(csp, ip, simulate_sigsegv)) {
+	prefix66 = 0;
+	pref_done = 0;
+	do {
+		switch (opcode = popb(csp, ip, simulate_sigsegv)) {
+			case 0x66:      /* operand prefix */  prefix66=1; break;
+			case 0x67:      /* address prefix */  break;
+			case 0x2e:      /* CS */              break;
+			case 0x3e:      /* DS */              break;
+			case 0x26:      /* ES */              break;
+			case 0x36:      /* SS */              break;
+			case 0x65:      /* GS */              break;
+			case 0x64:      /* FS */              break;
+			case 0xf2:      /* repnz */	      break;
+			case 0xf3:      /* rep */             break;
+			default: pref_done = 1;
+		}
 
-		/* pushfd */
-		case 0x9c:
-			pushl(ssp, sp, get_vflags(regs), simulate_sigsegv);
-			SP(regs) -= 4;
-			IP(regs) += 2;
-			VM86_FAULT_RETURN;
+		if (!pref_done)
+			IP(regs)++;	/* prefix found, adjust IP */
+		else
+			break;		/* no more prefixes */
+	} while (1);
 
-		/* popfd */
-		case 0x9d:
-			{
-			unsigned long newflags=popl(ssp, sp, simulate_sigsegv);
-			SP(regs) += 4;
-			IP(regs) += 2;
-			CHECK_IF_IN_TRAP;
-			set_vflags_long(newflags, regs);
-			VM86_FAULT_RETURN;
-			}
-
-		/* iretd */
-		case 0xcf:
-			{
-			unsigned long newip=popl(ssp, sp, simulate_sigsegv);
-			unsigned long newcs=popl(ssp, sp, simulate_sigsegv);
-			unsigned long newflags=popl(ssp, sp, simulate_sigsegv);
-			SP(regs) += 12;
-			IP(regs) = (unsigned short)newip;
-			regs->cs = (unsigned short)newcs;
-			CHECK_IF_IN_TRAP;
-			set_vflags_long(newflags, regs);
-			VM86_FAULT_RETURN;
-			}
-		/* need this to avoid a fallthrough */
-		default:
-			return_to_32bit(regs, VM86_UNKNOWN);
-		}
+	switch (opcode) {
 
 	/* pushf */
 	case 0x9c:
-		pushw(ssp, sp, get_vflags(regs), simulate_sigsegv);
-		SP(regs) -= 2;
+		if (prefix66) {
+			pushl(ssp, sp, get_vflags(regs), simulate_sigsegv);
+			SP(regs) -= 4;
+		} else {
+			pushw(ssp, sp, get_vflags(regs), simulate_sigsegv);
+			SP(regs) -= 2;
+		}
 		IP(regs)++;
 		VM86_FAULT_RETURN;
 
 	/* popf */
 	case 0x9d:
 		{
-		unsigned short newflags=popw(ssp, sp, simulate_sigsegv);
-		SP(regs) += 2;
+		unsigned long newflags;
+		if (prefix66) {
+			newflags=popl(ssp, sp, simulate_sigsegv);
+			SP(regs) += 4;
+		} else {
+			newflags = popw(ssp, sp, simulate_sigsegv);
+			SP(regs) += 2;
+		}
 		IP(regs)++;
 		CHECK_IF_IN_TRAP;
-		set_vflags_short(newflags, regs);
+		if (prefix66) {
+			set_vflags_long(newflags, regs);
+		} else {
+			set_vflags_short(newflags, regs);
+		}
 		VM86_FAULT_RETURN;
 		}
 
@@ -593,14 +592,28 @@
 	/* iret */
 	case 0xcf:
 		{
-		unsigned short newip=popw(ssp, sp, simulate_sigsegv);
-		unsigned short newcs=popw(ssp, sp, simulate_sigsegv);
-		unsigned short newflags=popw(ssp, sp, simulate_sigsegv);
-		SP(regs) += 6;
+		unsigned long newip;
+		unsigned long newcs;
+		unsigned long newflags;
+		if (prefix66) {
+			newip=popl(ssp, sp, simulate_sigsegv);
+			newcs=popl(ssp, sp, simulate_sigsegv);
+			newflags=popl(ssp, sp, simulate_sigsegv);
+			SP(regs) += 12;
+		} else {
+			newip = popw(ssp, sp, simulate_sigsegv);
+			newcs = popw(ssp, sp, simulate_sigsegv);
+			newflags = popw(ssp, sp, simulate_sigsegv);
+			SP(regs) += 6;
+		}
 		IP(regs) = newip;
 		regs->cs = newcs;
 		CHECK_IF_IN_TRAP;
-		set_vflags_short(newflags, regs);
+		if (prefix66) {
+			set_vflags_long(newflags, regs);
+		} else {
+			set_vflags_short(newflags, regs);
+		}
 		VM86_FAULT_RETURN;
 		}