Date: Thu, 31 Oct 2002 17:55:15 -0800 From: Marcel Moolenaar <marcel@xcllnt.net> To: arch@FreeBSD.org Subject: i386: Bug in prototype for rgs() Message-ID: <20021101015515.GA1707@dhcp01.pn.xcllnt.net>
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Gang,
The prototype for rgs() in sys/i386/include/cpufunc.h claims that
the result of the function is 32-bits (ie returns an u_int). As
such, when inlining the function the compiler happy generates
the following code:
11ed7: 8c 6d 80 movl %gs,0xffffff80(%ebp)
or
12175: 8c ad 14 fd ff ff movl %gs,0xfffffd14(%ebp)
where in this case the memory operand is 32-bit. The source location
that corresponds with this is sys/i386/linux/linux_sysvec.c:331 and
sys/i386/linux/linux_sysvec.c:451
If you actually look at the frame being created in the debugger, you'll
see:
Breakpoint 4, linux_sendsig (catcher=0x28091468, sig=11, mask=0xc2827d78,
code=30) at ../../../i386/linux/linux_sysvec.c:472
472 if (copyout(&frame, fp, sizeof(frame)) != 0) {
Current language: auto; currently c
(kgdb) p /x frame
$21 = {sf_sig = 0xb, sf_sc = {sc_gs = 0xcdd3002f, sc_fs = 0xf, sc_es = 0x2f,
sc_ds = 0x2f, sc_edi = 0x2809aca8, sc_esi = 0xbfbff0e0,
[snip]
In words: the upper 32-bit of sf_sc.sc_gs are garbage. Different CPU
implementations behave differently WRT to the upper 16-bits when the
destination is known to be a 32-bit operand (ie register).
The point: should we not do (whitespace corrupted diff):
Index: cpufunc.h
===================================================================
RCS file: /home/ncvs/src/sys/i386/include/cpufunc.h,v
retrieving revision 1.130
diff -u -r1.130 cpufunc.h
--- cpufunc.h 22 Sep 2002 04:45:21 -0000 1.130
+++ cpufunc.h 1 Nov 2002 01:08:45 -0000
@@ -449,10 +449,10 @@
return (sel);
}
-static __inline u_int
+static __inline u_int16_t
rgs(void)
{
- u_int sel;
+ u_int16_t sel;
__asm __volatile("movl %%gs,%0" : "=rm" (sel));
return (sel);
}
So that the compiler generates:
5c2: 8c e8 mov %gs,%eax
5c4: 0f b7 c0 movzwl %ax,%eax
5c7: 89 45 80 mov %eax,0xffffff80(%ebp)
--
Marcel Moolenaar USPA: A-39004 marcel@xcllnt.net
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