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SubjectRe: git commit 9fd67b4ed0714ab718f1f9bd14c344af336a6df7 (x86-64: Give vvars their own page) breaks Xen PV guests (64-bit).
> > However, this is what I get later on, any ideas?
>
> > [    0.585880] init[1] illegal int 0xcc from 32-bit mode ip:ffffffffff600400 cs:e033 sp:7fff230ca088 ax:ffffffffff600400 si:7faee3e822bf di:7fff230ca158
>
> That will, indeed, crash your system.
>
> 0xe033 is FLAT_RING3_CS64
>
> Jeremy / other Xen people: I'm trying to implement a lightweight
> check to distinguish a trap from a sane (i.e. allowable for syscalls)
> 64-bit user context from anything else. There seems to be precedent
> for using ->cs == __USER_CS to detect 64-bitness; for example, step.c
> contains:
>
> #ifdef CONFIG_X86_64
> case 0x40 ... 0x4f:
> if (regs->cs != __USER_CS)
> /* 32-bit mode: register increment */
> return 0;
> /* 64-bit mode: REX prefix */
> continue;
> #endif
>
> The prefetch opcode checker in mm/fault.c does something similar.
>
> Even the sysret code in xen/xen-asm_64.S does:
>
> pushq %r11
> pushq $__USER_CS
> pushq %rcx
>
> So I'm at a bit of a loss.
>
> You could probably hack it up and get your kernel to boot by allowing
> __USER_CS and 0xe033 in that check, but I'd rather understand it

Did this little hack:


diff --git a/arch/x86/kernel/vsyscall_64.c b/arch/x86/kernel/vsyscall_64.c
index dda7dff..5d0cf37 100644
--- a/arch/x86/kernel/vsyscall_64.c
+++ b/arch/x86/kernel/vsyscall_64.c
@@ -131,7 +131,7 @@ void dotraplinkage do_emulate_vsyscall(struct pt_regs *regs, long error_code)
* Real 64-bit user mode code has cs == __USER_CS. Anything else
* is bogus.
*/
- if (regs->cs != __USER_CS) {
+ if ((regs->cs != __USER_CS) && (regs->cs != FLAT_RING3_CS64)) {
/*
* If we trapped from kernel mode, we might as well OOPS now
* instead of returning to some random address and OOPSing
diff --git a/arch/x86/xen/mmu.c b/arch/x86/xen/mmu.c
index f987bde..0e4c13c 100644
--- a/arch/x86/xen/mmu.c
+++ b/arch/x86/xen/mmu.c
@@ -1916,6 +1916,7 @@ static void xen_set_fixmap(unsigned idx, phys_addr_t phys, pgprot_t prot)
# endif
#else
case VSYSCALL_LAST_PAGE ... VSYSCALL_FIRST_PAGE:
+ case VVAR_PAGE:
#endif
case FIX_TEXT_POKE0:
case FIX_TEXT_POKE1:
And getting this on 64-bit:

started: BusyBox v1.14.3 (2011-07-26 11:43:49 EDT)
[ 0.578603] rcS[1128]: segfault at ffffffffff5ff0a0 ip 00007fff40b7380a sp 00007fff40b5c0f0 error 4
[ 0.578847] rcS used greatest stack depth: 5024 bytes left
[ 0.581897] sh[1131]: segfault at ffffffffff5ff0a0 ip 00007fffb93ff80a sp 00007fffb92bbd70 error 4
[ 1.587637] sh[1137]: segfault at ffffffffff5ff0a0 ip 00007ffffa5ff80a sp 00007ffffa522560 error 4
[ 2.592295] sh[1141]: segfault at ffffffffff5ff0a0 ip 00007ffffcb3f80a sp 00007ffffca98af0 error 4
[ 3.596344] sh[1145]: segfault at ffffffffff5ff0a0 ip 00007fff2e3ff80a sp 00007fff2e3e3370 error 4
[ 4.599812] sh[1149]: segfault at ffffffffff5ff0a0 ip 00007fff62dff80a sp 00007fff62ca9f10 error 4
[ 5.605835] sh[1153]: segfault at ffffffffff5ff0a0 ip 00007fff117ff80a sp 00007fff1175e7f0 error 4
[ 6.609438] sh[1157]: segfault at ffffffffff5ff0a0 ip 00007fff91bff80a sp 00007fff91bd71c0 error 4
[ 7.614714] sh[1161]: segfault at ffffffffff5ff0a0 ip 00007fff396b280a sp 00007fff3968ede0 error 4
[ 8.620374] sh[1165]: segfault at ffffffffff5ff0a0 ip 00007fffd398b80a sp 00007fffd38ecd70 error 4
[ 9.625512] sh[1169]: segfault at ffffffffff5ff0a0 ip 00007fff617d980a sp 00007fff61776070 error 4
[ 10.630246] sh[1173]: segfault at ffffffffff5ff0a0 ip 00007fff89fff80a sp 00007fff89f7f3b0 error 4
[ 11.635588] sh[1177]: segfault at ffffffffff5ff0a0 ip 00007fffa95ff80a sp 00007fffa95ea7c0 error 4
[ 12.640491] sh[1181]: segfault at ffffffffff5ff0a0 ip 00007fff28cd180a sp 00007fff28c524f0 error 4
..
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