Messages in this thread | | | Date | Wed, 22 Aug 2001 08:11:40 -0400 | From | Brian Gerst <> | Subject | Re: [OOPS] repeatable 2.4.8-ac7, 2.4.7-ac6 just run xdos |
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Andi Kleen wrote: > > On Wed, Aug 22, 2001 at 07:57:59AM -0400, Brian Gerst wrote: > > Yes. What happened here is that %ds and %es were not being updated > > atomically. Under normal operation, this would just leave %es with > > USER_DS, which is sufficiently equivalent to KERNEL_DS to not cause a > > fault. Coming out of vm86 mode however forces the data segment > > registers to null after saving the real mode values on the stack. If an > > interrupt happened between setting %ds and %es (what are the odds?) then > > that assumption would fail and leave %es null, causing the next string > > instruction to go boom. The same fix should be applied to entry.S as > > well. > > No that's not the problem. interrupt gates come in with interrupts off, > so there are no other interrupts that could race here. The syscall entry > always updates %ds/%es unconditionally and %ds first, so there is no > race. > > It was much simpler. It assumed that __KERNEL_DS could not be loaded > from user space because of the segment register priviledge checking; and > that was obviously not true from vm86 mode. > > -Andi
The kernel was initially entered throught the general protection fault trap gate, with interupts on. The syscall entry was left on the stack because of the way sys_vm86 works.
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