amd64: Relax the assertion added in commit 4a59cbc12

[markj]

Summary

We only need to ensure that interrupts are disabled when handling a
fault from iret. Otherwise it's possible to trigger the assertion
legitimately, e.g., by copying in from an invalid address.

Fixes: 4a59cbc12
Reported by: pho

[kib]

Ok, but I think that segment load faults should not have interrupts enabled. Since we do not context-switch them, having indefinite state for any of the segments is probably not too healthy.

[markj]

In D30594#687126, @kib wrote:

Ok, but I think that segment load faults should not have interrupts enabled. Since we do not context-switch them, having indefinite state for any of the segments is probably not too healthy.

Yeah, that's why I wrote it this way originally. I missed that we handle pcb_onfault != NULL in the same block.

I can add the same assertion to all of the segment load fault cases. Do you see a better approach?

[kib]

In D30594#687134, @markj wrote:

In D30594#687126, @kib wrote:

Ok, but I think that segment load faults should not have interrupts enabled. Since we do not context-switch them, having indefinite state for any of the segments is probably not too healthy.

Yeah, that's why I wrote it this way originally. I missed that we handle pcb_onfault != NULL in the same block.

I can add the same assertion to all of the segment load fault cases. Do you see a better approach?

No, I think explicitly mark all cases with asserts is the only way. Or it could be a table of addresses and new %rips for segment faults, with one loop iterating over, and one assert in case of the match. But I think it is not needed (yet).

[markj]

Add assertions for segment register update faults. Convert to using a table.

[markj]

Actually I am having trouble convincing myself that

In D30594#687163, @kib wrote:

In D30594#687134, @markj wrote:

In D30594#687126, @kib wrote:

Ok, but I think that segment load faults should not have interrupts enabled. Since we do not context-switch them, having indefinite state for any of the segments is probably not too healthy.

Yeah, that's why I wrote it this way originally. I missed that we handle pcb_onfault != NULL in the same block.

I can add the same assertion to all of the segment load fault cases. Do you see a better approach?

No, I think explicitly mark all cases with asserts is the only way. Or it could be a table of addresses and new %rips for segment faults, with one loop iterating over, and one assert in case of the match. But I think it is not needed (yet).

I converted to a table, it seemed quite ugly to duplicate the assertion for each case. I tested with a program that uses setcontext() to set bogus segment register values.