__switch_to macro

DISCUSSION [email protected] 2 participants 2 messages Apr 6, 2001

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2001-04-06 09:21 UTC #0 lore raw

Jamie Lokier <[email protected]> on 04/06/2001 03:18:13 PM

To: Amol Lad/HSS@HSS
cc: [email protected]

Subject: Re: __switch_to macro

Quoted 7 lines · [email protected] wrote:

1) What exactly is meant by ' stale segment register values' in the note.
2) In the above macro, I think we recover gracefully from error condition while recovering fs and gs segment registers . The loadsegment(fs,next->tss.fs) and loadsegment(gs,next->tss.gs) does it. I am not able to understand loadsegment macro. The macro is as under
/** Load a segment. Fall back on loading a zero segment if something goes

wrong

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**/

#define loadsegment(seg,value)           \
     asm volatile("\n"             \
          "1:\t"                   \
          "movl %0,%%" #seg "\n"   \
          "2:\n"                   \
          "3:\t"                   \
          "pushl $0\n\t"           \
          "jmp 2b\n"               \
          ".previous\n"            \
          ".section __ex_table,\"a\"\n\t  \
          "/align 4\n\t"           \
          ".long 1b,3b\n"          \
          ".previous"              \
          : :"m" (*(unsigned int *)&(value)))

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I also want to know what is 'something' in the comment above the macro

The answers to 1. and 'something' are the same: stale segment values. You can't load any value into %ss, %ds, %es, %fs or %gs. They must be valid references into the GDT or LDT, with the appropriote protection level, or 0.

Usually the values stored in tss are ok, as they were valid values when they were stored. However for programs that use modify_ldt, it's possible for a valid LDT entry to be made invalid while some tss still refers to that segment.

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Thanks a lot.. good explanation..

At the next attempt to load the segment value into a segment register, you get a fault. The code in loadsegment traps this fault and loads zero into the segment register instead when this happens. Zero is always allowed. If the user program then tries to access data referenced by that segment register, user space will get a general_protection fault. As it's only user space that calls modify_ldt to invalidate an LDT entry, that's reasonable.

There's one thing that confuses me: don't you get a segment_not_present fault? If so, traps.c's do_segment_not_present doesn't appear to search the exception table, and the code in loadsegment would not work.

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well.. I peeked into traps.c.. I do seem a call to die_if_no_fixup

in this function. And I think loadsegment is already making an entry in exception table.

Amol

-- Jamie

Jamie Lokier

2001-04-06 11:40 UTC #1 lore raw

Quoted 7 lines · [email protected] wrote:

There's one thing that confuses me: don't you get a segment_not_present fault? If so, traps.c's do_segment_not_present doesn't appear to search the exception table, and the code in loadsegment would not work.
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well.. I peeked into traps.c.. I do seem a call to die_if_no_fixup
in this function. And I think loadsegment is already making an entry in exception table.

Ah, you're right. I didn't follow DO_ERROR all the way to do_trap and hency search_exception_table.

-- Jamie