Commit d60ad744 authored Nov 04, 2017 by Andy Lutomirski Committed by Ingo Molnar Nov 07, 2017

selftests/x86/ldt_gdt: Robustify against set_thread_area() and LAR oddities



Bits 19:16 of LAR's result are undefined, and some upcoming
improvements to the test case seem to trigger this.  Mask off those
bits to avoid spurious failures.

commit 5b781c7e ("x86/tls: Forcibly set the accessed bit in TLS
segments") adds a valid case in which LAR's output doesn't quite
agree with set_thread_area()'s input.  This isn't triggered in the
test as is, but it will be if we start calling set_thread_area()
with the accessed bit clear.  Work around this discrepency.

I've added a Fixes tag so that -stable can pick this up if neccesary.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bpetkov@suse.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Fixes: 5b781c7e ("x86/tls: Forcibly set the accessed bit in TLS segments")
Link: http://lkml.kernel.org/r/b82f3f89c034b53580970ac865139fd8863f44e2.1509794321.git.luto@kernel.org


Signed-off-by: Ingo Molnar <mingo@kernel.org>

parent 693cb558

tools/testing/selftests/x86/ldt_gdt.c

+9 −1

Original line number	Diff line number	Diff line
		@@ -115,7 +115,15 @@ static void check_valid_segment(uint16_t index, int ldt,
		return;
		}

		if (ar != expected_ar) {
		/* The SDM says "bits 19:16 are undefined". Thanks. */
		ar &= ~0xF0000;

		/*
		* NB: Different Linux versions do different things with the
		* accessed bit in set_thread_area().
		*/
		if (ar != expected_ar &&
		(ldt \|\| ar != (expected_ar \| AR_ACCESSED))) {
		printf("[FAIL]\t%s entry %hu has AR 0x%08X but expected 0x%08X\n",
		(ldt ? "LDT" : "GDT"), index, ar, expected_ar);
		nerrs++;