PERFORCE change 931244 for review

[Robert Watson]

http://p4web.freebsd.org/@@931244?ac=10

Change 931244 by rwatson at rwatson_zenith_cl_cam_ac_uk on 2013/10/02 13:51:29

	Shrink PCB-embedded trusted stack down to 2 entries from 10 for now;
	growth of the PCB shrunk the available kernel stack sufficiently to
	cause deeply nested VM paths to overflow.  In the case I encountered,
	triggered by a userspace NULL-pointer dereference, found the
	following on a manually unwound stack:
	
	user null pointer deref -> vm -> signal delivery -> coredump ->
	  vfs -> page fault -> vm page filling -> interrupt delivery ->
	  timer code -> sleepq processing -> <boom>
	
	Where <boom> consisted of taking a TLB invalid fault on the guard
	page and then keeling over.
	
	It would be helpful if (a) DDB's stack trace code on MIPS could
	walk past exceptions rather than requiring manual unwinding, and
	(b) code detecting stack overflow didn't promptly overwrite the
	register state required to debug it.
	
	In the longer term we'll need to move the TSC elsewhere.  We may
	also want to grow the default MIPS kernel stack for 64-bit somewhat
	as even before my change, it came very close to the limit.

Affected files ...

.. //depot/projects/ctsrd/cheribsd/src/sys/mips/include/cheri.h#24 edit

Differences ...

==== //depot/projects/ctsrd/cheribsd/src/sys/mips/include/cheri.h#24 (text+ko) ====

@@ -118,7 +118,7 @@
 	struct chericap	csf_idc;
 };
 
-#define	CHERI_STACK_DEPTH	10	/* XXXRW: 10 is a nice round number. */
+#define	CHERI_STACK_DEPTH	2	/* XXXRW: 2 is a nice round number. */
 struct cheri_stack {
 	u_int		cs_max;		/* Maximum frame depth. */
 	u_int		cs_pointer;	/* Current frame index. */