[PATCH] mm: make opt_bootscrub non-init

From: Roger Pau Monne <roger.pau@citrix.com>
To: xen-devel@lists.xenproject.org
Cc: Sergey Dyasli <sergey.dyasli@citrix.com>,
	Stefano Stabellini <sstabellini@kernel.org>,
	Wei Liu <wei.liu2@citrix.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
	George Dunlap <George.Dunlap@eu.citrix.com>,
	Andrew Cooper <andrew.cooper3@citrix.com>,
	Ian Jackson <ian.jackson@eu.citrix.com>, Tim Deegan <tim@xen.org>,
	Julien Grall <julien.grall@arm.com>,
	Jan Beulich <jbeulich@suse.com>,
	Roger Pau Monne <roger.pau@citrix.com>
Subject: [PATCH] mm: make opt_bootscrub non-init
Date: Fri, 23 Nov 2018 15:30:02 +0100	[thread overview]
Message-ID: <20181123143002.79743-1-roger.pau@citrix.com> (raw)

LLVM code generation can attempt to load from a variable in the next
condition of an expression under certain circumstances, thus turning
the following condition:

if ( system_state < SYS_STATE_active && opt_bootscrub == BOOTSCRUB_IDLE )

Into:

0xffff82d080223967 <+103>: cmpl   $0x3,0x37b032(%rip) # 0xffff82d08059e9a0 <system_state>
0xffff82d08022396e <+110>: setb   -0x29(%rbp)
0xffff82d080223972 <+114>: cmpl   $0x2,0x228a8b(%rip) # 0xffff82d08044c404 <opt_bootscrub>

Such code will trigger a page fault if system_state >=
SYS_STATE_active because opt_bootscrub will be unmapped.

Fix this by making opt_bootscrub non-init, thus preventing the page
fault. The LLVM bug with the discussion about this issue can be found
at:

https://bugs.llvm.org/show_bug.cgi?id=39707

I haven't been able to find any other instances of such conditional
expression that uses system_state together with an init variable or
function.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
---
Cc: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: George Dunlap <George.Dunlap@eu.citrix.com>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Jan Beulich <jbeulich@suse.com>
Cc: Julien Grall <julien.grall@arm.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Stefano Stabellini <sstabellini@kernel.org>
Cc: Tim Deegan <tim@xen.org>
Cc: Wei Liu <wei.liu2@citrix.com>
Cc: Sergey Dyasli <sergey.dyasli@citrix.com>
---
 xen/common/page_alloc.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/xen/common/page_alloc.c b/xen/common/page_alloc.c
index 08ee8cfbb9..56c0b24865 100644
--- a/xen/common/page_alloc.c
+++ b/xen/common/page_alloc.c
@@ -166,7 +166,7 @@ enum bootscrub_mode {
     BOOTSCRUB_ON,
     BOOTSCRUB_IDLE,
 };
-static enum bootscrub_mode __initdata opt_bootscrub = BOOTSCRUB_IDLE;
+static enum bootscrub_mode opt_bootscrub = BOOTSCRUB_IDLE;
 static int __init parse_bootscrub_param(const char *s)
 {
     /* Interpret 'bootscrub' alone in its positive boolean form */
-- 
2.19.1


_______________________________________________
Xen-devel mailing list
Xen-devel@lists.xenproject.org
https://lists.xenproject.org/mailman/listinfo/xen-devel
