[Xen-devel] [PATCH v2] mm: make opt_bootscrub non-init

[Roger Pau Monne <roger.pau@xxxxxxxxxx>]

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@xxxxxxxxxx>
Reviewed-by: Sergey Dyasli <sergey.dyasli@xxxxxxxxxx>
Acked-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Acked-by: Julien Grall <julien.grall@xxxxxxx>
Acked-by: Wei Liu <wei.liu2@xxxxxxxxxx>
---
Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Cc: George Dunlap <George.Dunlap@xxxxxxxxxxxxx>
Cc: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>
Cc: Jan Beulich <jbeulich@xxxxxxxx>
Cc: Julien Grall <julien.grall@xxxxxxx>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>
Cc: Tim Deegan <tim@xxxxxxx>
Cc: Wei Liu <wei.liu2@xxxxxxxxxx>
Cc: Sergey Dyasli <sergey.dyasli@xxxxxxxxxx>
---
Changes since v1:
 - Make opt_bootscrub read mostly.
 - Add a comment about why opt_bootscrub is not in the init section.
---
 xen/common/page_alloc.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/xen/common/page_alloc.c b/xen/common/page_alloc.c
index 08ee8cfbb9..b4086781c4 100644
--- a/xen/common/page_alloc.c
+++ b/xen/common/page_alloc.c
@@ -166,7 +166,14 @@ enum bootscrub_mode {
     BOOTSCRUB_ON,
     BOOTSCRUB_IDLE,
 };
-static enum bootscrub_mode __initdata opt_bootscrub = BOOTSCRUB_IDLE;
+/*
+ * opt_bootscrub should live in the init section, since it's not accessed
+ * afterwards. However at least LLVM assumes there are no side effects of
+ * accessing the variable, and optimizes the condition so opt_bootscrub is
+ * read regardless of the value of system_state:
+ * https://bugs.llvm.org/show_bug.cgi?id=39707
+ */
+static enum bootscrub_mode __read_mostly opt_bootscrub = BOOTSCRUB_IDLE;
 static int __init parse_bootscrub_param(const char *s)
 {
     /* Interpret 'bootscrub' alone in its positive boolean form */
-- 
2.19.1


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