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Re: [Xen-devel] [PATCH] mm: make opt_bootscrub non-init
- To: Roger Pau Monne <roger.pau@xxxxxxxxxx>, <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Fri, 23 Nov 2018 14:50:47 +0000
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- Cc: Sergey Dyasli <sergey.dyasli@xxxxxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Wei Liu <wei.liu2@xxxxxxxxxx>, Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>, George Dunlap <George.Dunlap@xxxxxxxxxxxxx>, Tim Deegan <tim@xxxxxxx>, Ian Jackson <ian.jackson@xxxxxxxxxxxxx>, Julien Grall <julien.grall@xxxxxxx>, Jan Beulich <jbeulich@xxxxxxxx>
- Delivery-date: Fri, 23 Nov 2018 14:51:01 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
- Openpgp: preference=signencrypt
On 23/11/2018 14:30, Roger Pau Monne wrote:
> 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>
Acked-by: Andrew Cooper <andrew.cooper3@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>
> ---
> 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 */
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