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

[Julien Grall <julien.grall@xxxxxxx>]

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: Julien Grall <julien.grall@xxxxxxx>

Cheers,

> ---
> 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 */
--
Julien Grall

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