Re: [PATCH] MIPS: Always page align TASK_SIZE

[David Daney]

On 02/08/2016 10:15 AM, Harvey Hunt wrote:
> Hi David,
>
> On 02/08/2016 10:11 AM, David Daney wrote:
> > On 02/08/2016 10:05 AM, Harvey Hunt wrote:
> > > STACK_TOP_MAX is aligned on a 32k boundary. When __bprm_mm_init()
> > > creates an
> > > initial stack for a process, it does so using STACK_TOP_MAX as the end
> > > of the
> > > vma. A process's arguments and environment information are placed on
> > > the stack
> > > and then the stack is relocated and aligned on a page boundary. When
> > > using a 32
> > > bit kernel with 64k pages, the relocated stack has the process's args
> > > erroneously stored in the middle of the stack. This means that processes
> > > receive no arguments or environment variables, preventing them from
> > > running
> > > correctly.
> > >
> > > Fix this by aligning TASK_SIZE on a page boundary.
> > >
> > > Signed-off-by: Harvey Hunt <harvey.hunt@xxxxxxxxxx>
> > > Cc: David Daney <david.daney@xxxxxxxxxx>
> > > Cc: Paul Burton <paul.burton@xxxxxxxxxx>
> > > Cc: James Hogan <james.hogan@xxxxxxxxxx>
> > > Cc: linux-kernel@xxxxxxxxxxxxxxx
> > > ---
> > >   arch/mips/include/asm/processor.h | 6 +++---
> > >   1 file changed, 3 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/arch/mips/include/asm/processor.h
> > > b/arch/mips/include/asm/processor.h
> > > index 3f832c3..b618b40 100644
> > > --- a/arch/mips/include/asm/processor.h
> > > +++ b/arch/mips/include/asm/processor.h
> > > @@ -39,13 +39,13 @@ extern unsigned int vced_count, vcei_count;
> > >   #ifdef CONFIG_32BIT
> > >   #ifdef CONFIG_KVM_GUEST
> > >   /* User space process size is limited to 1GB in KVM Guest Mode */
> > > -#define TASK_SIZE    0x3fff8000UL
> > > +#define TASK_SIZE    (0x40000000UL - PAGE_SIZE)
> > >   #else
> > >   /*
> > >    * User space process size: 2GB. This is hardcoded into a few places,
> > >    * so don't change it unless you know what you are doing.
> > >    */
> > > -#define TASK_SIZE    0x7fff8000UL
> > > +#define TASK_SIZE    (0x7fff8000UL & PAGE_SIZE)
> >
> > Can you check your math here.  This doesn't seem correct.
>
> Thanks for spotting that - it should have been:
>
> (0x7fff8000UL & PAGE_MASK)

This brings up an interesting point.  How was this tested?  Please note 
that in the change log.

Also look at the definition of PAGE_MASK in page.h

Is that correct?  Most of the other related symbols have an "_AC(1,UL)" 
in them.  Why is this not also appropriate for PAGE_MASK?

It may also be a good idea to prepare and test a patch that defines 
PAGE_MASK much in the same way HPAGE_MASK is defined.

David Daney


> I'll do a v2 now.
>
> > >   #endif
> > >
> > >   #define STACK_TOP_MAX    TASK_SIZE
> > > @@ -62,7 +62,7 @@ extern unsigned int vced_count, vcei_count;
> > >    * support 16TB; the architectural reserve for future expansion is
> > >    * 8192EB ...
> > >    */
> > > -#define TASK_SIZE32    0x7fff8000UL
> > > +#define TASK_SIZE32    (0x7fff8000UL & PAGE_SIZE)
> >
> > Same here.
>
> As above.
>
> > >   #define TASK_SIZE64    0x10000000000UL
> > >   #define TASK_SIZE (test_thread_flag(TIF_32BIT_ADDR) ? TASK_SIZE32 :
> > > TASK_SIZE64)
> > >   #define STACK_TOP_MAX    TASK_SIZE64
>
> Thanks,
>
> Harvey