Re: [Xen-devel] [PATCH v3 1/4] x86/mm: Adapt MODULES_END based on Fixmap section size

[Thomas Garnier <thgarnie@xxxxxxxxxx>]

On Wed, Feb 15, 2017 at 5:58 AM, Borislav Petkov <bp@xxxxxxx> wrote:
>
> On Tue, Feb 14, 2017 at 11:42:56AM -0800, Thomas Garnier wrote:
> > This patch aligns MODULES_END to the beginning of the Fixmap section.
> > It optimizes the space available for both sections. The address is
> > pre-computed based on the number of pages required by the Fixmap
> > section.
> >
> > It will allow GDT remapping in the Fixmap section. The current
> > MODULES_END static address does not provide enough space for the kernel
> > to support a large number of processors.
> >
> > Signed-off-by: Thomas Garnier <thgarnie@xxxxxxxxxx>
> > ---
> > Based on next-20170213
> > ---
> >  arch/x86/include/asm/fixmap.h           | 8 ++++++++
> >  arch/x86/include/asm/pgtable_64_types.h | 3 ---
> >  arch/x86/kernel/module.c                | 1 +
> >  arch/x86/mm/dump_pagetables.c           | 1 +
> >  arch/x86/mm/kasan_init_64.c             | 1 +
> >  5 files changed, 11 insertions(+), 3 deletions(-)
> >
> > diff --git a/arch/x86/include/asm/fixmap.h b/arch/x86/include/asm/fixmap.h
> > index 8554f960e21b..20231189e0e3 100644
> > --- a/arch/x86/include/asm/fixmap.h
> > +++ b/arch/x86/include/asm/fixmap.h
> > @@ -132,6 +132,14 @@ enum fixed_addresses {
> >
> >  extern void reserve_top_address(unsigned long reserve);
> >
> > +/* On 64-bit, the module sections ends with the start of the fixmap */
> > +#ifdef CONFIG_X86_64
> > +#define MODULES_VADDR    (__START_KERNEL_map + KERNEL_IMAGE_SIZE)
> > +#define MODULES_END   __fix_to_virt(__end_of_fixed_addresses + 1)
> > +#define MODULES_LEN   (MODULES_END - MODULES_VADDR)
> > +#endif /* CONFIG_X86_64 */
>
> JFYI: so there's another patchset which adds KERNEL_MAPPING_SIZE:
>
> https://lkml.kernel.org/r/1486040077-3719-1-git-send-email-bhe@xxxxxxxxxx
>
> and makes it a 1G, i.e., the KASLR default. I guess the above will have
> to be KERNEL_MAPPING_SIZE then.
>
> And why are you moving those to fixmap.h? What's wrong with
> including fixmap.h into pgtable_64_types.h so that you can get
> __end_of_fixed_addresses?
>
> FWIW, I didn't even have to add any includes with my .config, i.e., that 
> builds:
>

I tried to add fixmap.h and I thought I tried without too but maybe
not, I will try to adapt it on v4.

> ---
> diff --git a/arch/x86/include/asm/pgtable_64_types.h 
> b/arch/x86/include/asm/pgtable_64_types.h
> index 3a264200c62f..eda7fa856fa9 100644
> --- a/arch/x86/include/asm/pgtable_64_types.h
> +++ b/arch/x86/include/asm/pgtable_64_types.h
> @@ -67,7 +67,7 @@ typedef struct { pteval_t pte; } pte_t;
>  #endif /* CONFIG_RANDOMIZE_MEMORY */
>  #define VMALLOC_END    (VMALLOC_START + _AC((VMALLOC_SIZE_TB << 40) - 1, UL))
>  #define MODULES_VADDR    (__START_KERNEL_map + KERNEL_IMAGE_SIZE)
> -#define MODULES_END      _AC(0xffffffffff000000, UL)
> +#define MODULES_END   __fix_to_virt(__end_of_fixed_addresses + 1)
>  #define MODULES_LEN   (MODULES_END - MODULES_VADDR)
>  #define ESPFIX_PGD_ENTRY _AC(-2, UL)
>  #define ESPFIX_BASE_ADDR (ESPFIX_PGD_ENTRY << PGDIR_SHIFT)
> ---
>
> but I wouldn't be surprised if some strange configuration would need it.
>
> >  #define FIXADDR_SIZE (__end_of_permanent_fixed_addresses << PAGE_SHIFT)
> >  #define FIXADDR_START                (FIXADDR_TOP - FIXADDR_SIZE)
> >
> > diff --git a/arch/x86/include/asm/pgtable_64_types.h 
> > b/arch/x86/include/asm/pgtable_64_types.h
> > index 3a264200c62f..de8bace10200 100644
> > --- a/arch/x86/include/asm/pgtable_64_types.h
> > +++ b/arch/x86/include/asm/pgtable_64_types.h
> > @@ -66,9 +66,6 @@ typedef struct { pteval_t pte; } pte_t;
> >  #define VMEMMAP_START        __VMEMMAP_BASE
> >  #endif /* CONFIG_RANDOMIZE_MEMORY */
> >  #define VMALLOC_END  (VMALLOC_START + _AC((VMALLOC_SIZE_TB << 40) - 1, UL))
> > -#define MODULES_VADDR    (__START_KERNEL_map + KERNEL_IMAGE_SIZE)
> > -#define MODULES_END      _AC(0xffffffffff000000, UL)
>
> How much of an ABI breakage would that be? See
> Documentation/x86/x86_64/mm.txt.
>
> With my .config MODULES_END becomes 0xffffffffff1fe000 and it'll remain
> dynamic depending on .config. No idea how much in userspace relies on
> MODULES_END being static 0xffffffffff000000...
>
> Hmm.
>

Why do you think they rely on it being static? The VSYSCALL address is
not changed for example.

> --
> Regards/Gruss,
>     Boris.
>
> SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 
> 21284 (AG Nürnberg)
> --

Thanks for the feedback.


-- 
Thomas

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