Re: [PATCH v2 rebased 7/9] powerpc/mm: Convert to default topdown mmap layout

[Nicholas Piggin <npiggin@gmail.com>]

Excerpts from Christophe Leroy's message of November 26, 2021 3:52 am:
> Select CONFIG_ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT and
> remove arch/powerpc/mm/mmap.c
> 
> This change provides standard randomisation of mmaps.
> 
> See commit 8b8addf891de ("x86/mm/32: Enable full randomization on i386
> and X86_32") for all the benefits of mmap randomisation.

What's the actual difference before/after this patch? The mm/util.c
code looks very similar to the code removed with a few small 
differences. Can these differences be explained in this patch?

Thanks,
Nick

> 
> Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
> ---
> v2: Also remove selection of ARCH_HAS_ELF_RANDOMIZE as it is already selected by CONFIG_ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT
> ---
>  arch/powerpc/Kconfig                 |   2 +-
>  arch/powerpc/include/asm/processor.h |   2 -
>  arch/powerpc/mm/Makefile             |   2 +-
>  arch/powerpc/mm/mmap.c               | 105 ---------------------------
>  4 files changed, 2 insertions(+), 109 deletions(-)
>  delete mode 100644 arch/powerpc/mm/mmap.c
> 
> diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
> index fb48823ccd62..20504a9901f2 100644
> --- a/arch/powerpc/Kconfig
> +++ b/arch/powerpc/Kconfig
> @@ -122,7 +122,6 @@ config PPC
>  	select ARCH_HAS_DEBUG_WX		if STRICT_KERNEL_RWX
>  	select ARCH_HAS_DEVMEM_IS_ALLOWED
>  	select ARCH_HAS_DMA_MAP_DIRECT 		if PPC_PSERIES
> -	select ARCH_HAS_ELF_RANDOMIZE
>  	select ARCH_HAS_FORTIFY_SOURCE
>  	select ARCH_HAS_GCOV_PROFILE_ALL
>  	select ARCH_HAS_HUGEPD			if HUGETLB_PAGE
> @@ -158,6 +157,7 @@ config PPC
>  	select ARCH_USE_MEMTEST
>  	select ARCH_USE_QUEUED_RWLOCKS		if PPC_QUEUED_SPINLOCKS
>  	select ARCH_USE_QUEUED_SPINLOCKS	if PPC_QUEUED_SPINLOCKS
> +	select ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT
>  	select ARCH_WANT_IPC_PARSE_VERSION
>  	select ARCH_WANT_IRQS_OFF_ACTIVATE_MM
>  	select ARCH_WANT_LD_ORPHAN_WARN
> diff --git a/arch/powerpc/include/asm/processor.h b/arch/powerpc/include/asm/processor.h
> index e39bd0ff69f3..d906b14dd599 100644
> --- a/arch/powerpc/include/asm/processor.h
> +++ b/arch/powerpc/include/asm/processor.h
> @@ -378,8 +378,6 @@ static inline void prefetchw(const void *x)
>  
>  #define spin_lock_prefetch(x)	prefetchw(x)
>  
> -#define HAVE_ARCH_PICK_MMAP_LAYOUT
> -
>  /* asm stubs */
>  extern unsigned long isa300_idle_stop_noloss(unsigned long psscr_val);
>  extern unsigned long isa300_idle_stop_mayloss(unsigned long psscr_val);
> diff --git a/arch/powerpc/mm/Makefile b/arch/powerpc/mm/Makefile
> index d4c20484dad9..503a6e249940 100644
> --- a/arch/powerpc/mm/Makefile
> +++ b/arch/powerpc/mm/Makefile
> @@ -5,7 +5,7 @@
>  
>  ccflags-$(CONFIG_PPC64)	:= $(NO_MINIMAL_TOC)
>  
> -obj-y				:= fault.o mem.o pgtable.o mmap.o maccess.o pageattr.o \
> +obj-y				:= fault.o mem.o pgtable.o maccess.o pageattr.o \
>  				   init_$(BITS).o pgtable_$(BITS).o \
>  				   pgtable-frag.o ioremap.o ioremap_$(BITS).o \
>  				   init-common.o mmu_context.o drmem.o \
> diff --git a/arch/powerpc/mm/mmap.c b/arch/powerpc/mm/mmap.c
> deleted file mode 100644
> index 5972d619d274..000000000000
> --- a/arch/powerpc/mm/mmap.c
> +++ /dev/null
> @@ -1,105 +0,0 @@
> -// SPDX-License-Identifier: GPL-2.0-or-later
> -/*
> - *  flexible mmap layout support
> - *
> - * Copyright 2003-2004 Red Hat Inc., Durham, North Carolina.
> - * All Rights Reserved.
> - *
> - * Started by Ingo Molnar <mingo@elte.hu>
> - */
> -
> -#include <linux/personality.h>
> -#include <linux/mm.h>
> -#include <linux/random.h>
> -#include <linux/sched/signal.h>
> -#include <linux/sched/mm.h>
> -#include <linux/elf-randomize.h>
> -#include <linux/security.h>
> -#include <linux/mman.h>
> -
> -/*
> - * Top of mmap area (just below the process stack).
> - *
> - * Leave at least a ~128 MB hole.
> - */
> -#define MIN_GAP (128*1024*1024)
> -#define MAX_GAP (TASK_SIZE/6*5)
> -
> -static inline int mmap_is_legacy(struct rlimit *rlim_stack)
> -{
> -	if (current->personality & ADDR_COMPAT_LAYOUT)
> -		return 1;
> -
> -	if (rlim_stack->rlim_cur == RLIM_INFINITY)
> -		return 1;
> -
> -	return sysctl_legacy_va_layout;
> -}
> -
> -unsigned long arch_mmap_rnd(void)
> -{
> -	unsigned long shift, rnd;
> -
> -	shift = mmap_rnd_bits;
> -#ifdef CONFIG_COMPAT
> -	if (is_32bit_task())
> -		shift = mmap_rnd_compat_bits;
> -#endif
> -	rnd = get_random_long() % (1ul << shift);
> -
> -	return rnd << PAGE_SHIFT;
> -}
> -
> -static inline unsigned long stack_maxrandom_size(void)
> -{
> -	if (!(current->flags & PF_RANDOMIZE))
> -		return 0;
> -
> -	/* 8MB for 32bit, 1GB for 64bit */
> -	if (is_32bit_task())
> -		return (1<<23);
> -	else
> -		return (1<<30);
> -}
> -
> -static inline unsigned long mmap_base(unsigned long rnd,
> -				      struct rlimit *rlim_stack)
> -{
> -	unsigned long gap = rlim_stack->rlim_cur;
> -	unsigned long pad = stack_maxrandom_size() + stack_guard_gap;
> -
> -	/* Values close to RLIM_INFINITY can overflow. */
> -	if (gap + pad > gap)
> -		gap += pad;
> -
> -	if (gap < MIN_GAP)
> -		gap = MIN_GAP;
> -	else if (gap > MAX_GAP)
> -		gap = MAX_GAP;
> -
> -	return PAGE_ALIGN(DEFAULT_MAP_WINDOW - gap - rnd);
> -}
> -
> -/*
> - * This function, called very early during the creation of a new
> - * process VM image, sets up which VM layout function to use:
> - */
> -void arch_pick_mmap_layout(struct mm_struct *mm, struct rlimit *rlim_stack)
> -{
> -	unsigned long random_factor = 0UL;
> -
> -	if (current->flags & PF_RANDOMIZE)
> -		random_factor = arch_mmap_rnd();
> -
> -	/*
> -	 * Fall back to the standard layout if the personality
> -	 * bit is set, or if the expected stack growth is unlimited:
> -	 */
> -	if (mmap_is_legacy(rlim_stack)) {
> -		mm->mmap_base = TASK_UNMAPPED_BASE;
> -		mm->get_unmapped_area = arch_get_unmapped_area;
> -	} else {
> -		mm->mmap_base = mmap_base(random_factor, rlim_stack);
> -		mm->get_unmapped_area = arch_get_unmapped_area_topdown;
> -	}
> -}
> -- 
> 2.33.1
> 
> 
>