linux-kernel - Re: [PATCH 1/3] RISC-V: mm: fix mmap behavior in sv48 address space

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Date: Fri, 19 Jan 2024 17:50:36 -0800
From: Charlie Jenkins <charlie@...osinc.com>
To: Yangyu Chen <cyy@...self.name>
Cc: linux-riscv@...ts.infradead.org,
	Paul Walmsley <paul.walmsley@...ive.com>,
	Palmer Dabbelt <palmer@...belt.com>,
	Albert Ou <aou@...s.berkeley.edu>, Guo Ren <guoren@...nel.org>,
	Andy Chiu <andy.chiu@...ive.com>,
	Conor Dooley <conor.dooley@...rochip.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] RISC-V: mm: fix mmap behavior in sv48 address space

On Mon, Jan 15, 2024 at 03:58:30AM +0800, Yangyu Chen wrote:
> A commit add2cc6b6515 ("RISC-V: mm: Restrict address space for
> sv39,sv48,sv57") from patch[1] restricts regular mmap return address in the
> sv48 space if the address hint is not above the sv48 userspace address.
> However, this commit treats the address wrong which only use sv48 if the
> hint address is above sv48 user address space. Actually, it should use sv48
> if the address is above sv39 user address space. Moreover, the original

It is not valid to use any address in sv48 in this case. It is designed
to provide an address that is in sv39 to guarantee that the application
is able to handle any address that is returned by mmap. If, for example,
the application asks for the address 1<<42, it expects that all of the
bits above 42 are not used. The address provided by mmap must not
contain those bits, so it uses the sv39 address space.

- Charlie

> patch code looks very complex in logic, we can simplify it with min marco.
> 
> [1]. https://lore.kernel.org/r/20230809232218.849726-2-charlie@rivosinc.com
> 
> Signed-off-by: Yangyu Chen <cyy@...self.name>
> ---
>  arch/riscv/include/asm/processor.h | 39 ++++++------------------------
>  1 file changed, 8 insertions(+), 31 deletions(-)
> 
> diff --git a/arch/riscv/include/asm/processor.h b/arch/riscv/include/asm/processor.h
> index e1944ff0757a..7ead6a3e1f12 100644
> --- a/arch/riscv/include/asm/processor.h
> +++ b/arch/riscv/include/asm/processor.h
> @@ -9,6 +9,7 @@
>  #include <linux/const.h>
>  #include <linux/cache.h>
>  #include <linux/prctl.h>
> +#include <linux/minmax.h>
>  
>  #include <vdso/processor.h>
>  
> @@ -18,37 +19,13 @@
>  #define DEFAULT_MAP_WINDOW	(UL(1) << (MMAP_VA_BITS - 1))
>  #define STACK_TOP_MAX		TASK_SIZE
>  
> -#define arch_get_mmap_end(addr, len, flags)			\
> -({								\
> -	unsigned long mmap_end;					\
> -	typeof(addr) _addr = (addr);				\
> -	if ((_addr) == 0 || (IS_ENABLED(CONFIG_COMPAT) && is_compat_task())) \
> -		mmap_end = STACK_TOP_MAX;			\
> -	else if ((_addr) >= VA_USER_SV57)			\
> -		mmap_end = STACK_TOP_MAX;			\
> -	else if ((((_addr) >= VA_USER_SV48)) && (VA_BITS >= VA_BITS_SV48)) \
> -		mmap_end = VA_USER_SV48;			\
> -	else							\
> -		mmap_end = VA_USER_SV39;			\
> -	mmap_end;						\
> -})
> -
> -#define arch_get_mmap_base(addr, base)				\
> -({								\
> -	unsigned long mmap_base;				\
> -	typeof(addr) _addr = (addr);				\
> -	typeof(base) _base = (base);				\
> -	unsigned long rnd_gap = DEFAULT_MAP_WINDOW - (_base);	\
> -	if ((_addr) == 0 || (IS_ENABLED(CONFIG_COMPAT) && is_compat_task())) \
> -		mmap_base = (_base);				\
> -	else if (((_addr) >= VA_USER_SV57) && (VA_BITS >= VA_BITS_SV57)) \
> -		mmap_base = VA_USER_SV57 - rnd_gap;		\
> -	else if ((((_addr) >= VA_USER_SV48)) && (VA_BITS >= VA_BITS_SV48)) \
> -		mmap_base = VA_USER_SV48 - rnd_gap;		\
> -	else							\
> -		mmap_base = VA_USER_SV39 - rnd_gap;		\
> -	mmap_base;						\
> -})
> +#define arch_get_mmap_end(addr, len, flags) \
> +	((addr) >= DEFAULT_MAP_WINDOW ? STACK_TOP_MAX :\
> +	 min(DEFAULT_MAP_WINDOW, STACK_TOP_MAX))
> +
> +#define arch_get_mmap_base(addr, base) \
> +	((addr) >= DEFAULT_MAP_WINDOW ? base :\
> +	 min(base, DEFAULT_MAP_WINDOW))
>  
>  #else
>  #define DEFAULT_MAP_WINDOW	TASK_SIZE
> -- 
> 2.43.0
>