Re: [PATCH V2 2/4] riscv: mm: Fixup compat arch_get_mmap_end

[Charlie Jenkins]

On Fri, Dec 22, 2023 at 12:34:56AM -0300, Leonardo Bras wrote:
> On Thu, Dec 21, 2023 at 10:46:59AM -0500, guoren@xxxxxxxxxx wrote:
> > From: Guo Ren <guoren@xxxxxxxxxxxxxxxxx>
> > 
> > When the task is in COMPAT mode, the arch_get_mmap_end should be 2GB,
> > not TASK_SIZE_64. The TASK_SIZE has contained is_compat_mode()
> > detection, so change the definition of STACK_TOP_MAX to TASK_SIZE
> > directly.
> 
> ok
> 
> > 
> > Cc: stable@xxxxxxxxxxxxxxx
> > Fixes: add2cc6b6515 ("RISC-V: mm: Restrict address space for sv39,sv48,sv57")
> > Signed-off-by: Guo Ren <guoren@xxxxxxxxxxxxxxxxx>
> > Signed-off-by: Guo Ren <guoren@xxxxxxxxxx>
> > ---
> >  arch/riscv/include/asm/processor.h | 6 ++----
> >  1 file changed, 2 insertions(+), 4 deletions(-)
> > 
> > diff --git a/arch/riscv/include/asm/processor.h b/arch/riscv/include/asm/processor.h
> > index f19f861cda54..1f538fc4448d 100644
> > --- a/arch/riscv/include/asm/processor.h
> > +++ b/arch/riscv/include/asm/processor.h
> > @@ -16,15 +16,13 @@
> >  
> >  #ifdef CONFIG_64BIT
> >  #define DEFAULT_MAP_WINDOW	(UL(1) << (MMAP_VA_BITS - 1))
> > -#define STACK_TOP_MAX		TASK_SIZE_64
> > +#define STACK_TOP_MAX		TASK_SIZE
> 
> It means STACK_TOP_MAX will be in 64BIT:
> - TASK_SIZE_32 if compat_mode=y
> - TASK_SIZE_64 if compat_mode=n
> 
> Makes sense for me.
> 
> >  
> >  #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)			\
> > +	if ((_addr) == 0 || (_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;			\
> 
> 
> I don't think I got this change, or how it's connected to the commit msg.
> 
> Before:
> - addr == 0, or addr > 2^57, or compat: mmap_end = STACK_TOP_MAX
> - 2^48 < addr < 2^57: mmap_end = 2^48
> - 0 < addr < 2^48 : mmap_end = 2^39
> 
> Now:
> - addr == 0, or addr > 2^57: mmap_end = STACK_TOP_MAX
> - 2^48 < addr < 2^57: mmap_end = 2^48
> - 0 < addr < 2^48 : mmap_end = 2^39
> 
> IIUC compat mode addr will be < 2^32, so will always have mmap_end = 2^39 
> if addr != 0. Is that desireable? 
> (if not, above change is unneeded)

I agree, this change does not make sense for compat mode. Compat mode
should never return an address that is greater than 2^32, but this
change allows that.

> 
> Also, unrelated to the change:
> - 2^48 < addr < 2^57: mmap_end = 2^48
> Is the above correct?
> It looks like it should be 2^57 instead, and a new if clause for 
> 2^32 < addr < 2^48 should have mmap_end = 2^48.

That is not the case. I documented this behavior and reasoning in
Documentation/arch/riscv/vm-layout.rst in the "Userspace VAs" section.

I can reiterate here though. The hint address to mmap (defined here as
"addr") is the maximum userspace address that mmap should provide. What
you are describing is a minimum. The purpose of this change was to allow
applications that are not compatible with a larger virtual address (such
as applications like Java that use the upper bits of the VA to store
data) to have a consistent way of specifying how many bits they would
like to be left free in the VA. This requires to take the next lowest
address space to guaruntee that all of the most-significant bits left
clear in hint address do not end up populated in the virtual address
returned by mmap.

- Charlie

> 
> Do I get it wrong?
> 
> (I will send an RFC 'fixing' the code the way I am whinking it should look 
> like)
> 
> Thanks, 
> Leo
> 
> 
> 
> 
> 
> > -- 
> > 2.40.1
> > 
>