Re: [PATCH hotfix 6.12 v4 4/5] mm: refactor arch_calc_vm_flag_bits() and arm64 MTE handling

[Catalin Marinas <catalin.marinas@arm.com>]

On Tue, Oct 29, 2024 at 06:11:47PM +0000, Lorenzo Stoakes wrote:
> Currently MTE is permitted in two circumstances (desiring to use MTE having
> been specified by the VM_MTE flag) - where MAP_ANONYMOUS is specified, as
> checked by arch_calc_vm_flag_bits() and actualised by setting the
> VM_MTE_ALLOWED flag, or if the file backing the mapping is shmem, in which
> case we set VM_MTE_ALLOWED in shmem_mmap() when the mmap hook is activated
> in mmap_region().
> 
> The function that checks that, if VM_MTE is set, VM_MTE_ALLOWED is also set
> is the arm64 implementation of arch_validate_flags().
> 
> Unfortunately, we intend to refactor mmap_region() to perform this check
> earlier, meaning that in the case of a shmem backing we will not have
> invoked shmem_mmap() yet, causing the mapping to fail spuriously.
> 
> It is inappropriate to set this architecture-specific flag in general mm
> code anyway, so a sensible resolution of this issue is to instead move the
> check somewhere else.
> 
> We resolve this by setting VM_MTE_ALLOWED much earlier in do_mmap(), via
> the arch_calc_vm_flag_bits() call.
> 
> This is an appropriate place to do this as we already check for the
> MAP_ANONYMOUS case here, and the shmem file case is simply a variant of the
> same idea - we permit RAM-backed memory.
> 
> This requires a modification to the arch_calc_vm_flag_bits() signature to
> pass in a pointer to the struct file associated with the mapping, however
> this is not too egregious as this is only used by two architectures anyway
> - arm64 and parisc.
> 
> So this patch performs this adjustment and removes the unnecessary
> assignment of VM_MTE_ALLOWED in shmem_mmap().
> 
> Suggested-by: Catalin Marinas <catalin.marinas@arm.com>
> Reported-by: Jann Horn <jannh@google.com>
> Fixes: deb0f6562884 ("mm/mmap: undo ->mmap() when arch_validate_flags() fails")
> Cc: stable <stable@kernel.org>
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>

Thanks for respinning this. FTR,

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>

> @@ -151,13 +152,13 @@ calc_vm_prot_bits(unsigned long prot, unsigned long pkey)
>   * Combine the mmap "flags" argument into "vm_flags" used internally.
>   */
>  static inline unsigned long
> -calc_vm_flag_bits(unsigned long flags)
> +calc_vm_flag_bits(struct file *file, unsigned long flags)
>  {
>  	return _calc_vm_trans(flags, MAP_GROWSDOWN,  VM_GROWSDOWN ) |
>  	       _calc_vm_trans(flags, MAP_LOCKED,     VM_LOCKED    ) |
>  	       _calc_vm_trans(flags, MAP_SYNC,	     VM_SYNC      ) |
>  	       _calc_vm_trans(flags, MAP_STACK,	     VM_NOHUGEPAGE) |
> -	       arch_calc_vm_flag_bits(flags);
> +		arch_calc_vm_flag_bits(file, flags);

Nitpick (but please ignore, Andrew picked them up already): one space
alignment off.

-- 
Catalin