[PATCH hotfix 6.12 v2 4/8] mm: resolve faulty mmap_region() error path behaviour

[Lorenzo Stoakes]

On Mon, Oct 28, 2024 at 09:05:33PM +0000, Mark Brown wrote:
> On Mon, Oct 28, 2024 at 08:43:08PM +0000, Lorenzo Stoakes wrote:
>
> > +/*
> > + * We check VMA flag validity early in the mmap() process, however this can
> > + * cause issues for arm64 when using MTE, which requires that it be used with
> > + * shmem and in this instance and only then is VM_MTE_ALLOWED set permitting
> > + * this operation.
> > + *
> > + * To avoid having to tear down a partially complete mapping we do this ahead of
> > + * time.
> > + */
> > +static vm_flags_t arch_adjust_flags(struct file *file, vm_flags_t vm_flags)
> > +{
> > +	if (!IS_ENABLED(CONFIG_ARM64))
> > +		return vm_flags;
> > +
> > +	if (shmem_file(file))
> > +		return vm_flags | VM_MTE_ALLOWED;
> > +}
>
> This doesn't build:
>
> mm/mmap.c:1595:1: error: non-void function does not return a value in all control paths [-Werror,-Wreturn-type]
>  1595 | }
>       | ^

Doh that'll teach me for rushing this...

>
> with that corrected:
>
> diff --git a/mm/mmap.c b/mm/mmap.c
> index d1ab4301c671..cea051c5fef3 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -1587,11 +1587,10 @@ static unsigned long __mmap_region(struct file *file, unsigned long addr,
>   */
>  static vm_flags_t arch_adjust_flags(struct file *file, vm_flags_t vm_flags)
>  {
> -	if (!IS_ENABLED(CONFIG_ARM64))
> -		return vm_flags;
> +	if (IS_ENABLED(CONFIG_ARM64) && shmem_file(file))
> +		vm_flags |= VM_MTE_ALLOWED;
>
> -	if (shmem_file(file))
> -		return vm_flags | VM_MTE_ALLOWED;
> +	return vm_flags;
>  }
>
>  unsigned long mmap_region(struct file *file, unsigned long addr,
>
> the relevant tests all pass for me.
>
> Tested-by: Mark Brown <broonie at kernel.org>

Thanks!

>
> I'd have expected arch_adjust_flags() to be something overridden by the
> arch headers (probably like arch_calc_vm_prot_bits() and friends), but
> if this is juat a short lived fix it's probably not worth the trouble.

Yeah this is just a sample solution that I had put together when Linus
suggested a sensible alternative which I'll code up...

Good to confirm this is definitely the issue thanks for testing!