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

[Mark Brown]

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 | }
      | ^

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>

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.
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