Re: [PATCH v9 2/3] mm: add a field to store names for private anonymous memory

[Rasmus Villemoes]

On 03/09/2021 01.18, Suren Baghdasaryan wrote:
> From: Colin Cross <ccross@xxxxxxxxxx>
> 
> 
> changes in v9
> - Changed max anon vma name length from 64 to 256 (as in the original patch)
> because I found one case of the name length being 139 bytes. If anyone is
> curious, here it is:
> dalvik-/data/dalvik-cache/arm64/apex@com.android.permission@priv-app@GooglePermissionController@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx@classes.art

I'm not sure that's a very convincing argument. We don't add code
arbitrarily just because some userspace code running on some custom
kernel (ab)uses something in that kernel. Surely that user can come up
with a name that doesn't contain GooglePermissionController twice.

The argument for using strings and not just a 128 bit uuid was that it
should (also) be human readable, and 250-byte strings are not that.
Also, there's no natural law forcing this to be some power-of-two, and
in fact the implementation means that it's actually somewhat harmful
(give it a 256 char name, and we'll do a 260 byte alloc, which becomes a
512 byte alloc). So just make the limit 80, the kernel's definition of a
sane line length. As for the allowed chars, it can be relaxed later if
convincing arguments can be made.


> +/* mmap_lock should be read-locked */
> +static inline bool is_same_vma_anon_name(struct vm_area_struct *vma,
> +					 const char *name)
> +{
> +	const char *vma_name = vma_anon_name(vma);
> +
> +	if (likely(!vma_name))
> +		return name == NULL;
> +
> +	return name && !strcmp(name, vma_name);

It's probably preferable to spell this

  /* either both NULL, or pointers to same refcounted string */
  if (vma_name == name)
      return true;

  return name && vma_name && !strcmp(name, vma_name);

so you have one less conditional in the common case.

Rasmus