Re: [PATCH v2 4/6] mm/vma: make MAP_PRIVATE-mapped /dev/zero mappings truly anonymous

[David Hildenbrand (Arm)]

On 9/8/26 13:23, Lorenzo Stoakes (ARM) wrote:
> When mapping /dev/zero with MAP_PRIVATE, one ends up with strange VMAs
> originating from Linux's distant past.
> 
> These have vma->vm_file set but NULL vma->vm_ops, meaning they satisfy
> vma_is_anonymous() but otherwise resemble a file-backed VMA.
> 
> The introduction of anonymous page offsets and their subsequent use as
> indexes for MAP_PRIVATE-file-backed mappings mean the rmap does the right
> thing with these but we are left with inconsistencies.
> 
> The vma_start_pgoff(vma) == vma_start_anon_pgoff(vma) invariant is true for
> all other anonymous VMAs, but not these.
> 
> These VMAs are also observable as files in /proc/<pid>/[maps, smaps,
> map_files] but otherwise behave like anonymous mappings.
> 
> Therefore let's make these VMAs actually anonymous at mapping time which
> will activate the anonymous code path for mappings.
> 
> This means we no longer have to account for this discrepancy anywhere and
> no longer have to think about these at all.
> 
> This is user-observable, as MAP_PRIVATE-/dev/zero will no longer appear in
> procfs as a file-backed mapping, but the impact of this change should be
> low as likely nobody is relying upon this.
> 
> However in any case, in using MAP_PRIVATE-/dev/zero they are explicitly
> asking anonymous memory, so no longer seeing these as file mappings is in
> fact correct.
> 
> A previous commit gave us file_is_dev_zero() to positively identify these
> mappings, so we expressly only do so for these alone.
> 
> Update assert_sane_pgoff(), the comment for vma_start_pgoff() and
> linear_anon_page_index() to reflect the change.
> 
> We make this change in call_mmap_prepare() alone as /dev/zero has been
> converted to an mmap_prepare hook and we do not permit nested MAP_PRIVATE
> mapping of /dev/zero.
> 
> We also remove the now defunct vma_desc_set_anonymous() and eliminate the
> temporary bisection hazard fix from the previous commit.
> 
> Also update the VMA userland tests to reflect the change.
> 
> Finally, update the procfs self tests proc-self-map-files-001 and
> proc-self-map-files-002 which both intend to map an arbitrary file
> MAP_PRIVATE then assert procfs state, but happen to choose /dev/zero.
> 
> Fix them by updating these to /proc/self/exe which is guaranteed to be
> present if procfs is mounted.
> 
> Signed-off-by: Lorenzo Stoakes (ARM) <ljs@xxxxxxxxxx>
> ---

Acked-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>

-- 
Cheers,

David