Re: [PATCH] mm/filemap: fix NULL pointer dereference in do_read_cache_folio()

[Matthew Wilcox]

On Sat, Nov 15, 2025 at 01:07:29AM +0530, ssrane_b23@xxxxxxxxxxxxx wrote:
> When read_cache_folio() is called with a NULL filler function on a
> mapping that does not implement read_folio, a NULL pointer
> dereference occurs in filemap_read_folio().
> 
> The crash occurs when:
> 
> build_id_parse() is called on a VMA backed by a file from a
> filesystem that does not implement ->read_folio() (e.g. procfs,
> sysfs, or other virtual filesystems).

Not a fan of this approach, to be honest.  This should be caught at
a higher level.  In __build_id_parse(), there's already a check:

        /* only works for page backed storage  */
        if (!vma->vm_file)
                return -EINVAL;

which is funny because the comment is correct, but the code is not.
I suspect the right answer is to add right after it:

+	if (vma->vm_file->f_mapping->a_ops == &empty_aops)
+		return -EINVAL;

Want to test that out?