Re: [PATCH v2 1/3] mm: introduce new .mmap_prepare() file callback

[David Hildenbrand <david@redhat.com>]

On 09.05.25 14:13, Lorenzo Stoakes wrote:
> Provide a means by which drivers can specify which fields of those
> permitted to be changed should be altered to prior to mmap()'ing a
> range (which may either result from a merge or from mapping an entirely new
> VMA).
> 
> Doing so is substantially safer than the existing .mmap() calback which
> provides unrestricted access to the part-constructed VMA and permits
> drivers and file systems to do 'creative' things which makes it hard to
> reason about the state of the VMA after the function returns.
> 
> The existing .mmap() callback's freedom has caused a great deal of issues,
> especially in error handling, as unwinding the mmap() state has proven to
> be non-trivial and caused significant issues in the past, for instance
> those addressed in commit 5de195060b2e ("mm: resolve faulty mmap_region()
> error path behaviour").
> 
> It also necessitates a second attempt at merge once the .mmap() callback
> has completed, which has caused issues in the past, is awkward, adds
> overhead and is difficult to reason about.
> 
> The .mmap_prepare() callback eliminates this requirement, as we can update
> fields prior to even attempting the first merge. It is safer, as we heavily
> restrict what can actually be modified, and being invoked very early in the
> mmap() process, error handling can be performed safely with very little
> unwinding of state required.
> 
> The .mmap_prepare() and deprecated .mmap() callbacks are mutually
> exclusive, so we permit only one to be invoked at a time.
> 
> Update vma userland test stubs to account for changes.
> 
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> ---
>   include/linux/fs.h               | 25 ++++++++++++
>   include/linux/mm_types.h         | 24 +++++++++++
>   mm/memory.c                      |  3 +-
>   mm/mmap.c                        |  2 +-
>   mm/vma.c                         | 68 +++++++++++++++++++++++++++++++-
>   tools/testing/vma/vma_internal.h | 66 ++++++++++++++++++++++++++++---
>   6 files changed, 180 insertions(+), 8 deletions(-)
> 
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 016b0fe1536e..e2721a1ff13d 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -2169,6 +2169,7 @@ struct file_operations {
>   	int (*uring_cmd)(struct io_uring_cmd *ioucmd, unsigned int issue_flags);
>   	int (*uring_cmd_iopoll)(struct io_uring_cmd *, struct io_comp_batch *,
>   				unsigned int poll_flags);
> +	int (*mmap_prepare)(struct vm_area_desc *);
>   } __randomize_layout;
>   
>   /* Supports async buffered reads */
> @@ -2238,11 +2239,35 @@ struct inode_operations {
>   	struct offset_ctx *(*get_offset_ctx)(struct inode *inode);
>   } ____cacheline_aligned;
>   
> +/* Did the driver provide valid mmap hook configuration? */
> +static inline bool file_has_valid_mmap_hooks(struct file *file)
> +{
> +	bool has_mmap = file->f_op->mmap;
> +	bool has_mmap_prepare = file->f_op->mmap_prepare;
> +
> +	/* Hooks are mutually exclusive. */
> +	if (WARN_ON_ONCE(has_mmap && has_mmap_prepare))
> +		return false;
> +	if (WARN_ON_ONCE(!has_mmap && !has_mmap_prepare))
> +		return false;
> +
> +	return true;
> +}

So, if neither is set, it's also an invalid setting, understood.

So we want XOR.



const bool has_mmap = file->f_op->mmap;
const bool has_mmap_prepare = file->f_op->mmap_prepare;
const bool mutual_exclusive = has_mmap ^ has_mmap_prepare;

WARN_ON_ONCE(!mutual_exclusive)
return mutual_exclusive;

-- 
Cheers,

David / dhildenb