patches/mm-update-mem-char-driver-to-use-mmap_prepare.patch

From: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Subject: mm: update mem char driver to use mmap_prepare
Date: Mon, 20 Oct 2025 13:11:31 +0100

Update the mem char driver (backing /dev/mem and /dev/zero) to use
f_op->mmap_prepare hook rather than the deprecated f_op->mmap.

The /dev/zero implementation has a very unique and rather concerning
characteristic in that it converts MAP_PRIVATE mmap() mappings anonymous
when they are, in fact, not.

The new f_op->mmap_prepare() can support this, but rather than introducing
a helper function to perform this hack (and risk introducing other users),
utilise the success hook to do so.

We utilise the newly introduced shmem_zero_setup_desc() to allow for the
shared mapping case via an f_op->mmap_prepare() hook.

We also use the desc->action_error_hook to filter the remap error to
-EAGAIN to keep behaviour consistent.

Link: https://lkml.kernel.org/r/48f60764d7a6901819d1af778fa33b775d2e8c77.1760959442.git.lorenzo.stoakes@oracle.com
Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Cc: Alexander Gordeev <agordeev@linux.ibm.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Andreas Larsson <andreas@gaisler.com>
Cc: Andrey Konovalov <andreyknvl@gmail.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Baoquan He <bhe@redhat.com>
Cc: Chatre, Reinette <reinette.chatre@intel.com>
Cc: Christian Borntraeger <borntraeger@linux.ibm.com>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Dave Jiang <dave.jiang@intel.com>
Cc: Dave Martin <dave.martin@arm.com>
Cc: Dave Young <dyoung@redhat.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: David S. Miller <davem@davemloft.net>
Cc: Dmitriy Vyukov <dvyukov@google.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Guo Ren <guoren@kernel.org>
Cc: Heiko Carstens <hca@linux.ibm.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: James Morse <james.morse@arm.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Jann Horn <jannh@google.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Kevin Tian <kevin.tian@intel.com>
Cc: Konstantin Komarov <almaz.alexandrovich@paragon-software.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: "Luck, Tony" <tony.luck@intel.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Nicolas Pitre <nico@fluxnic.net>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Pedro Falcato <pfalcato@suse.de>
Cc: Robin Murohy <robin.murphy@arm.com>
Cc: Sumanth Korikkar <sumanthk@linux.ibm.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Sven Schnelle <svens@linux.ibm.com>
Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Cc: "Uladzislau Rezki (Sony)" <urezki@gmail.com>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Cc: Vishal Verma <vishal.l.verma@intel.com>
Cc: Vivek Goyal <vgoyal@redhat.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Will Deacon <will@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 drivers/char/mem.c |   88 +++++++++++++++++++++++++------------------
 1 file changed, 52 insertions(+), 36 deletions(-)

--- a/drivers/char/mem.c~mm-update-mem-char-driver-to-use-mmap_prepare
+++ a/drivers/char/mem.c
@@ -304,13 +304,13 @@ static unsigned zero_mmap_capabilities(s
 }
 
 /* can't do an in-place private mapping if there's no MMU */
-static inline int private_mapping_ok(struct vm_area_struct *vma)
+static inline int private_mapping_ok(struct vm_area_desc *desc)
 {
-	return is_nommu_shared_mapping(vma->vm_flags);
+	return is_nommu_shared_mapping(desc->vm_flags);
 }
 #else
 
-static inline int private_mapping_ok(struct vm_area_struct *vma)
+static inline int private_mapping_ok(struct vm_area_desc *desc)
 {
 	return 1;
 }
@@ -322,46 +322,49 @@ static const struct vm_operations_struct
 #endif
 };
 
-static int mmap_mem(struct file *file, struct vm_area_struct *vma)
+static int mmap_filter_error(int err)
 {
-	size_t size = vma->vm_end - vma->vm_start;
-	phys_addr_t offset = (phys_addr_t)vma->vm_pgoff << PAGE_SHIFT;
+	return -EAGAIN;
+}
+
+static int mmap_mem_prepare(struct vm_area_desc *desc)
+{
+	struct file *file = desc->file;
+	const size_t size = vma_desc_size(desc);
+	const phys_addr_t offset = (phys_addr_t)desc->pgoff << PAGE_SHIFT;
 
 	/* Does it even fit in phys_addr_t? */
-	if (offset >> PAGE_SHIFT != vma->vm_pgoff)
+	if (offset >> PAGE_SHIFT != desc->pgoff)
 		return -EINVAL;
 
 	/* It's illegal to wrap around the end of the physical address space. */
 	if (offset + (phys_addr_t)size - 1 < offset)
 		return -EINVAL;
 
-	if (!valid_mmap_phys_addr_range(vma->vm_pgoff, size))
+	if (!valid_mmap_phys_addr_range(desc->pgoff, size))
 		return -EINVAL;
 
-	if (!private_mapping_ok(vma))
+	if (!private_mapping_ok(desc))
 		return -ENOSYS;
 
-	if (!range_is_allowed(vma->vm_pgoff, size))
+	if (!range_is_allowed(desc->pgoff, size))
 		return -EPERM;
 
-	if (!phys_mem_access_prot_allowed(file, vma->vm_pgoff, size,
-						&vma->vm_page_prot))
+	if (!phys_mem_access_prot_allowed(file, desc->pgoff, size,
+					  &desc->page_prot))
 		return -EINVAL;
 
-	vma->vm_page_prot = phys_mem_access_prot(file, vma->vm_pgoff,
-						 size,
-						 vma->vm_page_prot);
-
-	vma->vm_ops = &mmap_mem_ops;
-
-	/* Remap-pfn-range will mark the range VM_IO */
-	if (remap_pfn_range(vma,
-			    vma->vm_start,
-			    vma->vm_pgoff,
-			    size,
-			    vma->vm_page_prot)) {
-		return -EAGAIN;
-	}
+	desc->page_prot = phys_mem_access_prot(file, desc->pgoff,
+					       size,
+					       desc->page_prot);
+
+	desc->vm_ops = &mmap_mem_ops;
+
+	/* Remap-pfn-range will mark the range VM_IO. */
+	mmap_action_remap_full(desc, desc->pgoff);
+	/* We filter remap errors to -EAGAIN. */
+	desc->action.error_hook = mmap_filter_error;
+
 	return 0;
 }
 
@@ -501,14 +504,26 @@ static ssize_t read_zero(struct file *fi
 	return cleared;
 }
 
-static int mmap_zero(struct file *file, struct vm_area_struct *vma)
+static int mmap_zero_private_success(const struct vm_area_struct *vma)
+{
+	/*
+	 * This is a highly unique situation where we mark a MAP_PRIVATE mapping
+	 * of /dev/zero anonymous, despite it not being.
+	 */
+	vma_set_anonymous((struct vm_area_struct *)vma);
+
+	return 0;
+}
+
+static int mmap_zero_prepare(struct vm_area_desc *desc)
 {
 #ifndef CONFIG_MMU
 	return -ENOSYS;
 #endif
-	if (vma->vm_flags & VM_SHARED)
-		return shmem_zero_setup(vma);
-	vma_set_anonymous(vma);
+	if (desc->vm_flags & VM_SHARED)
+		return shmem_zero_setup_desc(desc);
+
+	desc->action.success_hook = mmap_zero_private_success;
 	return 0;
 }
 
@@ -526,10 +541,11 @@ static unsigned long get_unmapped_area_z
 {
 	if (flags & MAP_SHARED) {
 		/*
-		 * mmap_zero() will call shmem_zero_setup() to create a file,
-		 * so use shmem's get_unmapped_area in case it can be huge;
-		 * and pass NULL for file as in mmap.c's get_unmapped_area(),
-		 * so as not to confuse shmem with our handle on "/dev/zero".
+		 * mmap_zero_prepare() will call shmem_zero_setup() to create a
+		 * file, so use shmem's get_unmapped_area in case it can be
+		 * huge; and pass NULL for file as in mmap.c's
+		 * get_unmapped_area(), so as not to confuse shmem with our
+		 * handle on "/dev/zero".
 		 */
 		return shmem_get_unmapped_area(NULL, addr, len, pgoff, flags);
 	}
@@ -632,7 +648,7 @@ static const struct file_operations __ma
 	.llseek		= memory_lseek,
 	.read		= read_mem,
 	.write		= write_mem,
-	.mmap		= mmap_mem,
+	.mmap_prepare	= mmap_mem_prepare,
 	.open		= open_mem,
 #ifndef CONFIG_MMU
 	.get_unmapped_area = get_unmapped_area_mem,
@@ -668,7 +684,7 @@ static const struct file_operations zero
 	.write_iter	= write_iter_zero,
 	.splice_read	= copy_splice_read,
 	.splice_write	= splice_write_zero,
-	.mmap		= mmap_zero,
+	.mmap_prepare	= mmap_zero_prepare,
 	.get_unmapped_area = get_unmapped_area_zero,
 #ifndef CONFIG_MMU
 	.mmap_capabilities = zero_mmap_capabilities,
_