Rather than providing a hook, simplify things by providing the ability to
override mmap action errors.  This allows us to more carefully validate the
value provided and thus ensure only a valid error code is specified, and
simplifies the interface.

This way, we eliminate all hooks but mmap_prepare and allow only mmap
actions to be specified (which core mm controls).

This significantly improves robustness and eliminates any unnecessary code
duplication in driver mmap hooks.

We also update the /dev/mem logic (the only user) to use
mmap_action->error_override instead.

Signed-off-by: Lorenzo Stoakes <ljs@kernel.org>
---
 drivers/char/mem.c              |  8 +-------
 include/linux/mm_types.h        |  9 +++------
 mm/util.c                       | 29 +++++++++++++++++++++--------
 tools/testing/vma/include/dup.h |  9 +++------
 4 files changed, 28 insertions(+), 27 deletions(-)

diff --git a/drivers/char/mem.c b/drivers/char/mem.c
index a4297eb39887..63253d1de5d7 100644
--- a/drivers/char/mem.c
+++ b/drivers/char/mem.c
@@ -322,11 +322,6 @@ static const struct vm_operations_struct mmap_mem_ops = {
 #endif
 };
 
-static int mmap_filter_error(int err)
-{
-	return -EAGAIN;
-}
-
 static int mmap_mem_prepare(struct vm_area_desc *desc)
 {
 	struct file *file = desc->file;
@@ -362,8 +357,7 @@ static int mmap_mem_prepare(struct vm_area_desc *desc)
 
 	/* Remap-pfn-range will mark the range with the I/O flag. */
 	mmap_action_remap_full(desc, desc->pgoff);
-	/* We filter remap errors to -EAGAIN. */
-	desc->action.error_hook = mmap_filter_error;
+	desc->action.error_override = -EAGAIN;
 
 	return 0;
 }
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 945c0a5386d6..5ef78617ce93 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -844,13 +844,10 @@ struct mmap_action {
 	enum mmap_action_type type;
 
 	/*
-	 * If specified, this hook is invoked when an error occurred when
-	 * attempting the selected action.
-	 *
-	 * The hook can return an error code in order to filter the error, but
-	 * it is not valid to clear the error here.
+	 * If non-zero, replace errors that arise from mmap actions with this
+	 * value instead. Only valid error codes may be specified.
 	 */
-	int (*error_hook)(int err);
+	int error_override;
 
 	/*
 	 * This should be set in rare instances where the operation required
diff --git a/mm/util.c b/mm/util.c
index 4e172990afcd..af2c2103f0d9 100644
--- a/mm/util.c
+++ b/mm/util.c
@@ -1414,16 +1414,22 @@ static int mmap_action_finish(struct vm_area_struct *vma,
 	 */
 	len = vma_pages(vma) << PAGE_SHIFT;
 	do_munmap(current->mm, vma->vm_start, len, NULL);
-	if (action->error_hook) {
-		/* We may want to filter the error. */
-		err = action->error_hook(err);
-		/* The caller should not clear the error. */
-		VM_WARN_ON_ONCE(!err);
-	}
-	return err;
+
+	return action->error_override ?: err;
 }
 
 #ifdef CONFIG_MMU
+
+static int check_mmap_action(struct mmap_action *action)
+{
+	const unsigned long override = action->error_override;
+
+	if (WARN_ON_ONCE(override && !IS_ERR_VALUE(override)))
+		return -EINVAL;
+
+	return 0;
+}
+
 /**
  * mmap_action_prepare - Perform preparatory setup for an VMA descriptor
  * action which need to be performed.
@@ -1433,7 +1439,14 @@ static int mmap_action_finish(struct vm_area_struct *vma,
  */
 int mmap_action_prepare(struct vm_area_desc *desc)
 {
-	switch (desc->action.type) {
+	struct mmap_action *action = &desc->action;
+	int err;
+
+	err = check_mmap_action(action);
+	if (err)
+		return err;
+
+	switch (action->type) {
 	case MMAP_NOTHING:
 		return 0;
 	case MMAP_REMAP_PFN:
diff --git a/tools/testing/vma/include/dup.h b/tools/testing/vma/include/dup.h
index fddfd1b57c09..bf26b3f48d3a 100644
--- a/tools/testing/vma/include/dup.h
+++ b/tools/testing/vma/include/dup.h
@@ -483,13 +483,10 @@ struct mmap_action {
 	enum mmap_action_type type;
 
 	/*
-	 * If specified, this hook is invoked when an error occurred when
-	 * attempting the selection action.
-	 *
-	 * The hook can return an error code in order to filter the error, but
-	 * it is not valid to clear the error here.
+	 * If non-zero, replace errors that arise from mmap actions with this
+	 * value instead. Only valid error codes may be specified.
 	 */
-	int (*error_hook)(int err);
+	int error_override;
 
 	/*
 	 * This should be set in rare instances where the operation required
-- 
2.54.0