[PATCH v4 3/3] drm/tyr: allow userspace buffers to be mismatched

[Deborah Brouwer]

Allow userspace to provide a buffer that is smaller or larger than the
object that the kernel expects to write into it.

Set a minimum buffer size to match those in PANTHOR_UOBJ_MIN_SIZE. If
userspace provides at least the minimum buffer size, then simply write
until the buffer is full.

If userspace provides a larger buffer than necessary, then zero out the
remaining buffer.

This ensures UAPI compatibility with Panthor.

Signed-off-by: Deborah Brouwer <deborah.brouwer@xxxxxxxxxxxxx>
---
 drivers/gpu/drm/tyr/file.rs | 49 +++++++++++++++++++++++++++++++++++++--------
 1 file changed, 41 insertions(+), 8 deletions(-)

diff --git a/drivers/gpu/drm/tyr/file.rs b/drivers/gpu/drm/tyr/file.rs
index ceadde976502..bf66238ea4c6 100644
--- a/drivers/gpu/drm/tyr/file.rs
+++ b/drivers/gpu/drm/tyr/file.rs
@@ -1,14 +1,18 @@
 // SPDX-License-Identifier: GPL-2.0 or MIT
 
+use core::mem::offset_of;
+
 use kernel::{
     drm::{
         self,
         Registered, //
     },
+    num::casts::FromSafeCast,
     prelude::*,
     uaccess::UserSlice,
     uapi, //
 };
+use zerocopy::Immutable;
 
 use crate::driver::{
     TyrDrmDevice,
@@ -16,6 +20,37 @@
     TyrDrmRegistrationData, //
 };
 
+trait UobjMinSize {
+    fn min_size(&self) -> usize;
+}
+
+impl UobjMinSize for uapi::drm_panthor_gpu_info {
+    fn min_size(&self) -> usize {
+        offset_of!(Self, tiler_present) + size_of_val(&self.tiler_present)
+    }
+}
+
+// If the user buffer is at least the minimum required size, then
+// copy as much of the kernel object as will fit into the user buffer.
+// Don't fail if the user buffer is smaller than the kernel object.
+// If the user buffer is bigger than needed, zero out the remainder.
+fn set_uobj<T: IntoBytes + Immutable>(
+    usr_dst: u64,
+    size: u32,
+    min_size: usize,
+    src_obj: &T,
+) -> Result {
+    let size = usize::from_safe_cast(size);
+    if size < min_size {
+        return Err(EINVAL);
+    }
+
+    let mut writer = UserSlice::new(UserPtr::from_u64(usr_dst), size).writer();
+    writer.write_slice_partial(src_obj.as_bytes(), 0)?;
+    writer.clear()?;
+    Ok(())
+}
+
 #[pin_data]
 pub(crate) struct TyrDrmFileData {}
 
@@ -48,14 +83,12 @@ pub(crate) fn dev_query(
         } else {
             match devquery.type_ {
                 uapi::drm_panthor_dev_query_type_DRM_PANTHOR_DEV_QUERY_GPU_INFO => {
-                    let mut writer = UserSlice::new(
-                        UserPtr::from_addr(devquery.pointer as usize),
-                        devquery.size as usize,
-                    )
-                    .writer();
-
-                    writer.write_slice(reg_data.gpu_info.as_bytes())?;
-
+                    set_uobj(
+                        devquery.pointer,
+                        devquery.size,
+                        reg_data.gpu_info.min_size(),
+                        &reg_data.gpu_info,
+                    )?;
                     Ok(0)
                 }
                 _ => Err(EINVAL),

-- 
2.56.0