vhost-user: honor requested config size and ignore offset

QEMU and Cloud-Hypervisor differ in how they request config:

- QEMU always uses offset 0 and may request a smaller, feature-dependent
  size.
- Cloud-Hypervisor uses VHOST_USER_CONFIG_OFFSET (0x100) and always
  requests the full config size.

The vhost-user spec requires the backend response size to exactly match
the frontend request. To satisfy both implementations, ignore the offset
and return the first N bytes of the config, where N is the requested
size.

backport of commit 788307c.

Author: pykello

.github/workflows/ci.yaml

@@ -37,6 +37,7 @@ jobs:
           meson setup build --prefix=/usr/local
           meson compile -C build
           sudo --preserve-env=PATH,RUSTUP_HOME,CARGO_HOME,HOME meson install -C build
+          sudo ldconfig
 
       - name: Clone and build isa-l_crypto
         run: |

Cargo.lock

@@ -1,6 +1,6 @@
 [package]
 name = "ubiblk"
-version = "0.2.1"
+version = "0.2.2"
 edition = "2021"
 
 [profile.release]

Cargo.toml

@@ -19,7 +19,7 @@ use super::{backend_thread::UbiBlkBackendThread, KeyEncryptionCipher, Options};
 use crate::block_device::{init_metadata as init_metadata_file, UbiMetadata};
 use crate::utils::aligned_buffer::BUFFER_ALIGNMENT;
 use crate::Result;
-use log::{debug, error, info};
+use log::{debug, error, info, warn};
 use nix::sys::statfs::statfs;
 use std::path::Path;
 use vhost::vhost_user::message::*;
@@ -271,8 +271,37 @@ impl VhostUserBackend for UbiBlkBackend {
         }
     }
 
-    fn get_config(&self, _offset: u32, _size: u32) -> Vec<u8> {
-        self.config.as_slice().to_vec()
+    fn get_config(&self, _offset: u32, size: u32) -> Vec<u8> {
+        // 1. QEMU (tag v8.2.2):
+        //    a. sets offset to 0 at vhost_user_get_config
+        //       (hw/virtio/vhost-user.c:2414)
+        //    b. calculates size dynamically based on available features in
+        //       virtio_get_config_size (hw/virtio/virtio.c:2956). This can be
+        //       less than the full config struct size.
+        // 2. Cloud-Hypervisor (tag v46.0):
+        //    a. sets offset to VHOST_USER_CONFIG_OFFSET (0x100), which is the
+        //       starting position of the device configuration space in virtio
+        //       devices (virtio-devices/src/vhost_user/blk.rs:147)
+        //    b. always sets size to the full config struct size
+        //       (virtio-devices/src/vhost_user/blk.rs:142)
+        // 3. vhost-user spec's requirement for VHOST_USER_GET_CONFIG:
+        //    > vhost-user back-end’s payload size MUST match the front-end’s
+        //    > request
+        // Based on above, we ignore the offset and return the first "size"
+        // bytes of the config struct, which works for both QEMU and
+        // Cloud-Hypervisor.
+        let config_bytes = self.config.as_slice();
+        let requested_size = size as usize;
+        if size as usize > config_bytes.len() {
+            warn!(
+                "get_config requested size {size} exceeds config struct size {}, padding with zeros",
+                config_bytes.len()
+            );
+        }
+        let mut buf = vec![0u8; requested_size];
+        let copy_len = std::cmp::min(requested_size, config_bytes.len());
+        buf[..copy_len].copy_from_slice(&config_bytes[..copy_len]);
+        buf
     }
 
     fn exit_event(&self, thread_index: usize) -> Option<EventFd> {