feat(storage): ext4-only disk via mkfs.ext4 -d; drop FAT16; feature-gated mount (ro on unknown ro_compat); ext4 read spans blocks, delete clears inode, mkdir counts dir; virtio-blk writes; test-qemu e2fsck gate

crates/drivers/block/virtio/src/lib.rs

@@ -26,6 +26,7 @@ pub struct VirtioBlockDevice {
     mmio: UserMmio,
     queue: VirtioQueue,
     dma: DmaAddrs,
+    sectors: usize,
 }
 
 impl VirtioBlockDevice {
@@ -52,20 +53,23 @@ impl VirtioBlockDevice {
             data_pa: PhysicalAddress::from_raw(hdr_pa + 32),
             data_va: VirtualAddress::from_raw(REQ_HDR_VA + 32),
         };
+        let capacity = regs.read(block::CONFIG_CAPACITY) as u64
+            | (regs.read(block::CONFIG_CAPACITY + 4) as u64) << 32;
         Some(Self {
             mmio: UserMmio,
             queue: VirtioQueue::new(QueueAddrs::from_base(VQUEUE_VA)),
             dma,
+            sectors: usize::try_from(capacity).ok()?,
         })
     }
 }
 
 impl BlockDevice for VirtioBlockDevice {
     fn sector_size(&self) -> usize { SECTOR_SIZE }
-    fn sector_count(&self) -> usize { 0xFFFF }
+    fn sector_count(&self) -> usize { self.sectors }
 
     fn read_sector(&mut self, sector: SectorId, buf: &mut [u8]) -> bool {
-        if buf.len() < SECTOR_SIZE { return false; }
+        if buf.len() < SECTOR_SIZE || sector.raw() as usize >= self.sectors { return false; }
         let mut sector_buf = [0u8; SECTOR_SIZE];
         if !block::read_sector(&self.mmio, &self.queue, &self.dma, sector.raw(), &mut sector_buf) {
             return false;
@@ -74,8 +78,8 @@ impl BlockDevice for VirtioBlockDevice {
         true
     }
 
-    fn write_sector(&mut self, _sector: SectorId, _buf: &[u8]) -> bool { false }
-
-    /// Writing is not implemented yet.
-    fn writable(&self) -> bool { false }
+    fn write_sector(&mut self, sector: SectorId, buf: &[u8]) -> bool {
+        if sector.raw() as usize >= self.sectors { return false; }
+        block::write_sector(&self.mmio, &self.queue, &self.dma, sector.raw(), buf)
+    }
 }