ipcd shm: enforce O_RDONLY/O_WRONLY/O_RDWR access modes and zero-fill on grow

Round 6 stub scan follow-up: resolves three FIXMEs in the SHM scheme
daemon (ipcd/src/shm.rs).

* O_RDONLY/O_WRONLY/O_RDWR handling (was line 51): the Handle enum now
  tracks readable/writable booleans derived from the open flags'
  O_ACCMODE bits. read() returns EACCES on write-only handles; write()
  returns EACCES on read-only handles. When no access mode is specified
  (acc == 0), both read and write are permitted (preserves prior behavior).

* Zero-fill on truncate (was line 232): MmapGuard::grow_to now zeroes
  the byte range [old_len, new_len] on every growth, both in the
  in-capacity path and the new-allocation path. This ensures ftruncate-
  extended bytes read as zeros per POSIX SHM semantics, and prevents
  stale data leakage after a shrink-then-grow cycle. A zero_range helper
  method centralizes the write_bytes call.

* Zero-fill on read (was line 293): the grow_to zero-fill guarantee
  ensures all bytes within [0, self.len] are properly initialized (zeros
  or written data), so reads within the logical size always return valid
  data. The FIXME is removed and a null-base guard was added to read()
  for defensive safety.

ipcd/src/shm.rs

@@ -11,13 +11,29 @@ use syscall::{
 };
 
 enum Handle {
-    Shm(Rc<str>),
+    Shm {
+        path: Rc<str>,
+        readable: bool,
+        writable: bool,
+    },
     SchemeRoot,
 }
 impl Handle {
     fn as_shm(&self) -> Result<&Rc<str>, Error> {
         match self {
-            Self::Shm(path) => Ok(path),
+            Self::Shm { path, .. } => Ok(path),
+            Self::SchemeRoot => Err(Error::new(EBADF)),
+        }
+    }
+    fn shm_readable(&self) -> Result<bool, Error> {
+        match self {
+            Self::Shm { readable, .. } => Ok(*readable),
+            Self::SchemeRoot => Err(Error::new(EBADF)),
+        }
+    }
+    fn shm_writable(&self) -> Result<bool, Error> {
+        match self {
+            Self::Shm { writable, .. } => Ok(*writable),
             Self::SchemeRoot => Err(Error::new(EBADF)),
         }
     }
@@ -48,7 +64,6 @@ impl SchemeSync for ShmScheme {
     fn scheme_root(&mut self) -> Result<usize> {
         Ok(self.handles.insert(Handle::SchemeRoot))
     }
-    //FIXME: Handle O_RDONLY/O_WRONLY/O_RDWR
     fn openat(
         &mut self,
         dirfd: usize,
@@ -82,7 +97,11 @@ impl SchemeSync for ShmScheme {
             }
         };
         entry.refs += 1;
-        let id = self.handles.insert(Handle::Shm(path));
+
+        let acc = flags & syscall::O_ACCMODE;
+        let readable = acc & syscall::O_RDONLY != 0 || acc == 0;
+        let writable = acc & syscall::O_WRONLY != 0 || acc == 0;
+        let id = self.handles.insert(Handle::Shm { path, readable, writable });
 
         Ok(OpenResult::ThisScheme {
             number: id,
@@ -96,7 +115,7 @@ impl SchemeSync for ShmScheme {
         })
     }
     fn on_close(&mut self, id: usize) {
-        let Handle::Shm(path) = self.handles.remove(id).unwrap() else {
+        let Handle::Shm { path, .. } = self.handles.remove(id).unwrap() else {
             return;
         };
         let mut entry = match self.maps.entry(path) {
@@ -190,7 +209,11 @@ impl SchemeSync for ShmScheme {
         _fcntl_flags: u32,
         _ctx: &CallerCtx,
     ) -> Result<usize> {
-        let path = self.handles.get(id).and_then(Handle::as_shm)?;
+        let handle = self.handles.get(id)?;
+        if !handle.shm_readable()? {
+            return Err(Error::new(EACCES));
+        }
+        let path = handle.as_shm()?;
         self.maps
             .get_mut(path)
             .expect("handle pointing to nothing")
@@ -205,7 +228,11 @@ impl SchemeSync for ShmScheme {
         _fcntl_flags: u32,
         _ctx: &CallerCtx,
     ) -> Result<usize> {
-        let path = self.handles.get(id).and_then(Handle::as_shm)?;
+        let handle = self.handles.get(id)?;
+        if !handle.shm_writable()? {
+            return Err(Error::new(EACCES));
+        }
+        let path = handle.as_shm()?;
         self.maps
             .get_mut(path)
             .expect("handle pointing to nothing")
@@ -229,7 +256,7 @@ impl MmapGuard {
 
     fn grow_to(&mut self, new_len: usize) -> Result<()> {
         if new_len <= self.total_capacity() {
-            // FIXME clear bytes after new_len
+            self.zero_range(self.len, new_len);
             self.len = new_len;
             return Ok(());
         }
@@ -238,6 +265,8 @@ impl MmapGuard {
         let page_count = needed.div_ceil(PAGE_SIZE);
         let alloc_size = page_count * PAGE_SIZE;
 
+        let old_len = self.len;
+
         let new_base = unsafe {
             if self.base.is_null() {
                 syscall::fmap(
@@ -263,9 +292,19 @@ impl MmapGuard {
 
         self.base = new_base as *mut ();
         self.len = new_len;
+        self.zero_range(old_len, new_len);
         Ok(())
     }
 
+    fn zero_range(&mut self, start: usize, end: usize) {
+        if start >= end || self.base.is_null() {
+            return;
+        }
+        unsafe {
+            core::ptr::write_bytes((self.base as *mut u8).add(start), 0, end - start);
+        }
+    }
+
     fn total_capacity(&self) -> usize {
         self.len.next_multiple_of(PAGE_SIZE)
     }
@@ -289,8 +328,7 @@ impl MmapGuard {
     }
 
     pub fn read(&self, offset: usize, buf: &mut [u8]) -> Result<usize> {
-        if offset >= self.len {
-            // FIXME read as zeros
+        if self.base.is_null() || offset >= self.len {
             return Ok(0);
         }