maps/xdp: drop the possibly-unsafe {Cpu,Dev}MapValue conversions

Making a `From` impl that hides an unsafe with some safety guarantee is
actually unsafe, because while we know how we use this impl, we don't
know how other will. And since it's on two public types, anything can
be done.

aya/src/maps/xdp/cpu_map.rs

@@ -77,7 +77,13 @@ impl<T: Borrow<MapData>> CpuMap<T> {
                 io_error,
             })?;
         let value: bpf_cpumap_val = value.ok_or(MapError::KeyNotFound)?;
-        Ok(value.into())
+
+        // SAFETY: map writes use fd, map reads use id.
+        // https://elixir.bootlin.com/linux/v6.2/source/include/uapi/linux/bpf.h#L6149
+        Ok(CpuMapValue {
+            qsize: value.qsize,
+            prog_id: unsafe { value.bpf_prog.id },
+        })
     }
 
     /// An iterator over the elements of the map.
@@ -148,14 +154,3 @@ pub struct CpuMapValue {
     pub qsize: u32,
     pub prog_id: u32,
 }
-
-impl From<bpf_cpumap_val> for CpuMapValue {
-    fn from(value: bpf_cpumap_val) -> Self {
-        // SAFETY: map writes use fd, map reads use id.
-        // https://elixir.bootlin.com/linux/v6.2/source/include/uapi/linux/bpf.h#L6149
-        CpuMapValue {
-            qsize: value.qsize,
-            prog_id: unsafe { value.bpf_prog.id },
-        }
-    }
-}

aya/src/maps/xdp/dev_map.rs

@@ -71,7 +71,13 @@ impl<T: Borrow<MapData>> DevMap<T> {
                 io_error,
             })?;
         let value: bpf_devmap_val = value.ok_or(MapError::KeyNotFound)?;
-        Ok(value.into())
+
+        // SAFETY: map writes use fd, map reads use id.
+        // https://elixir.bootlin.com/linux/v6.2/source/include/uapi/linux/bpf.h#L6136
+        Ok(DevMapValue {
+            ifindex: value.ifindex,
+            prog_id: unsafe { value.bpf_prog.id },
+        })
     }
 
     /// An iterator over the elements of the array.
@@ -141,14 +147,3 @@ pub struct DevMapValue {
     pub ifindex: u32,
     pub prog_id: u32,
 }
-
-impl From<bpf_devmap_val> for DevMapValue {
-    fn from(value: bpf_devmap_val) -> Self {
-        // SAFETY: map writes use fd, map reads use id.
-        // https://elixir.bootlin.com/linux/v6.2/source/include/uapi/linux/bpf.h#L6136
-        DevMapValue {
-            ifindex: value.ifindex,
-            prog_id: unsafe { value.bpf_prog.id },
-        }
-    }
-}

aya/src/maps/xdp/dev_map_hash.rs

@@ -60,7 +60,13 @@ impl<T: Borrow<MapData>> DevMapHash<T> {
             io_error,
         })?;
         let value: bpf_devmap_val = value.ok_or(MapError::KeyNotFound)?;
-        Ok(value.into())
+
+        // SAFETY: map writes use fd, map reads use id.
+        // https://elixir.bootlin.com/linux/v6.2/source/include/uapi/linux/bpf.h#L6136
+        Ok(DevMapValue {
+            ifindex: value.ifindex,
+            prog_id: unsafe { value.bpf_prog.id },
+        })
     }
 
     /// An iterator over the elements of the devmap in arbitrary order.