From 4c8575d571b28892bbf6ed3f7490ed6487cb6e2a Mon Sep 17 00:00:00 2001
From: yellowhatter <bannov.dy@gmail.com>
Date: Sun, 21 Jul 2024 17:57:01 +0300
Subject: [PATCH] Fix misaligned memory access bug (affects non-64 bit ARM)

---
 commons/zenoh-shm/src/posix_shm/segment.rs    | 22 ++++++++-----------
 .../src/unicast/establishment/ext/shm.rs      | 17 +++++++++-----
 2 files changed, 20 insertions(+), 19 deletions(-)

diff --git a/commons/zenoh-shm/src/posix_shm/segment.rs b/commons/zenoh-shm/src/posix_shm/segment.rs
index 5458ab3e3e..657976ece1 100644
--- a/commons/zenoh-shm/src/posix_shm/segment.rs
+++ b/commons/zenoh-shm/src/posix_shm/segment.rs
@@ -12,10 +12,7 @@
 //   ZettaScale Zenoh Team, <zenoh@zettascale.tech>
 //
 
-use std::{
-    fmt::{Debug, Display},
-    mem::size_of,
-};
+use std::fmt::{Debug, Display};
 
 use rand::Rng;
 use shared_memory::{Shmem, ShmemConf, ShmemError};
@@ -63,7 +60,7 @@ where
             // If creation fails because segment already exists for this id,
             // the creation attempt will be repeated with another id
             match ShmemConf::new()
-                .size(alloc_size + size_of::<usize>())
+                .size(alloc_size)
                 .os_id(Self::os_id(id.clone(), id_prefix))
                 .create()
             {
@@ -71,7 +68,6 @@ where
                     tracing::debug!(
                         "Created SHM segment, size: {alloc_size}, prefix: {id_prefix}, id: {id}"
                     );
-                    unsafe { *(shmem.as_ptr() as *mut usize) = alloc_size };
                     return Ok(Segment { shmem, id });
                 }
                 Err(ShmemError::LinkExists) => {}
@@ -94,10 +90,6 @@ where
                 )
             })?;
 
-        if shmem.len() <= size_of::<usize>() {
-            bail!("SHM segment too small")
-        }
-
         tracing::debug!("Opened SHM segment, prefix: {id_prefix}, id: {id}");
 
         Ok(Self { shmem, id })
@@ -110,17 +102,21 @@ where
     }
 
     pub fn as_ptr(&self) -> *mut u8 {
-        unsafe { self.shmem.as_ptr().add(size_of::<usize>()) }
+        self.shmem.as_ptr()
     }
 
+    /// Returns the length of this [`Segment<ID>`].
+    /// NOTE: one some platforms (at least windows) the returned len will be the actual length of an shm segment
+    /// (a required len rounded up to the nearest multiply of page size), on other (at least linux and macos) this
+    /// returns a value requested upon segment creation
     pub fn len(&self) -> usize {
-        unsafe { *(self.shmem.as_ptr() as *mut usize) }
+        self.shmem.len()
     }
 
     // TODO: dead code warning occurs because of `tested_crate_module!()` macro when feature `test` is not enabled. Better to fix that
     #[allow(dead_code)]
     pub fn is_empty(&self) -> bool {
-        unsafe { *(self.shmem.as_ptr() as *mut usize) == 0 }
+        self.len() == 0
     }
 
     pub fn id(&self) -> ID {
diff --git a/io/zenoh-transport/src/unicast/establishment/ext/shm.rs b/io/zenoh-transport/src/unicast/establishment/ext/shm.rs
index e2068af94a..025aaaef44 100644
--- a/io/zenoh-transport/src/unicast/establishment/ext/shm.rs
+++ b/io/zenoh-transport/src/unicast/establishment/ext/shm.rs
@@ -36,6 +36,10 @@ const AUTH_SEGMENT_PREFIX: &str = "auth";
 pub(crate) type AuthSegmentID = u32;
 pub(crate) type AuthChallenge = u64;
 
+const LEN_INDEX: usize = 0;
+const CHALLENGE_INDEX: usize = 1;
+const ID_START_INDEX: usize = 2;
+
 #[derive(Debug)]
 pub struct AuthSegment {
     array: ArrayInSHM<AuthSegmentID, AuthChallenge, usize>,
@@ -44,13 +48,14 @@ pub struct AuthSegment {
 impl AuthSegment {
     pub fn create(challenge: AuthChallenge, shm_protocols: &[ProtocolID]) -> ZResult<Self> {
         let array = ArrayInSHM::<AuthSegmentID, AuthChallenge, usize>::create(
-            1 + shm_protocols.len(),
+            ID_START_INDEX + shm_protocols.len(),
             AUTH_SEGMENT_PREFIX,
         )?;
         unsafe {
-            (*array.elem_mut(0)) = challenge;
-            for elem in 1..array.elem_count() {
-                (*array.elem_mut(elem)) = shm_protocols[elem - 1] as u64;
+            (*array.elem_mut(LEN_INDEX)) = shm_protocols.len() as AuthChallenge;
+            (*array.elem_mut(CHALLENGE_INDEX)) = challenge;
+            for elem in ID_START_INDEX..array.elem_count() {
+                (*array.elem_mut(elem)) = shm_protocols[elem - ID_START_INDEX] as u64;
             }
         };
         Ok(Self { array })
@@ -62,12 +67,12 @@ impl AuthSegment {
     }
 
     pub fn challenge(&self) -> AuthChallenge {
-        unsafe { *self.array.elem(0) }
+        unsafe { *self.array.elem(CHALLENGE_INDEX) }
     }
 
     pub fn protocols(&self) -> Vec<ProtocolID> {
         let mut result = vec![];
-        for elem in 1..self.array.elem_count() {
+        for elem in ID_START_INDEX..self.array.elem_count() {
             result.push(unsafe { *self.array.elem(elem) as u32 });
         }
         result