fix(linker): deref uninit TCB

The bug is described for `x86_64` and Linux but is the same for other architectures and on Redox.

`Tcb::current()` is used to retrieve the current TCB, which is done by by reading `fs:[0x10]`. The TCB layout describes that at offset `0x10`, there is `tcb_ptr: *mut GenericTcb<...>`, which is nothing more but a pointer to itself.

This is fine as otherwise a system call would be required to get the TCB (`arch_prctl(ARCH_GET_FS)` on Linux).

However, this is problematic as the function may be called when the FS base is not set, and in that case the expected output of the function should be [`None`], but we don't currently handle that.

To fix this, any code paths that maybe call this function on an uninitialized TCB are be switched to call `current_slow()`. Which just uses `arch_prctl(ARCH_GET_FS)` to get the FS base and check if it's non-zero.

Signed-off-by: Anhad Singh <andypython@protonmail.com>

src/header/dlfcn/mod.rs

@@ -47,7 +47,7 @@ pub unsafe extern "C" fn dlopen(cfilename: *const c_char, flags: c_int) -> *mut
         ))
     };
 
-    let tcb = match Tcb::current() {
+    let tcb = match Tcb::current_slow() {
         Some(tcb) => tcb,
         None => {
             ERROR.store(ERROR_NOT_SUPPORTED.as_ptr() as usize, Ordering::SeqCst);
@@ -83,7 +83,7 @@ pub unsafe extern "C" fn dlsym(handle: *mut c_void, symbol: *const c_char) -> *m
 
     let symbol_str = str::from_utf8_unchecked(CStr::from_ptr(symbol).to_bytes());
 
-    let tcb = match Tcb::current() {
+    let tcb = match Tcb::current_slow() {
         Some(tcb) => tcb,
         None => {
             ERROR.store(ERROR_NOT_SUPPORTED.as_ptr() as usize, Ordering::SeqCst);
@@ -110,7 +110,7 @@ pub unsafe extern "C" fn dlsym(handle: *mut c_void, symbol: *const c_char) -> *m
 
 #[no_mangle]
 pub unsafe extern "C" fn dlclose(handle: *mut c_void) -> c_int {
-    let tcb = match Tcb::current() {
+    let tcb = match Tcb::current_slow() {
         Some(tcb) => tcb,
         None => {
             ERROR.store(ERROR_NOT_SUPPORTED.as_ptr() as usize, Ordering::SeqCst);

src/ld_so/linker.rs

@@ -167,7 +167,7 @@ impl Linker {
         )?;
 
         unsafe {
-            let tcb = match Tcb::current() {
+            let tcb = match Tcb::current_slow() {
                 Some(some) => some,
                 None => Tcb::new(self.tls_size)?,
             };
@@ -230,7 +230,7 @@ impl Linker {
             if self.next_tls_module_id == 1 {
                 // Hack to allocate TCB on the first TLS module
                 unsafe {
-                    if Tcb::current().is_none() {
+                    if Tcb::current_slow().is_none() {
                         let tcb = Tcb::new(master.offset).expect_notls("failed to allocate TCB");
                         tcb.activate();
                     }

src/ld_so/tcb.rs

@@ -112,6 +112,46 @@ impl Tcb {
         Some(&mut *GenericTcb::<OsSpecific>::current_ptr()?.cast())
     }
 
+    // FIXME(andypython): move to platform/
+    pub unsafe fn current_slow() -> Option<&'static mut Self> {
+        #[cfg(all(target_os = "linux", target_arch = "x86_64"))]
+        {
+            use sc::nr::ARCH_PRCTL;
+            const ARCH_GET_FS: usize = 0x1003;
+
+            let mut fs_base = 0usize;
+            sc::syscall!(ARCH_PRCTL, ARCH_GET_FS, &mut fs_base as *mut usize);
+
+            if fs_base == 0 {
+                None
+            } else {
+                Some(&mut *(fs_base as *mut Self))
+            }
+        }
+
+        #[cfg(all(target_os = "redox", target_arch = "x86_64"))]
+        {
+            let mut env = syscall::EnvRegisters::default();
+
+            let file = syscall::open(
+                "/scheme/thisproc/current/regs/env",
+                syscall::O_CLOEXEC | syscall::O_RDONLY,
+            )
+            .expect_notls("failed to open handle for process registers");
+
+            let _ = syscall::read(file, &mut env).expect_notls("failed to read fsbase");
+            let _ = syscall::close(file);
+
+            if env.fsbase == 0 {
+                return None;
+            } else {
+                return Some(&mut *(env.fsbase as *mut Self));
+            }
+        }
+
+        // FIXME(andypython): Implement and test on other platforms
+    }
+
     /// A slice for all of the TLS data
     pub unsafe fn tls(&self) -> Option<&'static mut [u8]> {
         if self.tls_end.is_null() || self.tls_len == 0 {