Supporting Backtrace crate for x86_64-fortanix-unknown-sgx.

1. Backtracing is supported via libunwind which is linked to x86_64-fortanix-unknown-sgx.
2. To reduce enclave TCB size, we do not support symbol resolution for this target.
   We rather, display the offset of each function, which could be resolved later.

Cargo.toml

@@ -32,7 +32,7 @@ gimli = { version = "0.16.0", optional = true }
 memmap = { version = "0.7.0", optional = true }
 object = { version = "0.9.0", optional = true }
 
-[target.'cfg(unix)'.dependencies]
+[target.'cfg(any(unix, all(target_vendor="fortanix", target_env="sgx")))'.dependencies]
 libc = { version = "0.2.45", default-features = false }
 
 [target.'cfg(windows)'.dependencies]

src/backtrace/mod.rs

@@ -104,10 +104,11 @@ impl fmt::Debug for Frame {
 }
 
 cfg_if! {
-    if #[cfg(all(unix,
+    if #[cfg(any(all(unix,
                  not(target_os = "emscripten"),
                  not(all(target_os = "ios", target_arch = "arm")),
-                 feature = "libunwind"))] {
+                 feature = "libunwind"),
+                 target_env="sgx"))] {
         mod libunwind;
         use self::libunwind::trace as trace_imp;
         use self::libunwind::Frame as FrameImp;

src/capture.rs

@@ -229,7 +229,15 @@ impl fmt::Debug for Backtrace {
         };
 
         for (idx, frame) in iter.enumerate() {
-            let ip = frame.ip();
+            // To reduce TCB size in Sgx enclave, we do not want to implement symbol resolution functionality.
+            // Rather, we can print the offset of the address here, which could be later mapped to
+            // correct function.
+            let ip = if cfg!(target_env="sgx") {
+                usize::wrapping_sub(frame.ip() as _, std::os::fortanix_sgx::mem::image_base() as _) as _
+            } else {
+                frame.ip()
+            };
+
             write!(fmt, "\n{:4}: ", idx)?;
 
             let symbols = match frame.symbols {

src/lib.rs

@@ -72,10 +72,12 @@
 #![deny(missing_docs)]
 #![no_std]
 
+#![cfg_attr(target_env="sgx", feature(sgx_platform))]
+
 #[cfg(feature = "std")]
 #[macro_use] extern crate std;
 
-#[cfg(unix)]
+#[cfg(any(unix, target_env="sgx"))]
 extern crate libc;
 #[cfg(windows)]
 extern crate winapi;