Fix unneccessary stack copy

Author: Jake-Shadle

Cargo.lock

@@ -13,7 +13,7 @@ debug = 2
 
 [workspace.dependencies]
 cfg-if = "1.0"
-crash-context = { version = "0.6", path = "crash-context" }
+crash-context = "0.6"
 libc = "0.2"
 mach2 = "0.4"
 parking_lot = "0.12"

Cargo.toml

@@ -171,10 +171,6 @@ cfg_if::cfg_if! {
             pub uc_mcontext: mcontext_t,
         }
 
-        #[repr(C, align(16))]
-        #[derive(Clone)]
-        pub struct Reserved([u8; 4096]);
-
         // Note you might see this defined in C with `unsigned long` or
         // `unsigned long long` and think, WTF, those aren't the same! Except
         // `long` means either 32-bit _or_ 64-bit depending on the data model,
@@ -190,7 +186,9 @@ cfg_if::cfg_if! {
             pub sp: u64,
             pub pc: u64,
             pub pstate: u64,
-            //pub __reserved: Reserved,
+            // Note that u128 is ABI safe on aarch64, this is actually a
+            // `long double` in C which Rust doesn't have native support
+            pub __reserved: [u128; 256],
         }
 
         /// Magic value written by the kernel and our custom getcontext
@@ -267,9 +265,9 @@ mod test {
     #[cfg(not(target_env = "musl"))]
     #[test]
     fn matches_libc() {
-        // assert_eq!(
-        //     std::mem::size_of::<libc::ucontext_t>(),
-        //     std::mem::size_of::<super::ucontext_t>()
-        // );
+        assert_eq!(
+            std::mem::size_of::<libc::ucontext_t>(),
+            std::mem::size_of::<super::ucontext_t>()
+        );
     }
 }

crash-context/src/linux.rs

@@ -1,5 +1,5 @@
 use crate::{Error, Signal};
-use std::{mem, ptr};
+use std::{mem, ops::DerefMut, ptr};
 
 // std::cmp::max is not const :(
 const fn get_stack_size() -> usize {
@@ -402,8 +402,8 @@ unsafe extern "C" fn signal_handler(
 
 /// The size of `CrashContext` can be too big w.r.t the size of alternatate stack
 /// for `signal_handler`. Keep the crash context as a .bss field.
-static CRASH_CONTEXT: parking_lot::Mutex<mem::MaybeUninit<crash_context::CrashContext>> =
-    parking_lot::const_mutex(mem::MaybeUninit::uninit());
+static CRASH_CONTEXT: parking_lot::Mutex<crash_context::CrashContext> =
+    parking_lot::const_mutex(unsafe { mem::zeroed() });
 
 pub(super) struct HandlerInner {
     handler: Box<dyn crate::CrashEvent>,
@@ -435,12 +435,11 @@ impl HandlerInner {
             // Allow ourselves to be dumped, if that is what the user handler wishes to do
             let _set_dumpable = SetDumpable::new(self.dump_process);
             debug_print!("set dumpable");
-            let mut crash_ctx = CRASH_CONTEXT.lock();
+            let mut cc = CRASH_CONTEXT.lock();
 
             {
-                *crash_ctx = mem::MaybeUninit::zeroed();
+                ptr::write_bytes(cc.deref_mut(), 0, 1);
                 debug_print!("zeroed crashctx");
-                let cc = &mut *crash_ctx.as_mut_ptr();
 
                 ptr::copy_nonoverlapping(nix_info, &mut cc.siginfo, 1);
                 debug_print!("copied siginfo");
@@ -451,24 +450,23 @@ impl HandlerInner {
 
                 cfg_if::cfg_if! {
                     if #[cfg(target_arch = "aarch64")] {
-                        // let fp_ptr = uc_ptr.uc_mcontext.__reserved.as_ptr().cast::<crash_context::fpsimd_context>();
+                        let fp_ptr = uc_ptr.uc_mcontext.__reserved.as_ptr().cast::<crash_context::fpsimd_context>();
 
-                        // if (*fp_ptr).head.magic == crash_context::FPSIMD_MAGIC {
-                        //     ptr::copy_nonoverlapping(fp_ptr, &mut cc.float_state, 1);
-                        // }
+                        if (*fp_ptr).head.magic == crash_context::FPSIMD_MAGIC {
+                            ptr::copy_nonoverlapping(fp_ptr, &mut cc.float_state, 1);
+                        }
                     } else if #[cfg(not(target_arch = "arm"))] {
-                        // if !uc_ptr.uc_mcontext.fpregs.is_null() {
-                        //     ptr::copy_nonoverlapping(uc_ptr.uc_mcontext.fpregs, ((&mut cc.float_state) as *mut crash_context::fpregset_t).cast(), 1);
-
-                        // }
+                        if !uc_ptr.uc_mcontext.fpregs.is_null() {
+                            ptr::copy_nonoverlapping(uc_ptr.uc_mcontext.fpregs, ((&mut cc.float_state) as *mut crash_context::fpregset_t).cast(), 1);
+                        }
                     }
                 }
 
                 cc.pid = std::process::id() as i32;
                 cc.tid = libc::syscall(libc::SYS_gettid) as i32;
             }
 
-            self.handler.on_crash(&*crash_ctx.as_ptr())
+            self.handler.on_crash(&cc)
         }
     }
 }