diff --git a/redox-rt/src/arch/x86_64.rs b/redox-rt/src/arch/x86_64.rs
index eca4991f41..56cebb8f8c 100644
--- a/redox-rt/src/arch/x86_64.rs
+++ b/redox-rt/src/arch/x86_64.rs
@@ -12,7 +12,10 @@ use syscall::{
 
 use crate::{
     proc::{fork_inner, FdGuard},
-    signal::{inner_c, PosixStackt, RtSigarea, SigStack, PROC_CONTROL_STRUCT},
+    signal::{
+        get_sigaltstack, inner_c, PosixStackt, RtSigarea, SigStack, Sigaltstack,
+        PROC_CONTROL_STRUCT,
+    },
     RtTcb, Tcb,
 };
 
@@ -452,11 +455,7 @@ pub unsafe fn arch_pre(stack: &mut SigStack, area: &mut SigArea) -> PosixStackt
         stack.regs.rip = area.tmp_rip;
     }
 
-    PosixStackt {
-        sp: stack.regs.rsp as *mut (),
-        size: 0,  // TODO
-        flags: 0, // TODO
-    }
+    get_sigaltstack(area, stack.regs.rsp).into()
 }
 
 pub(crate) static SUPPORTS_AVX: AtomicU8 = AtomicU8::new(0);
diff --git a/redox-rt/src/signal.rs b/redox-rt/src/signal.rs
index 6a8b484bae..938f8ce3dc 100644
--- a/redox-rt/src/signal.rs
+++ b/redox-rt/src/signal.rs
@@ -52,6 +52,34 @@ pub struct PosixStackt {
     pub size: usize,
 }
 
+pub const SS_ONSTACK: usize = 1;
+pub const SS_DISABLE: usize = 2;
+
+impl From<Sigaltstack> for PosixStackt {
+    fn from(value: Sigaltstack) -> Self {
+        match value {
+            Sigaltstack::Disabled => PosixStackt {
+                sp: core::ptr::null_mut(),
+                size: 0,
+                flags: SS_DISABLE.try_into().unwrap(),
+            },
+            Sigaltstack::Enabled {
+                onstack,
+                base,
+                size,
+            } => PosixStackt {
+                sp: base.cast(),
+                size,
+                flags: if onstack {
+                    SS_ONSTACK.try_into().unwrap()
+                } else {
+                    0
+                },
+            },
+        }
+    }
+}
+
 #[repr(C)]
 // TODO: This struct is for practical reasons locked to Linux's ABI, but avoid redefining
 // it here. Alternatively, check at compile time that the structs are equivalent.
@@ -590,6 +618,19 @@ pub enum Sigaltstack {
         size: usize,
     },
 }
+
+pub(crate) fn get_sigaltstack(tcb: &SigArea, sp: usize) -> Sigaltstack {
+    if tcb.altstack_bottom == 0 && tcb.altstack_top == usize::MAX {
+        Sigaltstack::Disabled
+    } else {
+        Sigaltstack::Enabled {
+            base: tcb.altstack_bottom as *mut (),
+            size: tcb.altstack_top - tcb.altstack_bottom,
+            onstack: (tcb.altstack_bottom..tcb.altstack_top).contains(&sp),
+        }
+    }
+}
+
 pub unsafe fn sigaltstack(
     new: Option<&Sigaltstack>,
     old_out: Option<&mut Sigaltstack>,
@@ -597,15 +638,7 @@ pub unsafe fn sigaltstack(
     let _g = tmp_disable_signals();
     let tcb = &mut *Tcb::current().unwrap().os_specific.arch.get();
 
-    let old = if tcb.altstack_bottom == 0 && tcb.altstack_top == usize::MAX {
-        Sigaltstack::Disabled
-    } else {
-        Sigaltstack::Enabled {
-            base: tcb.altstack_bottom as *mut (),
-            size: tcb.altstack_top - tcb.altstack_bottom,
-            onstack: (tcb.altstack_bottom..tcb.altstack_top).contains(&crate::arch::current_sp()),
-        }
-    };
+    let old = get_sigaltstack(tcb, crate::arch::current_sp());
 
     if matches!(old, Sigaltstack::Enabled { onstack: true, .. }) && new != Some(&old) {
         return Err(Error::new(EPERM));
diff --git a/src/header/signal/redox.rs b/src/header/signal/redox.rs
index 3ed2033f55..fe08477605 100644
--- a/src/header/signal/redox.rs
+++ b/src/header/signal/redox.rs
@@ -50,6 +50,15 @@ pub const SA_NOCLDSTOP: usize = 0x4000_0000;
 pub const SS_ONSTACK: usize = 0x00000001;
 pub const SS_DISABLE: usize = 0x00000002;
 
+const _: () = {
+    if SS_ONSTACK != redox_rt::signal::SS_ONSTACK {
+        panic!();
+    }
+    if SS_DISABLE != redox_rt::signal::SS_DISABLE {
+        panic!();
+    }
+};
+
 // TODO: It's just a guess based on Linux
 pub const MINSIGSTKSZ: usize = 2048;
 pub const SIGSTKSZ: usize = 8096;
diff --git a/src/platform/redox/signal.rs b/src/platform/redox/signal.rs
index f9aed39380..de89f58455 100644
--- a/src/platform/redox/signal.rs
+++ b/src/platform/redox/signal.rs
@@ -1,6 +1,6 @@
 use core::mem::{self, offset_of};
 use redox_rt::signal::{
-    SigStack, Sigaction, SigactionFlags, SigactionKind, Sigaltstack, SignalHandler,
+    PosixStackt, SigStack, Sigaction, SigactionFlags, SigactionKind, Sigaltstack, SignalHandler,
 };
 use syscall::{self, Result};
 
@@ -235,21 +235,11 @@ impl PalSignal for Sys {
         redox_rt::signal::sigaltstack(new.as_ref(), old.as_mut())?;
 
         if let (Some(old_c_stack), Some(old)) = (old_c, old) {
-            *old_c_stack = match old {
-                Sigaltstack::Disabled => stack_t {
-                    ss_sp: core::ptr::null_mut(),
-                    ss_size: 0,
-                    ss_flags: SS_DISABLE.try_into().unwrap(),
-                },
-                Sigaltstack::Enabled {
-                    onstack,
-                    base,
-                    size,
-                } => stack_t {
-                    ss_sp: base.cast(),
-                    ss_size: size,
-                    ss_flags: SS_ONSTACK.try_into().unwrap(),
-                },
+            let c_stack = PosixStackt::from(old);
+            *old_c_stack = stack_t {
+                ss_sp: c_stack.sp.cast(),
+                ss_size: c_stack.size,
+                ss_flags: c_stack.flags,
             };
         }
         Ok(())