add helper function to declare an extern static for a weak symbol

Author: RalfJung

src/tools/miri/src/shims/extern_static.rs

@@ -29,6 +29,21 @@ impl<'mir, 'tcx> MiriMachine<'mir, 'tcx> {
         Ok(())
     }
 
+    /// Extern statics that are initialized with function pointers to the symbols of the same name.
+    fn weak_symbol_extern_statics(
+        this: &mut MiriInterpCx<'mir, 'tcx>,
+        names: &[&str],
+    ) -> InterpResult<'tcx> {
+        for name in names {
+            assert!(this.is_dyn_sym(name), "{name} is not a dynamic symbol");
+            let layout = this.machine.layouts.const_raw_ptr;
+            let ptr = this.fn_ptr(FnVal::Other(DynSym::from_str(name)));
+            let val = ImmTy::from_scalar(Scalar::from_pointer(ptr, this), layout);
+            Self::alloc_extern_static(this, name, val)?;
+        }
+        Ok(())
+    }
+
     /// Sets up the "extern statics" for this machine.
     pub fn init_extern_statics(this: &mut MiriInterpCx<'mir, 'tcx>) -> InterpResult<'tcx> {
         // "__rust_no_alloc_shim_is_unstable"
@@ -58,12 +73,7 @@ impl<'mir, 'tcx> MiriMachine<'mir, 'tcx> {
             }
             "android" => {
                 Self::null_ptr_extern_statics(this, &["bsd_signal"])?;
-                // "signal" -- just needs a non-zero pointer value (function does not even get called),
-                // but we arrange for this to call the `signal` function anyway.
-                let layout = this.machine.layouts.const_raw_ptr;
-                let ptr = this.fn_ptr(FnVal::Other(DynSym::from_str("signal")));
-                let val = ImmTy::from_scalar(Scalar::from_pointer(ptr, this), layout);
-                Self::alloc_extern_static(this, "signal", val)?;
+                Self::weak_symbol_extern_statics(this, &["signal"])?;
             }
             "windows" => {
                 // "_tls_used"

src/tools/miri/src/shims/foreign_items.rs

@@ -151,6 +151,15 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
         Ok(None)
     }
 
+    fn is_dyn_sym(&self, name: &str) -> bool {
+        let this = self.eval_context_ref();
+        match this.tcx.sess.target.os.as_ref() {
+            os if this.target_os_is_unix() => shims::unix::foreign_items::is_dyn_sym(name, os),
+            "windows" => shims::windows::foreign_items::is_dyn_sym(name),
+            _ => false,
+        }
+    }
+
     /// Emulates a call to a `DynSym`.
     fn emulate_dyn_sym(
         &mut self,

src/tools/miri/src/shims/unix/foreign_items.rs

@@ -15,7 +15,7 @@ use shims::unix::freebsd::foreign_items as freebsd;
 use shims::unix::linux::foreign_items as linux;
 use shims::unix::macos::foreign_items as macos;
 
-fn is_dyn_sym(name: &str, target_os: &str) -> bool {
+pub fn is_dyn_sym(name: &str, target_os: &str) -> bool {
     match name {
         // Used for tests.
         "isatty" => true,

src/tools/miri/src/shims/unix/linux/foreign_items.rs

@@ -113,9 +113,7 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
                 // have the right type.
 
                 let sys_getrandom = this.eval_libc("SYS_getrandom").to_target_usize(this)?;
-
                 let sys_statx = this.eval_libc("SYS_statx").to_target_usize(this)?;
-
                 let sys_futex = this.eval_libc("SYS_futex").to_target_usize(this)?;
 
                 if args.is_empty() {

src/tools/miri/src/shims/windows/foreign_items.rs

@@ -15,7 +15,7 @@ use crate::*;
 use shims::foreign_items::EmulateForeignItemResult;
 use shims::windows::handle::{Handle, PseudoHandle};
 
-fn is_dyn_sym(name: &str) -> bool {
+pub fn is_dyn_sym(name: &str) -> bool {
     // std does dynamic detection for these symbols
     matches!(
         name,