Merge pull request #4618 from vishruth-thimmaiah/add_memset

feat: add support for libc::memset

Author: RalfJung

src/shims/foreign_items.rs

@@ -827,6 +827,23 @@ trait EvalContextExtPriv<'tcx>: crate::MiriInterpCxExt<'tcx> {
                 this.mem_copy(ptr_src, ptr_dest, Size::from_bytes(n), true)?;
                 this.write_pointer(ptr_dest, dest)?;
             }
+            "memset" => {
+                let [ptr_dest, val, n] =
+                    this.check_shim_sig_lenient(abi, CanonAbi::C, link_name, args)?;
+                let ptr_dest = this.read_pointer(ptr_dest)?;
+                let val = this.read_scalar(val)?.to_i32()?;
+                let n = this.read_target_usize(n)?;
+                // The docs say val is "interpreted as unsigned char".
+                #[expect(clippy::as_conversions)]
+                let val = val as u8;
+
+                // C requires that this must always be a valid pointer, even if `n` is zero, so we better check that.
+                this.ptr_get_alloc_id(ptr_dest, 0)?;
+
+                let bytes = std::iter::repeat_n(val, n.try_into().unwrap());
+                this.write_bytes_ptr(ptr_dest, bytes)?;
+                this.write_pointer(ptr_dest, dest)?;
+            }
 
             // LLVM intrinsics
             "llvm.prefetch" => {

tests/fail-dep/libc/memset_null.rs

@@ -0,0 +1,8 @@
+use std::ptr;
+
+// null is explicitly called out as UB in the C docs for `memset`.
+fn main() {
+    unsafe {
+        libc::memset(ptr::null_mut(), 0, 0); //~ERROR: null pointer
+    }
+}

tests/fail-dep/libc/memset_null.stderr

@@ -0,0 +1,15 @@
+error: Undefined Behavior: pointer not dereferenceable: pointer must point to some allocation, but got null pointer
+  --> tests/fail-dep/libc/memset_null.rs:LL:CC
+   |
+LL |         libc::memset(ptr::null_mut(), 0, 0);
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Undefined Behavior occurred here
+   |
+   = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
+   = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
+   = note: BACKTRACE:
+   = note: inside `main` at tests/fail-dep/libc/memset_null.rs:LL:CC
+
+note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace
+
+error: aborting due to 1 previous error
+

tests/pass-dep/libc/libc-mem.rs

@@ -78,6 +78,47 @@ fn test_strcpy() {
     }
 }
 
+fn test_memset() {
+    unsafe {
+        let val = 1;
+        let dest = libc::calloc(3, 1);
+        libc::memset(dest, val, 3);
+        let slc = std::slice::from_raw_parts(dest as *const i8, 3);
+        assert_eq!(*slc, [1i8, 1, 1]);
+        libc::free(dest);
+    }
+
+    unsafe {
+        let val = 1;
+        let dest = libc::calloc(4, 1);
+        libc::memset(dest, val, 3);
+        let slc = std::slice::from_raw_parts(dest as *const i8, 4);
+        assert_eq!(*slc, [1i8, 1, 1, 0]);
+        libc::free(dest);
+    }
+
+    unsafe {
+        let val = 1;
+        let mut dest = 0_i8;
+        libc::memset(&mut dest as *mut i8 as *mut libc::c_void, val, mem::size_of::<i8>());
+        assert_eq!(dest, val as i8);
+    }
+
+    unsafe {
+        let val = 1;
+        let mut dest = 0_i16;
+        libc::memset(&mut dest as *mut i16 as *mut libc::c_void, val, mem::size_of::<i16>());
+        assert_eq!(dest, 257);
+    }
+
+    unsafe {
+        let val = 257;
+        let mut dest = 0_i16;
+        libc::memset(&mut dest as *mut i16 as *mut libc::c_void, val, mem::size_of::<i16>());
+        assert_eq!(dest, 257);
+    }
+}
+
 fn test_malloc() {
     // Test that small allocations sometimes *are* not very aligned.
     let saw_unaligned = (0..64).any(|_| unsafe {
@@ -310,4 +351,5 @@ fn main() {
 
     test_memcpy();
     test_strcpy();
+    test_memset();
 }