From 27afeb0085db8f066636a512241ed8e2003a3fe2 Mon Sep 17 00:00:00 2001
From: vishruth-thimmaiah <vishruththimmaiah@gmail.com>
Date: Wed, 8 Oct 2025 01:59:44 +0530
Subject: [PATCH] feat: add support for libc::memset

Signed-off-by: vishruth-thimmaiah <vishruththimmaiah@gmail.com>
---
 src/tools/miri/src/shims/foreign_items.rs     | 16 +++++++
 .../miri/tests/pass-dep/libc/libc-mem.rs      | 42 +++++++++++++++++++
 2 files changed, 58 insertions(+)

diff --git a/src/tools/miri/src/shims/foreign_items.rs b/src/tools/miri/src/shims/foreign_items.rs
index 1d086906e7a5..3fd57d3c8db2 100644
--- a/src/tools/miri/src/shims/foreign_items.rs
+++ b/src/tools/miri/src/shims/foreign_items.rs
@@ -827,6 +827,22 @@ 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;
+
+                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" => {
diff --git a/src/tools/miri/tests/pass-dep/libc/libc-mem.rs b/src/tools/miri/tests/pass-dep/libc/libc-mem.rs
index 727533a9de61..531d637d1f24 100644
--- a/src/tools/miri/tests/pass-dep/libc/libc-mem.rs
+++ b/src/tools/miri/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();
 }