Auto merge of #3484 - RalfJung:realloc, r=RalfJung

make realloc with a size of zero fail

Fixes https://github.com/rust-lang/miri/issues/2774

3 files changed, 27 insertions, 3 deletions

diff --git a/src/tools/miri/tests/fail-dep/realloc-zero.rs b/src/tools/miri/tests/fail-dep/realloc-zero.rs
new file mode 100644
index 00000000000..1482798e90c
--- /dev/null
+++ b/src/tools/miri/tests/fail-dep/realloc-zero.rs
@@ -0,0 +1,10 @@
+//@ignore-target-windows: No libc on Windows
+
+fn main() {
+    unsafe {
+        let p1 = libc::malloc(20);
+        // C made this UB...
+        let p2 = libc::realloc(p1, 0); //~ERROR: `realloc` with a size of zero
+        assert!(p2.is_null());
+    }
+}
diff --git a/src/tools/miri/tests/fail-dep/realloc-zero.stderr b/src/tools/miri/tests/fail-dep/realloc-zero.stderr
new file mode 100644
index 00000000000..749a61f7396
--- /dev/null
+++ b/src/tools/miri/tests/fail-dep/realloc-zero.stderr
@@ -0,0 +1,15 @@
+error: Undefined Behavior: `realloc` with a size of zero
+  --> $DIR/realloc-zero.rs:LL:CC
+   |
+LL |         let p2 = libc::realloc(p1, 0);
+   |                  ^^^^^^^^^^^^^^^^^^^^ `realloc` with a size of zero
+   |
+   = 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 $DIR/realloc-zero.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
+
diff --git a/src/tools/miri/tests/pass-dep/malloc.rs b/src/tools/miri/tests/pass-dep/malloc.rs
index f5e014c000d..35cd137931f 100644
--- a/src/tools/miri/tests/pass-dep/malloc.rs
+++ b/src/tools/miri/tests/pass-dep/malloc.rs
@@ -34,9 +34,8 @@ fn main() {
     }
 
     unsafe {
-        let p1 = libc::malloc(20);
-
-        let p2 = libc::realloc(p1, 0);
+        // Realloc with size 0 is okay for the null pointer
+        let p2 = libc::realloc(ptr::null_mut(), 0);
         assert!(p2.is_null());
     }