sched_setaffinity: test cpusetsize == 0

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

@@ -645,7 +645,9 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
                     this.set_last_error(einval)?;
                     this.write_scalar(Scalar::from_i32(-1), dest)?;
                 } else {
-                    // NOTE: cpusetsize might be smaller than `CpuAffinityMask::CPU_MASK_BYTES`
+                    // NOTE: cpusetsize might be smaller than `CpuAffinityMask::CPU_MASK_BYTES`.
+                    // Any unspecified bytes are treated as zero here (none of the CPUs are configured).
+                    // This is not exactly documented, so we assume that this is the behavior in practice.
                     let bits_slice = this.read_bytes_ptr_strip_provenance(mask, Size::from_bytes(cpusetsize))?;
                     // This ignores the bytes beyond `CpuAffinityMask::CPU_MASK_BYTES`
                     let bits_array: [u8; CpuAffinityMask::CPU_MASK_BYTES] =

src/tools/miri/tests/pass-dep/libc/libc-affinity.rs

@@ -105,6 +105,24 @@ fn get_small_cpu_mask() {
     }
 }
 
+fn set_small_cpu_mask() {
+    let mut cpuset: cpu_set_t = unsafe { core::mem::MaybeUninit::zeroed().assume_init() };
+
+    let err = unsafe { sched_getaffinity(PID, size_of::<cpu_set_t>(), &mut cpuset) };
+    assert_eq!(err, 0);
+
+    // setting a mask of size 0 is invalid
+    let err = unsafe { sched_setaffinity(PID, 0, &cpuset) };
+    assert_eq!(err, -1);
+    assert_eq!(std::io::Error::last_os_error().kind(), std::io::ErrorKind::InvalidInput);
+
+    // any other number of bytes (at least up to `size_of<cpu_set_t>()` will work
+    for i in 1..24 {
+        let err = unsafe { sched_setaffinity(PID, i, &cpuset) };
+        assert_eq!(err, 0, "fail for {i}");
+    }
+}
+
 fn set_custom_cpu_mask() {
     let cpu_count = std::thread::available_parallelism().unwrap().get();
 
@@ -189,6 +207,7 @@ fn main() {
     configure_unavailable_cpu();
     large_set();
     get_small_cpu_mask();
+    set_small_cpu_mask();
     set_custom_cpu_mask();
     parent_child();
 }