Make mmap not use expose semantics

src/shims/unix/linux/mem.rs

@@ -15,14 +15,14 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
     ) -> InterpResult<'tcx, Scalar<Provenance>> {
         let this = self.eval_context_mut();
 
-        let old_address = this.read_target_usize(old_address)?;
+        let old_address = this.read_pointer(old_address)?;
         let old_size = this.read_target_usize(old_size)?;
         let new_size = this.read_target_usize(new_size)?;
         let flags = this.read_scalar(flags)?.to_i32()?;
 
         // old_address must be a multiple of the page size
         #[allow(clippy::arithmetic_side_effects)] // PAGE_SIZE is nonzero
-        if old_address % this.machine.page_size != 0 || new_size == 0 {
+        if old_address.addr().bytes() % this.machine.page_size != 0 || new_size == 0 {
             this.set_last_error(Scalar::from_i32(this.eval_libc_i32("EINVAL")))?;
             return Ok(this.eval_libc("MAP_FAILED"));
         }
@@ -41,7 +41,6 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
             return Ok(Scalar::from_maybe_pointer(Pointer::null(), this));
         }
 
-        let old_address = Machine::ptr_from_addr_cast(this, old_address)?;
         let align = this.machine.page_align();
         let ptr = this.reallocate_ptr(
             old_address,
@@ -59,8 +58,6 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
             )
             .unwrap();
         }
-        // Memory mappings are always exposed
-        Machine::expose_ptr(this, ptr)?;
 
         Ok(Scalar::from_pointer(ptr, this))
     }

src/shims/unix/mem.rs

@@ -6,6 +6,13 @@
 //! mmap/munmap behave a lot like alloc/dealloc, and for simple use they are exactly
 //! equivalent. That is the only part we support: no MAP_FIXED or MAP_SHARED or anything
 //! else that goes beyond a basic allocation API.
+//!
+//! Note that in addition to only supporting malloc-like calls to mmap, we only support free-like
+//! calls to munmap, but for a very different reason. In principle, according to the man pages, it
+//! is possible to unmap arbitrary regions of address space. But in a high-level language like Rust
+//! this amounts to partial deallocation, which LLVM does not support. So any attempt to call our
+//! munmap shim which would partily unmap a region of address space previously mapped by mmap will
+//! report UB.
 
 use crate::{helpers::round_to_next_multiple_of, *};
 use rustc_target::abi::Size;
@@ -100,8 +107,6 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
             std::iter::repeat(0u8).take(usize::try_from(map_length).unwrap()),
         )
         .unwrap();
-        // Memory mappings don't use provenance, and are always exposed.
-        Machine::expose_ptr(this, ptr)?;
 
         Ok(Scalar::from_pointer(ptr, this))
     }
@@ -113,43 +118,21 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
     ) -> InterpResult<'tcx, Scalar<Provenance>> {
         let this = self.eval_context_mut();
 
-        let addr = this.read_target_usize(addr)?;
+        let addr = this.read_pointer(addr)?;
         let length = this.read_target_usize(length)?;
 
-        // addr must be a multiple of the page size
+        // addr must be a multiple of the page size, but apart from that munmap is just implemented
+        // as a dealloc.
         #[allow(clippy::arithmetic_side_effects)] // PAGE_SIZE is nonzero
-        if addr % this.machine.page_size != 0 {
+        if addr.addr().bytes() % this.machine.page_size != 0 {
             this.set_last_error(Scalar::from_i32(this.eval_libc_i32("EINVAL")))?;
             return Ok(Scalar::from_i32(-1));
         }
 
-        let length = round_to_next_multiple_of(length, this.machine.page_size);
-
-        let ptr = Machine::ptr_from_addr_cast(this, addr)?;
-
-        let Ok(ptr) = ptr.into_pointer_or_addr() else {
-            throw_unsup_format!("Miri only supports munmap on memory allocated directly by mmap");
-        };
-        let Some((alloc_id, offset, _prov)) = Machine::ptr_get_alloc(this, ptr) else {
-            throw_unsup_format!("Miri only supports munmap on memory allocated directly by mmap");
-        };
-
-        // Elsewhere in this function we are careful to check what we can and throw an unsupported
-        // error instead of Undefined Behavior when use of this function falls outside of the
-        // narrow scope we support. We deliberately do not check the MemoryKind of this allocation,
-        // because we want to report UB on attempting to unmap memory that Rust "understands", such
-        // the stack, heap, or statics.
-        let (_kind, alloc) = this.memory.alloc_map().get(alloc_id).unwrap();
-        if offset != Size::ZERO || alloc.len() as u64 != length {
-            throw_unsup_format!(
-                "Miri only supports munmap calls that exactly unmap a region previously returned by mmap"
-            );
-        }
-
-        let len = Size::from_bytes(alloc.len() as u64);
+        let length = Size::from_bytes(round_to_next_multiple_of(length, this.machine.page_size));
         this.deallocate_ptr(
-            ptr.into(),
-            Some((len, this.machine.page_align())),
+            addr,
+            Some((length, this.machine.page_align())),
             MemoryKind::Machine(MiriMemoryKind::Mmap),
         )?;
 

tests/fail-dep/shims/mmap_use_after_munmap.stderr

@@ -1,18 +1,3 @@
-warning: integer-to-pointer cast
-  --> $DIR/mmap_use_after_munmap.rs:LL:CC
-   |
-LL |         libc::munmap(ptr, 4096);
-   |         ^^^^^^^^^^^^^^^^^^^^^^^ integer-to-pointer cast
-   |
-   = help: This program is using integer-to-pointer casts or (equivalently) `ptr::from_exposed_addr`,
-   = help: which means that Miri might miss pointer bugs in this program.
-   = help: See https://doc.rust-lang.org/nightly/std/ptr/fn.from_exposed_addr.html for more details on that operation.
-   = help: To ensure that Miri does not miss bugs in your program, use Strict Provenance APIs (https://doc.rust-lang.org/nightly/std/ptr/index.html#strict-provenance, https://crates.io/crates/sptr) instead.
-   = help: You can then pass the `-Zmiri-strict-provenance` flag to Miri, to ensure you are not relying on `from_exposed_addr` semantics.
-   = help: Alternatively, the `-Zmiri-permissive-provenance` flag disables this warning.
-   = note: BACKTRACE:
-   = note: inside `main` at $DIR/mmap_use_after_munmap.rs:LL:CC
-
 error: Undefined Behavior: memory access failed: ALLOC has been freed, so this pointer is dangling
   --> $DIR/mmap_use_after_munmap.rs:LL:CC
    |
@@ -43,5 +28,5 @@ LL |         libc::munmap(ptr, 4096);
 
 note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace
 
-error: aborting due to 1 previous error; 1 warning emitted
+error: aborting due to 1 previous error
 

tests/fail-dep/shims/munmap_partial.rs

@@ -1,5 +1,6 @@
-//! Our mmap/munmap support is a thin wrapper over Interpcx::allocate_ptr. Since the underlying
-//! layer has much more UB than munmap does, we need to be sure we throw an unsupported error here.
+//! The man pages for mmap/munmap suggest that it is possible to partly unmap a previously-mapped
+//! region of addres space, but to LLVM that would be partial deallocation, which LLVM does not
+//! support. So even though the man pages say this sort of use is possible, we must report UB.
 //@ignore-target-windows: No libc on Windows
 
 fn main() {
@@ -13,6 +14,6 @@ fn main() {
             0,
         );
         libc::munmap(ptr, 1);
-        //~^ ERROR: unsupported operation
+        //~^ ERROR: Undefined Behavior
     }
 }

tests/fail-dep/shims/munmap_partial.stderr

@@ -1,29 +1,15 @@
-warning: integer-to-pointer cast
+error: Undefined Behavior: incorrect layout on deallocation: ALLOC has size 8192 and alignment ALIGN, but gave size 4096 and alignment ALIGN
   --> $DIR/munmap_partial.rs:LL:CC
    |
 LL |         libc::munmap(ptr, 1);
-   |         ^^^^^^^^^^^^^^^^^^^^ integer-to-pointer cast
+   |         ^^^^^^^^^^^^^^^^^^^^ incorrect layout on deallocation: ALLOC has size 8192 and alignment ALIGN, but gave size 4096 and alignment ALIGN
    |
-   = help: This program is using integer-to-pointer casts or (equivalently) `ptr::from_exposed_addr`,
-   = help: which means that Miri might miss pointer bugs in this program.
-   = help: See https://doc.rust-lang.org/nightly/std/ptr/fn.from_exposed_addr.html for more details on that operation.
-   = help: To ensure that Miri does not miss bugs in your program, use Strict Provenance APIs (https://doc.rust-lang.org/nightly/std/ptr/index.html#strict-provenance, https://crates.io/crates/sptr) instead.
-   = help: You can then pass the `-Zmiri-strict-provenance` flag to Miri, to ensure you are not relying on `from_exposed_addr` semantics.
-   = help: Alternatively, the `-Zmiri-permissive-provenance` flag disables this warning.
-   = note: BACKTRACE:
-   = note: inside `main` at $DIR/munmap_partial.rs:LL:CC
-
-error: unsupported operation: Miri only supports munmap calls that exactly unmap a region previously returned by mmap
-  --> $DIR/munmap_partial.rs:LL:CC
-   |
-LL |         libc::munmap(ptr, 1);
-   |         ^^^^^^^^^^^^^^^^^^^^ Miri only supports munmap calls that exactly unmap a region previously returned by mmap
-   |
-   = help: this is likely not a bug in the program; it indicates that the program performed an operation that the interpreter does not support
+   = 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/munmap_partial.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; 1 warning emitted
+error: aborting due to 1 previous error
 

tests/pass-dep/shims/mmap.rs

@@ -28,9 +28,8 @@ fn test_mmap() {
     }
     assert!(slice.iter().all(|b| *b == 1));
 
-    // Ensure that we can munmap with just an integer
-    let just_an_address = ptr::invalid_mut(ptr.addr());
-    let res = unsafe { libc::munmap(just_an_address, page_size) };
+    // Ensure that we can munmap
+    let res = unsafe { libc::munmap(ptr, page_size) };
     assert_eq!(res, 0i32);
 }