Add pread and pwrite shims

src/shims/unix/fd.rs

@@ -36,6 +36,30 @@ pub trait FileDescription: std::fmt::Debug + Any {
         throw_unsup_format!("cannot write to {}", self.name());
     }
 
+    /// Reads as much as possible into the given buffer from a given offset,
+    /// and returns the number of bytes read.
+    fn pread<'tcx>(
+        &mut self,
+        _communicate_allowed: bool,
+        _bytes: &mut [u8],
+        _offset: u64,
+        _ecx: &mut MiriInterpCx<'tcx>,
+    ) -> InterpResult<'tcx, io::Result<usize>> {
+        throw_unsup_format!("cannot pread from {}", self.name());
+    }
+
+    /// Writes as much as possible from the given buffer starting at a given offset,
+    /// and returns the number of bytes written.
+    fn pwrite<'tcx>(
+        &mut self,
+        _communicate_allowed: bool,
+        _bytes: &[u8],
+        _offset: u64,
+        _ecx: &mut MiriInterpCx<'tcx>,
+    ) -> InterpResult<'tcx, io::Result<usize>> {
+        throw_unsup_format!("cannot pwrite to {}", self.name());
+    }
+
     /// Seeks to the given offset (which can be relative to the beginning, end, or current position).
     /// Returns the new position from the start of the stream.
     fn seek<'tcx>(
@@ -380,7 +404,18 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
         Ok((-1).into())
     }
 
-    fn read(&mut self, fd: i32, buf: Pointer, count: u64) -> InterpResult<'tcx, i64> {
+    /// Read data from `fd` into buffer specified by `buf` and `count`.
+    ///
+    /// If `offset` is `None`, reads data from current cursor position associated with `fd`
+    /// and updates cursor position on completion. Otherwise, reads from the specified offset
+    /// and keeps the cursor unchanged.
+    fn read(
+        &mut self,
+        fd: i32,
+        buf: Pointer,
+        count: u64,
+        offset: Option<i128>,
+    ) -> InterpResult<'tcx, i64> {
         let this = self.eval_context_mut();
 
         // Isolation check is done via `FileDescriptor` trait.
@@ -398,25 +433,31 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
         let communicate = this.machine.communicate();
 
         // We temporarily dup the FD to be able to retain mutable access to `this`.
-        let Some(file_descriptor) = this.machine.fds.dup(fd) else {
+        let Some(fd) = this.machine.fds.dup(fd) else {
             trace!("read: FD not found");
             return this.fd_not_found();
         };
 
-        trace!("read: FD mapped to {:?}", file_descriptor);
+        trace!("read: FD mapped to {fd:?}");
         // We want to read at most `count` bytes. We are sure that `count` is not negative
         // because it was a target's `usize`. Also we are sure that its smaller than
         // `usize::MAX` because it is bounded by the host's `isize`.
         let mut bytes = vec![0; usize::try_from(count).unwrap()];
-        // `File::read` never returns a value larger than `count`,
-        // so this cannot fail.
-        let result = file_descriptor
-            .borrow_mut()
-            .read(communicate, &mut bytes, this)?
-            .map(|c| i64::try_from(c).unwrap());
-        drop(file_descriptor);
-
-        match result {
+        let result = match offset {
+            None => fd.borrow_mut().read(communicate, &mut bytes, this),
+            Some(offset) => {
+                let Ok(offset) = u64::try_from(offset) else {
+                    let einval = this.eval_libc("EINVAL");
+                    this.set_last_error(einval)?;
+                    return Ok(-1);
+                };
+                fd.borrow_mut().pread(communicate, &mut bytes, offset, this)
+            }
+        };
+        drop(fd);
+
+        // `File::read` never returns a value larger than `count`, so this cannot fail.
+        match result?.map(|c| i64::try_from(c).unwrap()) {
             Ok(read_bytes) => {
                 // If reading to `bytes` did not fail, we write those bytes to the buffer.
                 // Crucially, if fewer than `bytes.len()` bytes were read, only write
@@ -434,7 +475,13 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
         }
     }
 
-    fn write(&mut self, fd: i32, buf: Pointer, count: u64) -> InterpResult<'tcx, i64> {
+    fn write(
+        &mut self,
+        fd: i32,
+        buf: Pointer,
+        count: u64,
+        offset: Option<i128>,
+    ) -> InterpResult<'tcx, i64> {
         let this = self.eval_context_mut();
 
         // Isolation check is done via `FileDescriptor` trait.
@@ -451,16 +498,24 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
 
         let bytes = this.read_bytes_ptr_strip_provenance(buf, Size::from_bytes(count))?.to_owned();
         // We temporarily dup the FD to be able to retain mutable access to `this`.
-        let Some(file_descriptor) = this.machine.fds.dup(fd) else {
+        let Some(fd) = this.machine.fds.dup(fd) else {
             return this.fd_not_found();
         };
 
-        let result = file_descriptor
-            .borrow_mut()
-            .write(communicate, &bytes, this)?
-            .map(|c| i64::try_from(c).unwrap());
-        drop(file_descriptor);
+        let result = match offset {
+            None => fd.borrow_mut().write(communicate, &bytes, this),
+            Some(offset) => {
+                let Ok(offset) = u64::try_from(offset) else {
+                    let einval = this.eval_libc("EINVAL");
+                    this.set_last_error(einval)?;
+                    return Ok(-1);
+                };
+                fd.borrow_mut().pwrite(communicate, &bytes, offset, this)
+            }
+        };
+        drop(fd);
 
+        let result = result?.map(|c| i64::try_from(c).unwrap());
         this.try_unwrap_io_result(result)
     }
 }

src/shims/unix/foreign_items.rs

@@ -92,7 +92,7 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
                 let fd = this.read_scalar(fd)?.to_i32()?;
                 let buf = this.read_pointer(buf)?;
                 let count = this.read_target_usize(count)?;
-                let result = this.read(fd, buf, count)?;
+                let result = this.read(fd, buf, count, None)?;
                 this.write_scalar(Scalar::from_target_isize(result, this), dest)?;
             }
             "write" => {
@@ -101,7 +101,47 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
                 let buf = this.read_pointer(buf)?;
                 let count = this.read_target_usize(n)?;
                 trace!("Called write({:?}, {:?}, {:?})", fd, buf, count);
-                let result = this.write(fd, buf, count)?;
+                let result = this.write(fd, buf, count, None)?;
+                // Now, `result` is the value we return back to the program.
+                this.write_scalar(Scalar::from_target_isize(result, this), dest)?;
+            }
+            "pread" => {
+                let [fd, buf, count, offset] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
+                let fd = this.read_scalar(fd)?.to_i32()?;
+                let buf = this.read_pointer(buf)?;
+                let count = this.read_target_usize(count)?;
+                let offset = this.read_scalar(offset)?.to_int(this.libc_ty_layout("off_t").size)?;
+                let result = this.read(fd, buf, count, Some(offset))?;
+                this.write_scalar(Scalar::from_target_isize(result, this), dest)?;
+            }
+            "pwrite" => {
+                let [fd, buf, n, offset] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
+                let fd = this.read_scalar(fd)?.to_i32()?;
+                let buf = this.read_pointer(buf)?;
+                let count = this.read_target_usize(n)?;
+                let offset = this.read_scalar(offset)?.to_int(this.libc_ty_layout("off_t").size)?;
+                trace!("Called pwrite({:?}, {:?}, {:?}, {:?})", fd, buf, count, offset);
+                let result = this.write(fd, buf, count, Some(offset))?;
+                // Now, `result` is the value we return back to the program.
+                this.write_scalar(Scalar::from_target_isize(result, this), dest)?;
+            }
+            "pread64" => {
+                let [fd, buf, count, offset] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
+                let fd = this.read_scalar(fd)?.to_i32()?;
+                let buf = this.read_pointer(buf)?;
+                let count = this.read_target_usize(count)?;
+                let offset = this.read_scalar(offset)?.to_int(this.libc_ty_layout("off64_t").size)?;
+                let result = this.read(fd, buf, count, Some(offset))?;
+                this.write_scalar(Scalar::from_target_isize(result, this), dest)?;
+            }
+            "pwrite64" => {
+                let [fd, buf, n, offset] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
+                let fd = this.read_scalar(fd)?.to_i32()?;
+                let buf = this.read_pointer(buf)?;
+                let count = this.read_target_usize(n)?;
+                let offset = this.read_scalar(offset)?.to_int(this.libc_ty_layout("off64_t").size)?;
+                trace!("Called pwrite64({:?}, {:?}, {:?}, {:?})", fd, buf, count, offset);
+                let result = this.write(fd, buf, count, Some(offset))?;
                 // Now, `result` is the value we return back to the program.
                 this.write_scalar(Scalar::from_target_isize(result, this), dest)?;
             }

src/shims/unix/fs.rs

@@ -49,6 +49,54 @@ impl FileDescription for FileHandle {
         Ok(self.file.write(bytes))
     }
 
+    fn pread<'tcx>(
+        &mut self,
+        communicate_allowed: bool,
+        bytes: &mut [u8],
+        offset: u64,
+        _ecx: &mut MiriInterpCx<'tcx>,
+    ) -> InterpResult<'tcx, io::Result<usize>> {
+        assert!(communicate_allowed, "isolation should have prevented even opening a file");
+        // Emulates pread using seek + read + seek to restore cursor position.
+        // Correctness of this emulation relies on sequential nature of Miri execution.
+        // The closure is used to emulate `try` block, since we "bubble" `io::Error` using `?`.
+        let mut f = || {
+            let cursor_pos = self.file.stream_position()?;
+            self.file.seek(SeekFrom::Start(offset))?;
+            let res = self.file.read(bytes);
+            // Attempt to restore cursor position even if the read has failed
+            self.file
+                .seek(SeekFrom::Start(cursor_pos))
+                .expect("failed to restore file position, this shouldn't be possible");
+            res
+        };
+        Ok(f())
+    }
+
+    fn pwrite<'tcx>(
+        &mut self,
+        communicate_allowed: bool,
+        bytes: &[u8],
+        offset: u64,
+        _ecx: &mut MiriInterpCx<'tcx>,
+    ) -> InterpResult<'tcx, io::Result<usize>> {
+        assert!(communicate_allowed, "isolation should have prevented even opening a file");
+        // Emulates pwrite using seek + write + seek to restore cursor position.
+        // Correctness of this emulation relies on sequential nature of Miri execution.
+        // The closure is used to emulate `try` block, since we "bubble" `io::Error` using `?`.
+        let mut f = || {
+            let cursor_pos = self.file.stream_position()?;
+            self.file.seek(SeekFrom::Start(offset))?;
+            let res = self.file.write(bytes);
+            // Attempt to restore cursor position even if the write has failed
+            self.file
+                .seek(SeekFrom::Start(cursor_pos))
+                .expect("failed to restore file position, this shouldn't be possible");
+            res
+        };
+        Ok(f())
+    }
+
     fn seek<'tcx>(
         &mut self,
         communicate_allowed: bool,

tests/pass/shims/fs.rs

@@ -30,6 +30,8 @@ fn main() {
     test_directory();
     test_canonicalize();
     test_from_raw_os_error();
+    #[cfg(unix)]
+    test_pread_pwrite();
 }
 
 fn test_path_conversion() {
@@ -303,3 +305,42 @@ fn test_from_raw_os_error() {
     // Make sure we can also format this.
     let _ = format!("{error:?}");
 }
+
+#[cfg(unix)]
+fn test_pread_pwrite() {
+    use std::os::unix::fs::FileExt;
+
+    let bytes = b"hello world";
+    let path = utils::prepare_with_content("miri_test_fs_pread_pwrite.txt", bytes);
+    let mut f = OpenOptions::new().read(true).write(true).open(path).unwrap();
+
+    let mut buf1 = [0u8; 3];
+    f.seek(SeekFrom::Start(5)).unwrap();
+
+    // Check that we get expected result after seek
+    f.read_exact(&mut buf1).unwrap();
+    assert_eq!(&buf1, b" wo");
+    f.seek(SeekFrom::Start(5)).unwrap();
+
+    // Check pread
+    f.read_exact_at(&mut buf1, 2).unwrap();
+    assert_eq!(&buf1, b"llo");
+    f.read_exact_at(&mut buf1, 6).unwrap();
+    assert_eq!(&buf1, b"wor");
+
+    // Ensure that cursor position is not changed
+    f.read_exact(&mut buf1).unwrap();
+    assert_eq!(&buf1, b" wo");
+    f.seek(SeekFrom::Start(5)).unwrap();
+
+    // Check pwrite
+    f.write_all_at(b" mo", 6).unwrap();
+
+    let mut buf2 = [0u8; 11];
+    f.read_exact_at(&mut buf2, 0).unwrap();
+    assert_eq!(&buf2, b"hello  mold");
+
+    // Ensure that cursor position is not changed
+    f.read_exact(&mut buf1).unwrap();
+    assert_eq!(&buf1, b"  m");
+}