Auto merge of #1159 - divergentdave:shim-seek, r=RalfJung

Add shim for seeking files

This adds a shim for `lseek64` (`lseek` on macOS). This enables the use of `<File as Seek>::seek`. Testing is included.

src/shims/foreign_items.rs

@@ -489,6 +489,13 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
                 this.write_scalar(Scalar::from_int(result, dest.layout.size), dest)?;
             }
 
+            | "lseek64"
+            | "lseek"
+            => {
+                let result = this.lseek64(args[0], args[1], args[2])?;
+                this.write_scalar(Scalar::from_int(result, dest.layout.size), dest)?;
+            }
+
             "unlink" => {
                 let result = this.unlink(args[0])?;
                 this.write_scalar(Scalar::from_int(result, dest.layout.size), dest)?;

src/shims/fs.rs

@@ -1,7 +1,7 @@
 use std::collections::HashMap;
 use std::convert::{TryFrom, TryInto};
 use std::fs::{remove_file, File, OpenOptions};
-use std::io::{Read, Write};
+use std::io::{Read, Seek, SeekFrom, Write};
 use std::path::PathBuf;
 use std::time::SystemTime;
 
@@ -264,6 +264,40 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
         }
     }
 
+    fn lseek64(
+        &mut self,
+        fd_op: OpTy<'tcx, Tag>,
+        offset_op: OpTy<'tcx, Tag>,
+        whence_op: OpTy<'tcx, Tag>,
+    ) -> InterpResult<'tcx, i64> {
+        let this = self.eval_context_mut();
+
+        this.check_no_isolation("lseek64")?;
+
+        let fd = this.read_scalar(fd_op)?.to_i32()?;
+        let offset = this.read_scalar(offset_op)?.to_i64()?;
+        let whence = this.read_scalar(whence_op)?.to_i32()?;
+
+        let seek_from = if whence == this.eval_libc_i32("SEEK_SET")? {
+            SeekFrom::Start(offset as u64)
+        } else if whence == this.eval_libc_i32("SEEK_CUR")? {
+            SeekFrom::Current(offset)
+        } else if whence == this.eval_libc_i32("SEEK_END")? {
+            SeekFrom::End(offset)
+        } else {
+            let einval = this.eval_libc("EINVAL")?;
+            this.set_last_error(einval)?;
+            return Ok(-1);
+        };
+
+        if let Some(handle) = this.machine.file_handler.handles.get_mut(&fd) {
+            let result = handle.file.seek(seek_from).map(|offset| offset as i64);
+            this.try_unwrap_io_result(result)
+        } else {
+            this.handle_not_found()
+        }
+    }
+
     fn unlink(&mut self, path_op: OpTy<'tcx, Tag>) -> InterpResult<'tcx, i32> {
         let this = self.eval_context_mut();
 

tests/run-pass/fs.rs

@@ -2,7 +2,7 @@
 // compile-flags: -Zmiri-disable-isolation
 
 use std::fs::{File, remove_file};
-use std::io::{Read, Write, ErrorKind, Result};
+use std::io::{Read, Write, ErrorKind, Result, Seek, SeekFrom};
 use std::path::{PathBuf, Path};
 
 fn test_metadata(bytes: &[u8], path: &Path) -> Result<()> {
@@ -42,6 +42,23 @@ fn main() {
     file.read_to_end(&mut contents).unwrap();
     assert_eq!(bytes, contents.as_slice());
 
+    // Test that seeking to the beginning and reading until EOF gets the text again.
+    file.seek(SeekFrom::Start(0)).unwrap();
+    let mut contents = Vec::new();
+    file.read_to_end(&mut contents).unwrap();
+    assert_eq!(bytes, contents.as_slice());
+    // Test seeking relative to the end of the file.
+    file.seek(SeekFrom::End(-1)).unwrap();
+    let mut contents = Vec::new();
+    file.read_to_end(&mut contents).unwrap();
+    assert_eq!(&bytes[bytes.len() - 1..], contents.as_slice());
+    // Test seeking relative to the current position.
+    file.seek(SeekFrom::Start(5)).unwrap();
+    file.seek(SeekFrom::Current(-3)).unwrap();
+    let mut contents = Vec::new();
+    file.read_to_end(&mut contents).unwrap();
+    assert_eq!(&bytes[2..], contents.as_slice());
+
     // Test that metadata of an absolute path is correct.
     test_metadata(bytes, &path).unwrap();
     // Test that metadata of a relative path is correct.