simplify path checking and reword comments

Author: pvdrz

src/shims/fs.rs

@@ -288,7 +288,7 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
 
         // Under normal circumstances, we would use `deref_operand(statxbuf_op)` to produce a
         // proper `MemPlace` and then write the results of this function to it. However, the
-        // syscall API in Rust is untyped. This means that all the `statx` parameters are provided
+        // `syscall` function is untyped. This means that all the `statx` parameters are provided
         // as `isize`s instead of having the proper types. Thus, we have to recover the layout of
         // `statxbuf_op` by using the `libc::statx` struct type.
         let statxbuf_place = {
@@ -303,39 +303,23 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
             this.ref_to_mplace(statxbuf_imm)?
         };
 
-        let pathname: PathBuf = this.read_os_str_from_c_str(pathname_scalar)?.into();
-        // This argument should be a `c_int` but the syscall API just returns a pointer sized
-        // integer.
+        let path: PathBuf = this.read_os_str_from_c_str(pathname_scalar)?.into();
+        // `flags` should be a `c_int` but the `syscall` function provides an `isize`.
         let flags: i32 = this.try_from_ptr_sized_operand(flags_op)?;
 
-        let at_fdcwd = this.eval_libc_i32("AT_FDCWD")?;
-        // Now we need to resolve the path of the target file. This was done following the manual
-        // entry for `statx`
-
-        // If `pathname` is absolute we can ignore `dirfd`.
-        let path = if pathname.is_absolute() {
-            pathname
-        } else {
-            // This argument should be a `c_int` but the syscall API just returns a pointer sized
-            // integer.
-            let dirfd: i32 = this.try_from_ptr_sized_operand(dirfd_op)?;
-            // If `dirfd` is `AT_FDCWD`, `pathname` is relative to the current directory.
-            if dirfd == at_fdcwd {
-                match std::env::current_dir() {
-                    Ok(cwd) => cwd.join(pathname),
-                    Err(e) => {
-                        this.set_last_error_from_io_error(e)?;
-                        return Ok(-1);
-                    }
-                }
-            } else {
-                // This behavior is specified but cannot be tested from `libstd`. If you found this
-                // error please open an issue reporting it.
-                throw_unsup_format!(
-                    "Using statx with a fildes different from `AT_FDCWD` is not supported"
-                )
-            }
-        };
+        // we only support interpreting `path` as an absolute directory or as a directory relative
+        // to `dirfd` when the latter is `AT_FDCWD`. Other behavior is specified but it cannot be
+        // tested from `libstd`. If you found this error, please open an issue reporting it.
+
+        // `dirfd` should be a `c_int` but the `syscall` function provides an `isize`.
+        if !(path.is_absolute()
+            || this.try_from_ptr_sized_operand::<i32>(dirfd_op)?
+                == this.eval_libc_i32("AT_FDCWD")?)
+        {
+            throw_unsup_format!(
+                "Using statx with a file descriptor different from `AT_FDCWD` is not supported"
+            )
+        }
 
         // the `_mask_op` paramter specifies the file information that the caller requested.
         // However `statx` is allowed to return information that was not requested or to not
@@ -366,8 +350,7 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
         // The `mode` field specifies the type of the file and the permissions over the file for
         // the owner, its group and other users. Given that we can only provide the file type
         // without using platform specific methods, we only set the bits corresponding to the file
-        // type. This argument should be an `__u16` but the syscall API just returns a pointer sized
-        // integer.
+        // type. This should be an `__u16` but `libc` provides its values as `u32`.
         let mode = if file_type.is_file() {
             this.eval_libc("S_IFREG")?
         } else if file_type.is_dir() {
@@ -377,7 +360,7 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
         }.to_u32()?;
 
         let mode: u16 = mode.try_into().map_err(|e|
-            err_unsup_format!("Failed to convert 32-bit operand to u16: {}",
+            err_unsup_format!("Failed to convert 32-bit value to u16: {}",
             e
         ))?;