vexos: seek to end of file to determine stream end position

Author: tropicaaal

library/std/src/sys/fs/vexos.rs

@@ -16,6 +16,18 @@ pub use unsupported_fs::{
     unlink,
 };
 
+/// VEXos file descriptor.
+///
+/// This stores an opaque pointer to a [FatFs file object structure] managed by VEXos
+/// representing an open file on disk.
+///
+/// [FatFs file object structure]: https://github.com/Xilinx/embeddedsw/blob/master/lib/sw_services/xilffs/src/include/ff.h?rgh-link-date=2025-09-23T20%3A03%3A43Z#L215
+///
+/// # Safety
+///
+/// Since this platform uses a pointer to to an internal filesystem structure with a lifetime
+/// associated with it (rather than a UNIX-style file descriptor table), care must be taken to
+/// ensure that the pointer held by `FileDesc` is valid for as long as it exists.
 #[derive(Debug)]
 struct FileDesc(*mut vex_sdk::FIL);
 
@@ -436,9 +448,16 @@ impl File {
                 } else {
                     // `vexFileSeek` does not support seeking with negative offset, meaning
                     // we have to calculate the offset from the end of the file ourselves.
+
+                    // Seek to the end of the file to get the end position in the open buffer.
+                    map_fresult(vex_sdk::vexFileSeek(self.fd.0, 0, SEEK_END))?;
+                    let end_position = self.tell()?;
+
                     map_fresult(vex_sdk::vexFileSeek(
                         self.fd.0,
-                        try_convert_offset(self.file_attr()?.size() as i64 + offset)?,
+                        // NOTE: Files internally use a 32-bit representation for stream
+                        // position, so `end_position as i64` should never overflow.
+                        try_convert_offset(end_position as i64 + offset)?,
                         SEEK_SET,
                     ))?
                 }
@@ -517,12 +536,11 @@ pub fn lstat(p: &Path) -> io::Result<FileAttr> {
     stat(p)
 }
 
-// NOTE: Cannot use `copy` from `common` here, since we have no support
-// for `File::set_permissions`.
+// Cannot use `copy` from `common` here, since `File::set_permissions` is unsupported on this target.
 pub fn copy(from: &Path, to: &Path) -> io::Result<u64> {
     use crate::fs::File;
 
-    // If `from` is a directory, this call should fail.
+    // NOTE: If `from` is a directory, this call should fail due to vexFileOpen* returning null.
     let mut reader = File::open(from)?;
     let mut writer = File::create(to)?;