Fix a possible overflow due to use of fionread in poll_oneoff on Unix. (#881)

Closes #578.

crates/wasi-common/src/sys/unix/hostcalls_impl/misc.rs

@@ -124,9 +124,24 @@ fn poll_oneoff_handle_fd_event<'a>(
     ready_events: impl Iterator<Item = (FdEventData<'a>, yanix::poll::PollFd)>,
     events: &mut Vec<wasi::__wasi_event_t>,
 ) -> Result<()> {
+    use crate::fdentry::Descriptor;
     use std::{convert::TryInto, os::unix::prelude::AsRawFd};
     use yanix::{file::fionread, poll::PollFlags};
 
+    fn query_nbytes(fd: &Descriptor) -> Result<u64> {
+        // fionread may overflow for large files, so use another way for regular files.
+        if let Descriptor::OsHandle(os_handle) = fd {
+            let meta = os_handle.metadata()?;
+            if meta.file_type().is_file() {
+                use yanix::file::tell;
+                let len = meta.len();
+                let host_offset = unsafe { tell(os_handle.as_raw_fd())? };
+                return Ok(len - host_offset);
+            }
+        }
+        unsafe { Ok(fionread(fd.as_raw_fd())?.into()) }
+    }
+
     for (fd_event, poll_fd) in ready_events {
         log::debug!("poll_oneoff_handle_fd_event fd_event = {:?}", fd_event);
         log::debug!("poll_oneoff_handle_fd_event poll_fd = {:?}", poll_fd);
@@ -139,7 +154,7 @@ fn poll_oneoff_handle_fd_event<'a>(
         log::debug!("poll_oneoff_handle_fd_event revents = {:?}", revents);
 
         let nbytes = if fd_event.r#type == wasi::__WASI_EVENTTYPE_FD_READ {
-            unsafe { fionread(fd_event.descriptor.as_raw_fd())? }
+            query_nbytes(fd_event.descriptor)?
         } else {
             0
         };

crates/wasi-common/yanix/src/file.rs

@@ -235,8 +235,15 @@ pub unsafe fn fstat(fd: RawFd) -> Result<libc::stat> {
 }
 
 /// `fionread()` function, equivalent to `ioctl(fd, FIONREAD, *bytes)`.
-pub unsafe fn fionread(fd: RawFd) -> Result<usize> {
+pub unsafe fn fionread(fd: RawFd) -> Result<u32> {
     let mut nread: libc::c_int = 0;
     Errno::from_result(libc::ioctl(fd, libc::FIONREAD, &mut nread as *mut _))?;
     Ok(nread.try_into()?)
 }
+
+/// This function is unsafe because it operates on a raw file descriptor.
+/// It's provided, because std::io::Seek requires a mutable borrow.
+pub unsafe fn tell(fd: RawFd) -> Result<u64> {
+    let offset: i64 = Errno::from_result(libc::lseek(fd, 0, libc::SEEK_CUR))?;
+    Ok(offset.try_into()?)
+}