pipe: writeLocked has to return ErrWouldBlock if the pipe is full

PiperOrigin-RevId: 356450303

pkg/sentry/fsimpl/tmpfs/regular_file.go

@@ -656,6 +656,9 @@ func (rw *regularFileReadWriter) WriteFromBlocks(srcs safemem.BlockSeq) (uint64,
 
 			// Write to that memory as usual.
 			seg, gap = rw.file.data.Insert(gap, gapMR, fr.Start), fsutil.FileRangeGapIterator{}
+
+		default:
+			panic("unreachable")
 		}
 	}
 exitLoop:

pkg/sentry/kernel/pipe/pipe.go

@@ -247,11 +247,15 @@ func (p *Pipe) writeLocked(count int64, f func(safemem.BlockSeq) (uint64, error)
 		return 0, syscall.EPIPE
 	}
 
-	// POSIX requires that a write smaller than atomicIOBytes (PIPE_BUF) be
-	// atomic, but requires no atomicity for writes larger than this.
 	avail := p.max - p.size
+	if avail == 0 {
+		return 0, syserror.ErrWouldBlock
+	}
 	short := false
 	if count > avail {
+		// POSIX requires that a write smaller than atomicIOBytes
+		// (PIPE_BUF) be atomic, but requires no atomicity for writes
+		// larger than this.
 		if count <= atomicIOBytes {
 			return 0, syserror.ErrWouldBlock
 		}

test/syscalls/linux/sendfile.cc

@@ -551,6 +551,29 @@ TEST(SendFileTest, SendPipeEOF) {
               SyscallSucceedsWithValue(0));
 }
 
+TEST(SendFileTest, SendToFullPipeReturnsEAGAIN) {
+  // Create and open an empty input file.
+  const TempPath in_file = ASSERT_NO_ERRNO_AND_VALUE(TempPath::CreateFile());
+  const FileDescriptor in_fd =
+      ASSERT_NO_ERRNO_AND_VALUE(Open(in_file.path(), O_RDWR));
+
+  // Set up the output pipe.
+  int fds[2];
+  ASSERT_THAT(pipe2(fds, O_NONBLOCK), SyscallSucceeds());
+  const FileDescriptor rfd(fds[0]);
+  const FileDescriptor wfd(fds[1]);
+
+  int pipe_size = -1;
+  ASSERT_THAT(pipe_size = fcntl(wfd.get(), F_GETPIPE_SZ), SyscallSucceeds());
+  int data_size = pipe_size * 8;
+  ASSERT_THAT(ftruncate(in_fd.get(), data_size), SyscallSucceeds());
+
+  ASSERT_THAT(sendfile(wfd.get(), in_fd.get(), 0, data_size),
+              SyscallSucceeds());
+  EXPECT_THAT(sendfile(wfd.get(), in_fd.get(), 0, data_size),
+              SyscallFailsWithErrno(EAGAIN));
+}
+
 TEST(SendFileTest, SendPipeBlocks) {
   // Create temp file.
   constexpr char kData[] =