Don't report partialResult errors from sendfile

The input file descriptor is always a regular file, so sendfile can't lose any
data if it will not be able to write them to the output file descriptor.

Reported-by: syzbot+22d22330a35fa1c02155@syzkaller.appspotmail.com
PiperOrigin-RevId: 272730357

pkg/sentry/syscalls/linux/sys_splice.go

@@ -159,9 +159,14 @@ func Sendfile(t *kernel.Task, args arch.SyscallArguments) (uintptr, *kernel.Sysc
 		}, outFile.Flags().NonBlocking)
 	}
 
+	// Sendfile can't lose any data because inFD is always a regual file.
+	if n != 0 {
+		err = nil
+	}
+
 	// We can only pass a single file to handleIOError, so pick inFile
 	// arbitrarily. This is used only for debugging purposes.
-	return uintptr(n), nil, handleIOError(t, n != 0, err, kernel.ERESTARTSYS, "sendfile", inFile)
+	return uintptr(n), nil, handleIOError(t, false, err, kernel.ERESTARTSYS, "sendfile", inFile)
 }
 
 // Splice implements splice(2).
@@ -305,6 +310,11 @@ func Tee(t *kernel.Task, args arch.SyscallArguments) (uintptr, *kernel.SyscallCo
 		Dup:    true,
 	}, nonBlock)
 
+	// Tee doesn't change a state of inFD, so it can't lose any data.
+	if n != 0 {
+		err = nil
+	}
+
 	// See above; inFile is chosen arbitrarily here.
-	return uintptr(n), nil, handleIOError(t, n != 0, err, kernel.ERESTARTSYS, "tee", inFile)
+	return uintptr(n), nil, handleIOError(t, false, err, kernel.ERESTARTSYS, "tee", inFile)
 }

test/syscalls/linux/BUILD

@@ -1905,6 +1905,7 @@ cc_binary(
     srcs = ["sendfile.cc"],
     linkstatic = 1,
     deps = [
+        "//test/util:eventfd_util",
         "//test/util:file_descriptor",
         "//test/util:temp_path",
         "//test/util:test_main",

test/syscalls/linux/sendfile.cc

@@ -13,6 +13,7 @@
 // limitations under the License.
 
 #include <fcntl.h>
+#include <sys/eventfd.h>
 #include <sys/sendfile.h>
 #include <unistd.h>
 
@@ -21,6 +22,7 @@
 #include "absl/strings/string_view.h"
 #include "absl/time/clock.h"
 #include "absl/time/time.h"
+#include "test/util/eventfd_util.h"
 #include "test/util/file_descriptor.h"
 #include "test/util/temp_path.h"
 #include "test/util/test_util.h"
@@ -511,6 +513,23 @@ TEST(SendFileTest, SendPipeBlocks) {
               SyscallSucceedsWithValue(kDataSize));
 }
 
+TEST(SendFileTest, SendToSpecialFile) {
+  // Create temp file.
+  const TempPath in_file = ASSERT_NO_ERRNO_AND_VALUE(TempPath::CreateFileWith(
+      GetAbsoluteTestTmpdir(), "", TempPath::kDefaultFileMode));
+
+  const FileDescriptor inf =
+      ASSERT_NO_ERRNO_AND_VALUE(Open(in_file.path(), O_RDWR));
+  constexpr int kSize = 0x7ff;
+  ASSERT_THAT(ftruncate(inf.get(), kSize), SyscallSucceeds());
+
+  auto eventfd = ASSERT_NO_ERRNO_AND_VALUE(NewEventFD());
+
+  // eventfd can accept a number of bytes which is a multiple of 8.
+  EXPECT_THAT(sendfile(eventfd.get(), inf.get(), nullptr, 0xfffff),
+              SyscallSucceedsWithValue(kSize & (~7)));
+}
+
 }  // namespace
 
 }  // namespace testing