[vfs2][gofer] Fix file creation flags sent to gofer.

Fixes php runtime test ext/standard/tests/file/readfile_basic.phpt
Fixes #3516

fsgofers only want the access mode in the OpenFlags passed to Create(). If more
flags are supplied (like O_APPEND in this case), read/write from that fd will
fail with EBADF. See runsc/fsgofer/fsgofer.go:WriteAt()
VFS2 was providing more than just access modes. So filtering the flags using
p9.OpenFlagsModeMask == linux.O_ACCMODE fixes the issue.

Gofer in VFS1 also only extracts the access mode flags while making the create
RPC. See pkg/sentry/fs/gofer/path.go:Create()

Even in VFS2, when we open a handle, we extract out only the access mode flags
+ O_TRUNC.
See third_party/gvisor/pkg/sentry/fsimpl/gofer/handle.go:openHandle()

Added a test for this.

PiperOrigin-RevId: 326574829

2 files changed, 22 insertions, 4 deletions

diff --git a/pkg/sentry/fsimpl/gofer/filesystem.go b/pkg/sentry/fsimpl/gofer/filesystem.go
index 40fec890a..610a7ed78 100644
--- a/pkg/sentry/fsimpl/gofer/filesystem.go
+++ b/pkg/sentry/fsimpl/gofer/filesystem.go
@@ -1082,10 +1082,8 @@ func (d *dentry) createAndOpenChildLocked(ctx context.Context, rp *vfs.Resolving
 	}
 	creds := rp.Credentials()
 	name := rp.Component()
-	// Filter file creation flags and O_LARGEFILE out; the create RPC already
-	// has the semantics of O_CREAT|O_EXCL, while some servers will choke on
-	// O_LARGEFILE.
-	createFlags := p9.OpenFlags(opts.Flags &^ (vfs.FileCreationFlags | linux.O_LARGEFILE))
+	// We only want the access mode for creating the file.
+	createFlags := p9.OpenFlags(opts.Flags) & p9.OpenFlagsModeMask
 	fdobj, openFile, createQID, _, err := dirfile.create(ctx, name, createFlags, (p9.FileMode)(opts.Mode), (p9.UID)(creds.EffectiveKUID), (p9.GID)(creds.EffectiveKGID))
 	if err != nil {
 		dirfile.close(ctx)
diff --git a/test/syscalls/linux/open.cc b/test/syscalls/linux/open.cc
index c7147c20b..8f0c9cb49 100644
--- a/test/syscalls/linux/open.cc
+++ b/test/syscalls/linux/open.cc
@@ -147,6 +147,26 @@ TEST_F(OpenTest, WriteOnly) {
   EXPECT_THAT(write(wo_file.get(), &buf, 1), SyscallSucceedsWithValue(1));
 }
 
+TEST_F(OpenTest, CreateWithAppend) {
+  std::string data = "text";
+  std::string new_file = NewTempAbsPath();
+  const FileDescriptor file = ASSERT_NO_ERRNO_AND_VALUE(
+      Open(new_file, O_WRONLY | O_APPEND | O_CREAT, 0666));
+  EXPECT_THAT(write(file.get(), data.c_str(), data.size()),
+              SyscallSucceedsWithValue(data.size()));
+  EXPECT_THAT(lseek(file.get(), 0, SEEK_SET), SyscallSucceeds());
+  EXPECT_THAT(write(file.get(), data.c_str(), data.size()),
+              SyscallSucceedsWithValue(data.size()));
+
+  // Check that the size of the file is correct and that the offset has been
+  // incremented to that size.
+  struct stat s0;
+  EXPECT_THAT(fstat(file.get(), &s0), SyscallSucceeds());
+  EXPECT_EQ(s0.st_size, 2 * data.size());
+  EXPECT_THAT(lseek(file.get(), 0, SEEK_CUR),
+              SyscallSucceedsWithValue(2 * data.size()));
+}
+
 TEST_F(OpenTest, ReadWrite) {
   char buf;
   const FileDescriptor rw_file =