From d84607762889d999f93b89e64e09d2a4dda63bf7 Mon Sep 17 00:00:00 2001 From: Jamie Liu Date: Wed, 13 May 2020 10:51:50 -0700 Subject: Enable overlayfs_stale_read by default for runsc. Linux 4.18 and later make reads and writes coherent between pre-copy-up and post-copy-up FDs representing the same file on an overlay filesystem. However, memory mappings remain incoherent: - Documentation/filesystems/overlayfs.rst, "Non-standard behavior": "If a file residing on a lower layer is opened for read-only and then memory mapped with MAP_SHARED, then subsequent changes to the file are not reflected in the memory mapping." - fs/overlay/file.c:ovl_mmap() passes through to the underlying FD without any management of coherence in the overlay. - Experimentally on Linux 5.2: ``` $ cat mmap_cat_page.c #include #include #include #include #include #include int main(int argc, char **argv) { if (argc < 2) { errx(1, "syntax: %s [FILE]", argv[0]); } const int fd = open(argv[1], O_RDONLY); if (fd < 0) { err(1, "open(%s)", argv[1]); } const size_t page_size = sysconf(_SC_PAGE_SIZE); void* page = mmap(NULL, page_size, PROT_READ, MAP_SHARED, fd, 0); if (page == MAP_FAILED) { err(1, "mmap"); } for (;;) { write(1, page, strnlen(page, page_size)); if (getc(stdin) == EOF) { break; } } return 0; } $ gcc -O2 -o mmap_cat_page mmap_cat_page.c $ mkdir lowerdir upperdir workdir overlaydir $ echo old > lowerdir/file $ sudo mount -t overlay -o "lowerdir=lowerdir,upperdir=upperdir,workdir=workdir" none overlaydir $ ./mmap_cat_page overlaydir/file old ^Z [1]+ Stopped ./mmap_cat_page overlaydir/file $ echo new > overlaydir/file $ cat overlaydir/file new $ fg ./mmap_cat_page overlaydir/file old ``` Therefore, while the VFS1 gofer client's behavior of reopening read FDs is only necessary pre-4.18, replacing existing memory mappings (in both sentry and application address spaces) with mappings of the new FD is required regardless of kernel version, and this latter behavior is common to both VFS1 and VFS2. Re-document accordingly, and change the runsc flag to enabled by default. New test: - Before this CL: https://source.cloud.google.com/results/invocations/5b222d2c-e918-4bae-afc4-407f5bac509b - After this CL: https://source.cloud.google.com/results/invocations/f28c747e-d89c-4d8c-a461-602b33e71aab PiperOrigin-RevId: 311361267 --- runsc/boot/config.go | 6 ++++-- runsc/container/container_test.go | 37 ------------------------------------- runsc/main.go | 2 +- 3 files changed, 5 insertions(+), 40 deletions(-) (limited to 'runsc') diff --git a/runsc/boot/config.go b/runsc/boot/config.go index 6d6a705f8..bcec7e4db 100644 --- a/runsc/boot/config.go +++ b/runsc/boot/config.go @@ -241,8 +241,10 @@ type Config struct { // ReferenceLeakMode sets reference leak check mode ReferenceLeakMode refs.LeakMode - // OverlayfsStaleRead causes cached FDs to reopen after a file is opened for - // write to workaround overlayfs limitation on kernels before 4.19. + // OverlayfsStaleRead instructs the sandbox to assume that the root mount + // is on a Linux overlayfs mount, which does not necessarily preserve + // coherence between read-only and subsequent writable file descriptors + // representing the "same" file. OverlayfsStaleRead bool // TestOnlyAllowRunAsCurrentUserWithoutChroot should only be used in diff --git a/runsc/container/container_test.go b/runsc/container/container_test.go index acf988aa0..7ba301331 100644 --- a/runsc/container/container_test.go +++ b/runsc/container/container_test.go @@ -2126,43 +2126,6 @@ func TestNetRaw(t *testing.T) { } } -// TestOverlayfsStaleRead most basic test that '--overlayfs-stale-read' works. -func TestOverlayfsStaleRead(t *testing.T) { - conf := testutil.TestConfig(t) - conf.OverlayfsStaleRead = true - - in, err := ioutil.TempFile(testutil.TmpDir(), "stale-read.in") - if err != nil { - t.Fatalf("ioutil.TempFile() failed: %v", err) - } - defer in.Close() - if _, err := in.WriteString("stale data"); err != nil { - t.Fatalf("in.Write() failed: %v", err) - } - - out, err := ioutil.TempFile(testutil.TmpDir(), "stale-read.out") - if err != nil { - t.Fatalf("ioutil.TempFile() failed: %v", err) - } - defer out.Close() - - const want = "foobar" - cmd := fmt.Sprintf("cat %q >&2 && echo %q> %q && cp %q %q", in.Name(), want, in.Name(), in.Name(), out.Name()) - spec := testutil.NewSpecWithArgs("/bin/bash", "-c", cmd) - if err := run(spec, conf); err != nil { - t.Fatalf("Error running container: %v", err) - } - - gotBytes, err := ioutil.ReadAll(out) - if err != nil { - t.Fatalf("out.Read() failed: %v", err) - } - got := strings.TrimSpace(string(gotBytes)) - if want != got { - t.Errorf("Wrong content in out file, got: %q. want: %q", got, want) - } -} - // TestTTYField checks TTY field returned by container.Processes(). func TestTTYField(t *testing.T) { stop := testutil.StartReaper() diff --git a/runsc/main.go b/runsc/main.go index 0625a06e0..920ed84a5 100644 --- a/runsc/main.go +++ b/runsc/main.go @@ -76,7 +76,7 @@ var ( fileAccess = flag.String("file-access", "exclusive", "specifies which filesystem to use for the root mount: exclusive (default), shared. Volume mounts are always shared.") fsGoferHostUDS = flag.Bool("fsgofer-host-uds", false, "allow the gofer to mount Unix Domain Sockets.") overlay = flag.Bool("overlay", false, "wrap filesystem mounts with writable overlay. All modifications are stored in memory inside the sandbox.") - overlayfsStaleRead = flag.Bool("overlayfs-stale-read", false, "reopen cached FDs after a file is opened for write to workaround overlayfs limitation on kernels before 4.19.") + overlayfsStaleRead = flag.Bool("overlayfs-stale-read", true, "assume root mount is an overlay filesystem") watchdogAction = flag.String("watchdog-action", "log", "sets what action the watchdog takes when triggered: log (default), panic.") panicSignal = flag.Int("panic-signal", -1, "register signal handling that panics. Usually set to SIGUSR2(12) to troubleshoot hangs. -1 disables it.") profile = flag.Bool("profile", false, "prepares the sandbox to use Golang profiler. Note that enabling profiler loosens the seccomp protection added to the sandbox (DO NOT USE IN PRODUCTION).") -- cgit v1.2.3