enable falloc on btrfs if CoW is disabled

2 files changed, 41 insertions, 3 deletions

diff --git a/internal/fusefrontend/root_node.go b/internal/fusefrontend/root_node.go
index aa26b9c..38d070d 100644
--- a/internal/fusefrontend/root_node.go
+++ b/internal/fusefrontend/root_node.go
@@ -92,7 +92,9 @@ func NewRootNode(args Args, c *contentenc.ContentEnc, n *nametransform.NameTrans
 	}
 	// Suppress the message if the user has already specified -noprealloc
 	if rn.quirks&syscallcompat.QuirkBtrfsBrokenFalloc != 0 && !args.NoPrealloc {
-		syscallcompat.LogQuirk("Btrfs detected, forcing -noprealloc. See https://github.com/rfjakob/gocryptfs/issues/395 for why.")
+		syscallcompat.LogQuirk("Btrfs detected, forcing -noprealloc. " +
+			"Use \"chattr +C\" on the backing directory to enable NOCOW and allow preallocation. " +
+			"See https://github.com/rfjakob/gocryptfs/issues/395 for details.")
 	}
 	if statErr == nil {
 		rn.inoMap.TranslateStat(&st)
diff --git a/internal/syscallcompat/quirks_linux.go b/internal/syscallcompat/quirks_linux.go
index 87af03d..35f754d 100644
--- a/internal/syscallcompat/quirks_linux.go
+++ b/internal/syscallcompat/quirks_linux.go
@@ -1,11 +1,38 @@
 package syscallcompat
 
 import (
+	"syscall"
+
 	"golang.org/x/sys/unix"
 
 	"github.com/rfjakob/gocryptfs/v2/internal/tlog"
 )
 
+// FS_NOCOW_FL is the flag set by "chattr +C" to disable copy-on-write on
+// btrfs. Not exported by golang.org/x/sys/unix, value from linux/fs.h.
+const FS_NOCOW_FL = 0x00800000
+
+// dirHasNoCow checks whether the directory at the given path has the
+// NOCOW (No Copy-on-Write) attribute set (i.e. "chattr +C").
+// When a directory has this attribute, files created within it inherit
+// NOCOW, which makes fallocate work correctly on btrfs because writes
+// go in-place rather than through COW.
+func dirHasNoCow(path string) bool {
+	fd, err := syscall.Open(path, syscall.O_RDONLY|syscall.O_DIRECTORY, 0)
+	if err != nil {
+		tlog.Debug.Printf("dirHasNoCow: Open %q failed: %v", path, err)
+		return false
+	}
+	defer syscall.Close(fd)
+
+	flags, err := unix.IoctlGetInt(fd, unix.FS_IOC_GETFLAGS)
+	if err != nil {
+		tlog.Debug.Printf("dirHasNoCow: FS_IOC_GETFLAGS on %q failed: %v", path, err)
+		return false
+	}
+	return flags&FS_NOCOW_FL != 0
+}
+
 // DetectQuirks decides if there are known quirks on the backing filesystem
 // that need to be workarounded.
 //
@@ -21,10 +48,19 @@ func DetectQuirks(cipherdir string) (q uint64) {
 	// Preallocation on Btrfs is broken ( https://github.com/rfjakob/gocryptfs/issues/395 )
 	// and slow ( https://github.com/rfjakob/gocryptfs/issues/63 ).
 	//
+	// The root cause is that btrfs COW allocates new blocks on write even for
+	// preallocated extents, defeating the purpose of fallocate. However, if the
+	// backing directory has the NOCOW attribute (chattr +C), writes go in-place
+	// and fallocate works correctly.
+	//
 	// Cast to uint32 avoids compile error on arm: "constant 2435016766 overflows int32"
 	if uint32(st.Type) == unix.BTRFS_SUPER_MAGIC {
-		// LogQuirk is called in fusefrontend/root_node.go
-		q |= QuirkBtrfsBrokenFalloc
+		if dirHasNoCow(cipherdir) {
+			tlog.Debug.Printf("DetectQuirks: Btrfs detected but cipherdir has NOCOW attribute (chattr +C), fallocate should work correctly")
+		} else {
+			// LogQuirk is called in fusefrontend/root_node.go
+			q |= QuirkBtrfsBrokenFalloc
+		}
 	}
 
 	return q