Merge branch 'master' into freebsd-support

5 files changed, 53 insertions, 6 deletions

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
diff --git a/internal/syscallcompat/sys_common.go b/internal/syscallcompat/sys_common.go
index e95373a..4f84d98 100644
--- a/internal/syscallcompat/sys_common.go
+++ b/internal/syscallcompat/sys_common.go
@@ -54,10 +54,10 @@ func Openat(dirfd int, path string, flags int, mode uint32) (fd int, err error)
 			flags |= syscall.O_EXCL
 		}
 	} else {
-		// If O_CREAT is not used, we should use O_NOFOLLOW
-		if flags&syscall.O_NOFOLLOW == 0 {
-			tlog.Warn.Printf("Openat: O_NOFOLLOW missing: flags = %#x", flags)
-			flags |= syscall.O_NOFOLLOW
+		// If O_CREAT is not used, we should use O_NOFOLLOW or O_SYMLINK
+		if flags&(unix.O_NOFOLLOW|OpenatFlagNofollowSymlink) == 0 {
+			tlog.Warn.Printf("Openat: O_NOFOLLOW/O_SYMLINK missing: flags = %#x", flags)
+			flags |= unix.O_NOFOLLOW
 		}
 	}
 
diff --git a/internal/syscallcompat/sys_darwin.go b/internal/syscallcompat/sys_darwin.go
index 248b5a2..52d4e0f 100644
--- a/internal/syscallcompat/sys_darwin.go
+++ b/internal/syscallcompat/sys_darwin.go
@@ -29,6 +29,10 @@ const (
 	// Only exists on Linux. Define here to fix build failure, even though
 	// we will never see this flag.
 	RENAME_WHITEOUT = 1 << 30
+
+	// On Darwin we use O_SYMLINK which allows opening a symlink itself.
+	// On Linux, we only have O_NOFOLLOW.
+	OpenatFlagNofollowSymlink = unix.O_SYMLINK
 )
 
 // Unfortunately fsetattrlist does not have a syscall wrapper yet.
diff --git a/internal/syscallcompat/sys_freebsd.go b/internal/syscallcompat/sys_freebsd.go
index ec2c402..874e920 100644
--- a/internal/syscallcompat/sys_freebsd.go
+++ b/internal/syscallcompat/sys_freebsd.go
@@ -26,6 +26,9 @@ const (
 
 	// ENODATA is only defined on Linux, but FreeBSD provides ENOATTR
 	ENODATA = unix.ENOATTR
+
+	// On FreeBSD, we only have O_NOFOLLOW.
+	OpenatFlagNofollowSymlink = unix.O_NOFOLLOW
 )
 
 // EnospcPrealloc is supposed to preallocate ciphertext space without
diff --git a/internal/syscallcompat/sys_linux.go b/internal/syscallcompat/sys_linux.go
index faa7ffc..4669d8a 100644
--- a/internal/syscallcompat/sys_linux.go
+++ b/internal/syscallcompat/sys_linux.go
@@ -31,6 +31,10 @@ const (
 
 	// Only defined on Linux
 	ENODATA = unix.ENODATA
+
+	// On Darwin we use O_SYMLINK which allows opening a symlink itself.
+	// On Linux, we only have O_NOFOLLOW.
+	OpenatFlagNofollowSymlink = unix.O_NOFOLLOW
 )
 
 var preallocWarn sync.Once