Re: [PATCH 0/7] fix up issues from djwong/fuse4fs-fork

["Theodore Tso" <tytso@mit.edu>]

On Tue, May 05, 2026 at 03:56:35PM -0700, Darrick J. Wong wrote:
> FUSE kABI 7.20 added FUSE_AUTO_INVAL_DATA, which is bit 12.  It looks to
> me as though they decided to add their own MacOS-specific feature flags
> at the end of the u32 want field.  Then Linux FUSE added 11 more feature
> flags, at which point they unthinkingly ported over FUSE_CACHE_SYMLINKS,
> which collides with FUSE_DARWIN_ACCESS_EXT.  Apparently nobody on the
> macfuse end noticed, so on your machine you're getting whatever
> "ACCESS_EXT" does.

Yeah....

What MacFuse needs to do is to steal some extra fields from struct
fuse_init_in and fuse_init_out for the darwin-specific capabilities.
It turns out it already has conn->{want,capable}_darwin, but there's
no way to pass it in and out of op_init....

#ifdef __APPLE__
	/*
	 * TODO(bf)
	 *
	 * Resolve conflict with vanilla API. We need a separate field flags for
	 * Darwin-only flags. As long as we don't support anything beyond ABI
	 * version 7.19 on the kernel-side this should not be an issue, though.
	 * We need to clean this up when moving to 7.20 or later.
	 */
	if (se->conn.want_darwin & FUSE_DARWIN_CAP_ACCESS_EXT)
		outargflags |= FUSE_DARWIN_ACCESS_EXT;

So I *guess* what MacFuse needs to do is to do something like:

struct fuse_init_in {
	uint32_t	major;
	uint32_t	minor;
	uint32_t	max_readahead;
	uint32_t	flags;
	uint32_t	flags2;
	uint32_t	unused[9];
	uint32_t	darwin_flags;
	uint32_t	darwin_flags2;
};

am I right in understanding that fuse_*_{in,out} is private between
the OS's libfuse and OS's fuse driver or kernel extension, so
it's not disastrous for fuse_kernel.h for Mac and Linux to drift?

> Does this work?
>
> /* MacFUSE overlays feature bits with LinuxFUSE, this is fcked up */
> #if defined(FUSE_CAP_CACHE_SYMLINKS) && !defined(FUSE_CACHE_SYMLINKS)
> 	fuse_set_feature_flag(conn, FUSE_CAP_CACHE_SYMLINKS);
> #endif

What I'm thinking about doing is adding at the beginning of
fuse[24]fs.c:

#ifdef __APPLE__
/*
 * Sigh.... MacFuse is overloading the top bits of the flags field of
 * struct fuse_init_{out} so we have to avoid using these capability
 * flags until this gets fixed in MacFUSE
 */
#undef FUSE_CACHE_SYMLINKS
#undef FUSE_NO_OPENDIR_SUPPORT
#undef FUSE_EXPLICIT_INVAL_DATA
#undef FUSE_MAP_ALIGNMENT
#undef FUSE_SUBMOUNTS
#undef FUSE_HANDLE_KILLPRIV_V2
#undef FUSE_SETXATTR_EXT
#undef FUSE_INIT_EXT
#endif

						- Ted