Re: [fuse-devel] Secondary groups

[Emmanuel D. <ma...@ne...>]

On Mon, Dec 05, 2011 at 01:44:17PM +0100, Miklos Szeredi wrote:
> But please take into account that in most cases the filesystem isn't
> interested in the secondary groups and so care is needed not to
> introduce a performance regression in the majority for the sake of a
> performance improvement in the minority.

It could be enabled by a FUSE_XHEADER flag in the init message. 
If the kernel returns the flag in the reply, then this means subsequent 
messages will start by struct fuse_in_xheader instead of 
struct fuse_in_header. 

Here there is a choice to make: I can just add secondary groups to 
struct fuse_in_xheader, or I can make an extensible struct fuse_in_xheader
where secondary groups or other information can be added. In that later
case, another init flag (FUSE_SECONDARY_GROUPS) would cause secondary 
groups to be appended.

Here is a proposal for the extensible header. Note that without option,
it remains binary compatible with the original one. Also note that header_len
will make sure that future changes can be handled gracefully by the 
filesystem: it will just have to skip the extra data.

struct fuse_in_xheader {         
        __u32   len;
        __u32   opcode;         
        __u64   unique;
        __u64   nodeid;
        __u32   uid;
        __u32   gid;
        __u32   pid;
        __u8    version:       /* header version for later changes */
        __u8    padding;
	__u16   header_len;    /* header length including options */
        /* More optionnal data, starting by struct fuse_in_xheader_option */
}; 

Each option would start by struct fuse_in_xhopt, which would give the
option type and length. If the filesystem does not care about one, it
can skip:

struct fuse_in_xhopt {
	__u16   type;          /* FUSE_XHOPT_* */
        __u16   option_len;
}

And here is the structure for secondary groups:

#define FUSE_XHOPT_GROUPS 1
struct fuse_in_xhopt_groups {
	__u16   type;          /* FUSE_XHOPT_GROUPS */
        __u16   option_len;    /* sizeof(struct fuse_in_xhopt_groups) */
        __u32   ngroups;
        __u32   groups[NGROUPS];
}

OTOH, if this is considered overkill, I can just do an unextensible new 
header, with just groups added:

struct fuse_in_xheader {         
        __u32   len;
        __u32   opcode;         
        __u64   unique;
        __u64   nodeid;
        __u32   uid;
        __u32   gid;
        __u32   pid;
        __u32   ngroups; 
        __u32   groups[NGROUPS];
}; 

-- 
Emmanuel Dreyfus
ma...@ne...