man/man9/genfs_rename.9

.\"	$NetBSD: genfs_rename.9,v 1.5 2022/01/17 22:27:10 wiz Exp $
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.\" This documentation is derived from text contributed to The NetBSD
.\" Foundation by Taylor R. Campbell.
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.Dd May 1, 2013
.Dt GENFS_RENAME 9
.Os
.Sh NAME
.Nm genfs_rename ,
.Nm genfs_insane_rename ,
.Nm genfs_sane_rename
.Nd generic framework for implementing
.Xr VOP_RENAME 9
.Sh SYNOPSIS
.Ft int
.Fo genfs_insane_rename
.Fa "struct vop_rename_args *v"
.Fa "int (*sane_rename)(struct vnode *fdvp, struct componentname *fcnp, struct vnode *tdvp, struct componentname *tcnp, kauth_cred_t, bool)"
.Fc
.Ft int
.Fo genfs_sane_rename
.Fa "const struct genfs_rename_ops *gro"
.Fa "struct vnode *fdvp"
.Fa "struct componentname *fcnp"
.Fa "void *fde"
.Fa "struct vnode *tdvp"
.Fa "struct componentname *tcnp"
.Fa "void *tde"
.Fa "kauth_cred_t cred"
.Fa "bool posixly_correct"
.Fc
.Ft int
.Fo genfs_rename_knote
.Fa "struct vnode *fdvp"
.Fa "struct vnode *fvp"
.Fa "struct vnode *tdvp"
.Fa "struct vnode *tvp"
.Fc
.Ft void
.Fo genfs_rename_cache_purge
.Fa "struct vnode *fdvp"
.Fa "struct vnode *fvp"
.Fa "struct vnode *tdvp"
.Fa "struct vnode *tvp"
.Fc
.Ft int
.Fo genfs_ufslike_rename_check_possible
.Fa "unsigned long fdflags"
.Fa "unsigned long fflags"
.Fa "unsigned long tdflags"
.Fa "unsigned long tflags"
.Fa "bool clobber"
.Fa "unsigned long immutable"
.Fa "unsigned long append"
.Fc
.Ft int
.Fo genfs_ufslike_rename_check_permitted
.Fa "kauth_cred_t cred"
.Fa "struct vnode *fdvp"
.Fa "mode_t fdmode"
.Fa "uid_t fduid"
.Fa "struct vnode *fvp"
.Fa "uid_t fuid"
.Fa "struct vnode *tdvp"
.Fa "mode_t tdmode"
.Fa "uid_t tduid"
.Fa "struct vnode *tvp"
.Fa "uid_t tuid"
.Fc
.Ft int
.Fo genfs_ufslike_remove_check_possible
.Fa "unsigned long dflags"
.Fa "unsigned long flags"
.Fa "unsigned long immutable"
.Fa "unsigned long append"
.Fc
.Ft int
.Fo genfs_ufslike_remove_check_permitted
.Fa "kauth_cred_t cred"
.Fa "struct vnode *dvp"
.Fa "mode_t dmode"
.Fa "uid_t duid"
.Fa "struct vnode *vp"
.Fa "uid_t uid"
.Fc
.Sh DESCRIPTION
The
.Nm
functions provide a file-system-independent framework for implementing
.Xr VOP_RENAME 9
with correct locking and error-checking.
.Pp
Implementing rename is nontrivial.
If you are doing it for a new file system, you should consider starting
from
.Fn tmpfs_rename
as implemented in
.Pa sys/fs/tmpfs/tmpfs_rename.c
and adapting it to your file system's physical operations.
.Pp
Because there are so many moving parts to a rename operation,
.Nm
uses the following naming conventions:
.Bl -tag -width indent
.It Fa mp Pq "mount point"
mount point of the file system in question
.It Fa fdvp Pq "from directory vnode pointer"
directory from which we are removing an entry
.It Fa fcnp Pq "from componentname pointer"
name of entry to remove from
.Fa fdvp
.It Fa fde Pq "from directory entry"
fs-specific data about the entry in
.Fa fdvp
.It Fa fvp Pq "from vnode pointer"
file at the entry named
.Fa fcnp
in
.Fa fdvp
.It Fa tdvp Pq "to directory vnode pointer"
directory to which we are adding an entry
.It Fa tcnp Pq "to componentname pointer"
name of entry to add to
.Fa tdvp
.It Fa tde Pq "to directory entry"
fs-specific data about the entry in
.Fa tdvp
.It Fa tvp Pq "to vnode pointer"
file previously at the entry named
.Fa tcnp
in
.Fa tdvp ,
to be replaced, if any, or
.Dv NULL
if there was no entry before
.It Fa vp Pq "vnode pointer"
any file
.It Fa dvp Pq "directory vnode pointer"
any directory with an entry for
.Fa vp
.El
.Pp
A file system mumblefs should implement various file-system-dependent
parts of the rename operation in a
.Vt struct genfs_rename_ops ,
and use
.Nm
to implement
.Fn mumblefs_rename
for
.Xr VOP_RENAME 9
as follows:
.Bd -literal -offset indent
static const struct genfs_rename_ops mumblefs_genfs_rename_ops;

static int
mumblefs_sane_rename(
    struct vnode *fdvp, struct componentname *fcnp,
    struct vnode *tdvp, struct componentname *tcnp,
    kauth_cred_t cred, bool posixly_correct)
{
	struct mumblefs_lookup_results fulr, tulr;

	return genfs_sane_rename(&mumblefs_genfs_rename_ops,
	    fdvp, fcnp, &fulr, tdvp, tcnp, &tulr,
	    cred, posixly_correct);
}

int
mumblefs_rename(void *v)
{

	return genfs_insane_rename(v, &mumblefs_sane_rename);
}
.Ed
.Pp
The split between
.Fn mumblefs_rename
and
.Fn mumblefs_sane_rename
is designed to enable us to easily change the
.Xr VOP_RENAME 9
interface, which is currently designed for a broken (hence
.Sq insane )
locking scheme, to a more sensible locking scheme once all the file
systems have their rename operations split up thus.
.Pp
The
.Vt struct mumblefs_lookup_results
structure is storage for information about directory entries which
needs to pass from the lookups of the children (see the
.Fa gro_lookup
member of
.Vt "struct genfs_rename_ops" )
to the physical on-disk rename or remove operations (see the
.Fa gro_rename
and
.Fa gro_remove
members of
.Vt "struct genfs_rename_ops" ) .
.Pp
Callers must implement the following operations as members in a
.Vt struct genfs_rename_ops
structure passed to
.Nm :
.Bl -tag -width indent
.It Ft int Fn "(*gro_genealogy)" "struct mount *mp" "kauth_cred_t cred" "struct vnode *fdvp" "struct vnode *tdvp" "struct vnode **intermediate_node_ret"
Walk up the directory tree from the directory vnode
.Fa tdvp
until hitting either
.Fa fdvp
or the root.
If
.Fa fdvp
is hit, store the child of
.Fa fdvp
through which the path from
.Fa tdvp
passed in
.Fa *intermediate_node_ret ,
referenced but unlocked.
If
.Fa fdvp
is not hit, store
.Dv NULL
in
.Fa *intermediate_node_ret .
Return zero on success or error on failure.
(Failure means file-system-specific failures, not hitting or missing
.Fa fdvp . )
.Pp
.Fa fdvp
and
.Fa tdvp
are guaranteed to be distinct, non-null, referenced, and unlocked.
Since no locks are held on entry except for the file-system-wide rename
lock,
.Fa gro_genealogy
may take any locks it pleases.
.It Ft int Fn "(*gro_lock_directory)" "struct mount *mp" "struct vnode *vp"
Lock the directory vnode
.Fa vp ,
but fail if it has been rmdired already.
Return zero on success or error on failure.
.It Ft int Fn "(*gro_lookup)" "struct mount *mp" "struct vnode *dvp" "struct componentname *cnp" "void *de" "struct vnode **vpp"
Look up the entry in
.Fa dvp
for
.Fa cnp ,
storing the vnode in
.Fa "*vpp"
and using
.Fa de ,
one of the pointers passed to
.Nm genfs_sane_rename ,
to store information about the directory entry as needed by the file
system's
.Fa gro_rename
operation, and return zero.
If there is no such entry, return error.
.Pp
.Fa dvp
is guaranteed to be locked, and the vnode returned in
.Fa "*vpp"
must be unlocked.
However,
.Fa gro_lookup
may temporarily lock the vnode without causing deadlock.
.It Ft bool Fn "(*gro_directory_empty_p)" "struct mount *mp" "kauth_cred_t cred" "struct vnode *vp" "struct vnode *dvp"
Return true if the directory vnode
.Fa vp
is empty.
The argument
.Fa dvp
is the parent of
.Fa vp ,
as required for this check by some file systems.
.Pp
.Fa dvp
and
.Fa vp
are guaranteed to be distinct, non-null, referenced, and locked.
.It Ft int Fn "(*gro_rename_check_possible)" "struct mount *mp" "struct vnode *fdvp" "struct vnode *fvp" "struct vnode *tdvp" "struct vnode *tvp"
Return zero if the file system might allow the rename independent of
credentials, or error if not.
This should check, for example, any immutability flags in the vnodes in
question, and should use
.Fn genfs_ufslike_rename_check_possible
for file systems similar to UFS/FFS.
.Pp
.Fa fdvp
and
.Fa tdvp
may be the same; every other pair of vnodes is guaranteed to be
distinct.
.Fa tvp
may be
.Dv NULL ;
every other vnode is guaranteed to be non-null.
All three or four vnodes are guaranteed to be referenced and locked.
.It Ft int Fn "(*gro_rename_check_permitted)" "struct mount *mp" "kauth_cred_t cred" "struct vnode *fdvp" "struct vnode *fvp" "struct vnode *tdvp" "struct vnode *tvp"
Return zero if the file system allows the rename given the credentials
.Fa cred ,
or error if not.
This should check, for example, the ownership and permissions bits of
the vnodes in question, and should use
.Fn genfs_ufslike_rename_check_permitted
for file systems similar to UFS/FFS.
.Pp
.Fa fdvp
and
.Fa tdvp
may be the same; every other pair of vnodes is guaranteed to be
distinct.
.Fa tvp
may be
.Dv NULL ;
every other vnode is guaranteed to be non-null.
All three or four vnodes are guaranteed to be referenced and locked.
.It Ft int Fn "(*gro_rename)" "struct mount *mp" "kauth_cred_t cred" "struct vnode *fdvp" "struct componentname *fcnp" "void *fde" "struct vnode *fvp" "struct vnode *tdvp" "struct componentname *tcnp" "void *tde" "struct vnode *tvp"
Perform the physical file system rename operation, report any knotes,
and purge the namecache entries.
Return zero on success or error on failure.
All file-system-independent error cases have been handled already.
.Pp
File systems using
.Xr fstrans 9
should use
.Xr fstrans_start 9
and
.Xr fstrans_done 9
here.
.Fa fde
and
.Fa tde
are the pointers that were supplied to
.Fn genfs_sane_rename
and got passed to the
.Fa gro_lookup
operation to find information about directory entries.
.Pp
This may use
.Fn genfs_rename_knote
to report any knotes, if the various file-system-dependent routines it
uses to edit links don't do that already.
This should use
.Fn genfs_rename_cache_purge
to purge the namecache.
.Pp
.Fa fdvp
and
.Fa tdvp
may be the same; every other pair of vnodes is guaranteed to be
distinct.
.Fa tvp
may be null; every other vnode is guaranteed to be non-null.
All three or four vnodes are guaranteed to be referenced and locked.
.It Ft int Fn "(*gro_remove_check_possible)" "struct mount *mp" "struct vnode *dvp" "struct vnode *vp"
Return zero if the file system might allow removing an entry in
.Fa dvp
for
.Fa vp
independent of credentials, or error if not.
This should use
.Fn genfs_ufslike_remove_check_possible
for file systems similar to UFS/FFS.
.Pp
.Fa dvp
and
.Fa vp
are guaranteed to be distinct, non-null, referenced, and locked.
.Pp
This, and
.Fa gro_remove_check_permitted
below, are for renames that reduce to a remove; that is, renaming one
entry to another when both entries refer to the same file.
For reasons of locking insanity,
.Nm
cannot simply call
.Xr VOP_REMOVE 9
instead.
.It Ft int Fn "(*gro_remove_check_permitted)" "struct mount *mp" "kauth_cred_t cred" "struct vnode *dvp" "struct vnode *vp"
Return zero if the file system allows removing an entry in
.Fa dvp
for
.Fa vp
given the credentials
.Fa cred ,
or error if not.
This should use
.Fn genfs_ufslike_remove_check_permitted
for file systems similar to UFS/FFS.
.Pp
.Fa dvp
and
.Fa vp
are guaranteed to be distinct, non-null, referenced, and locked.
.It Ft int Fn "(*gro_remove)" "struct mount *mp" "kauth_cred_t cred" "struct vnode *dvp" "struct componentname *cnp" "void *de" "struct vnode *vp"
For a rename that is effectively a remove, perform the physical file
system remove operation, report any knotes, and purge the namecache
entries.
Return zero on success or error on failure.
All file-system-independent error cases have been handled already.
.Pp
File systems using
.Xr fstrans 9
should use
.Xr fstrans_start 9
and
.Xr fstrans_done 9
here.
.Fa de
is one of the pointers that were supplied to
.Fn genfs_sane_rename
and got passed to the
.Fa gro_lookup
operation to find information about directory entries.
.Pp
This should signal a
.Dv NOTE_WRITE
knote for
.Fa dvp ,
and either a
.Dv NOTE_DELETE
or a
.Dv NOTE_LINK
knote for
.Fa vp ,
depending on whether this removed the last link to it or not.
.Pp
.Fa dvp
and
.Fa vp
are guaranteed to be distinct, non-null, referenced, and locked.
.El
.Pp
The following utilities are provided for implementing the
.Vt struct genfs_rename_ops
operations:
.Bl -tag -width indent
.It Fn genfs_rename_knote fdvp fvp tdvp tvp
Signal all the knotes relevant for the rename operation.
.It Fn genfs_rename_cache_purge fdvp fvp tdvp tvp
Purge any namecache entries that the rename operation invalidates.
.It Fn genfs_ufslike_rename_check_possible fdflags fflags tdflags tflags clobber immutable append
Check whether the UFS/FFS-like flags of the files involved a rename
allow it.
Return zero if allowed or error if not.
.Pp
.Bl -tag -width immutable -compact
.It Fa fdflags
flags of source directory
.It Fa fflags
flags of source file
.It Fa tdflags
flags of target directory
.It Fa tflags
flags of target file, if there is one and
.Fa clobber
is true, or ignored otherwise
.It Fa clobber
true if there is a target file whose entry will be clobbered or false
if not
.It Fa immutable
bit mask for the file system's immutable bit, like the UFS/FFS
.Dv IMMUTABLE
.It Fa append
bit mask for the file system's append-only bit, like the UFS/FFS
.Dv APPEND
.El
.It Fn genfs_ufslike_rename_check_permitted cred fdvp fdmode fduid fvp fuid tdvp tdmode tduid tvp tuid
Check whether the credentials
.Fa cred
are permitted by the file ownership and permissions bits to perform a
rename.
Return zero if permitted or error if not.
.Pp
.Bl -tag -width fdmode -compact
.It Fa cred
caller's credentials
.It Fa fdvp
source directory
.It Fa fdmode
file permissions bits of
.Fa fdvp
.It Fa fduid
uid of the owner of
.Fa fdvp
.It Fa fvp
source file
.It Fa fuid
uid of owner of
.Fa fvp
.It Fa tdvp
target directory
.It Fa tdmode
file permissions bits of
.Fa tdvp
.It Fa tduid
uid of owner of
.Fa tdvp
.It Fa tvp
target file, if there is one, or
.Dv NULL
if not
.It Fa tuid
uid of owner of
.Fa tvp ,
if there is a target file, or ignored otherwise
.El
.It Fn genfs_ufslike_remove_check_possible dflags flags immutable append
Check whether the UFS/FFS-like flags of the files involved a remove
allow it.
Return zero if allowed or error if not.
.Pp
.Bl -tag -width immutable -compact
.It Fa dflags
flags of the directory
.It Fa flags
flags of the file in the directory
.It Fa immutable
bit mask for the file system's immutable bit, like the UFS/FFS
.Dv IMMUTABLE
.It Fa append
bit mask for the file system's append-only bit, like the UFS/FFS
.Dv APPEND
.El
.It Fn genfs_ufslike_remove_check_permitted cred dvp dmode duid vp uid
Check whether the credentials
.Fa cred
are permitted by the file ownership and permissions bits to perform a
remove.
Return zero if permitted or error if not.
.Pp
.Bl -tag -width fdmode -compact
.It Fa cred
caller's credentials
.It Fa dvp
directory
.It Fa dmode
file permissions bits of
.Fa dvp
.It Fa duid
uid of owner of
.Fa dvp
.It Fa vp
file in
.Fa dvp
.It Fa uid
uid of owner of
.Fa vp
.El
.El
.Sh NOTES
Because there are so many cases of rename, it cannot be assumed a
priori that any pairs of
.Fa fdvp ,
.Fa fvp ,
.Fa tdvp ,
or
.Fa fvp
are distinct:
.Bl -column -offset indent \
"fdvp = tdvp" "rename(\*qa/b\*q, \*qa/c\*q)"
.It Li "fdvp = fvp"     Ta Li "rename(\*qa/.\*q, \*qb\*q)"
.It Li "fdvp = tdvp"    Ta Li "rename(\*qa/b\*q, \*qa/c\*q)"
.It Li "fdvp = tvp"     Ta Li "rename(\*qa/b\*q, \*qa\*q)"
.It Li "fvp = tdvp"     Ta Li "rename(\*qa\*q, \*qa/b\*q)"
.It Li "fvp = tvp"      Ta Li "rename(\*qa\*q, \*qa\*q)"
.It Li "tdvp = tvp"     Ta Li "rename(\*qa\*q, \*qb/.\*q)"
.El
.Pp
Handling all these cases correctly, and getting the locking correct and
deadlock-free, is very tricky, which is why
.Nm
exists.
The interface to
.Nm
is very complicated because it must fit the insane
.Xr VOP_RENAME 9
and
.Xr VOP_LOOKUP 9
protocols until we can fix them, and because it must accommodate a
variety of crufty file systems.
.Sh SEE ALSO
.Xr genfs 9 ,
.Xr vfs 9 ,
.Xr vnodeops 9
.Sh HISTORY
.Nm
was designed and implemented by
.An Taylor R. Campbell Aq Mt riastradh@NetBSD.org
after many discussions with
.An David Holland Aq Mt dholland@NetBSD.org ,
and first appeared in
.Nx 6.0 .