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Subject[RFC] __d_path() API change (was Re: [PATCH] Remove use of mnt_ns->root and fix a couple of bugs in d_namespace_path)
	I think I have a saner solution; the root of the problem is that
__d_path() API is asking for trouble. What I'm proposing is:

* Give prepend_path() an extra argument for reporting which
fatherless thing has it reached if it has missed the root - struct
path *bastard. struct path *root becomes const on that *and* what
we put into *bastard is properly grabbed. We do that only if that
pointer is not NULL, of course. In any case we return 1 if we go
out through global_root:

* modify the callers accordingly; everything except __d_path()
will simply pass NULL as that extra argument and instead of checking
if root has been changed, just check if return value was positive.

* __d_path() gets the similar extra argument itself; if it's non-NULL
and we miss the root, the caller can expect to get (referenced) point where
the walk has stopped stored there. _Maybe_ it ought to fill it with
NULL/NULL otherwise; I've just done that in the only such caller before
calling __d_path(). If it is NULL *and* root has not been NULL/NULL (i.e.
we asked for subtree explicitly and have nowhere to put the stopping point),
we return NULL instead of pointer to relative pathname.

* seq_path_root() does _NOT_ return -ENAMETOOLONG (it's stupid anyway -
the normal seq_file logics will take care of growing the buffer and redoing
the call of ->show() just fine). However, if it gets path not reachable
from root, it returns SEQ_SKIP. The only caller adjusted (i.e. stopped
ignoring the return value as it used to do).

* unlike seq_path_root(), the caller in tomoyo passes NULL/NULL in
*root (it wants absolute path). It still calls with bastard == NULL, so
no references are grabbed.

* apparmor d_namespace_path() calls __d_path() passing it a pointer
to local struct path to fill. It's been earlier initialized to NULL/NULL
(see above in part about __d_path()), so the check for "has it been outside
of chroot jail" is simply bastard.mnt != NULL after the call. Moreover,
in this case it's safe to access vfsmount and dentry; we can do the checks
just fine, they won't be freed under us. One more thing - I've exported
uninlined variant of check_mnt() (called our_mnt(), for obvious reasons)
and used it here.

Completely untested patch follows:

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
---
diff --git a/fs/dcache.c b/fs/dcache.c
index 2ac8e7e..d8a1783 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -2440,7 +2440,8 @@ static int prepend_name(char **buffer, int *buflen, struct qstr *name)
/**
* prepend_path - Prepend path string to a buffer
* @path: the dentry/vfsmount to report
- * @root: root vfsmnt/dentry (may be modified by this function)
+ * @root: root vfsmnt/dentry
+ * @bastard: the fatherless thing we'd walked up to if we missed root
* @buffer: pointer to the end of the buffer
* @buflen: pointer to buffer length
*
@@ -2449,7 +2450,9 @@ static int prepend_name(char **buffer, int *buflen, struct qstr *name)
* If path is not reachable from the supplied root, then the value of
* root is changed (without modifying refcounts).
*/
-static int prepend_path(const struct path *path, struct path *root,
+static int prepend_path(const struct path *path,
+ const struct path *root,
+ struct path *bastard,
char **buffer, int *buflen)
{
struct dentry *dentry = path->dentry;
@@ -2483,10 +2486,10 @@ static int prepend_path(const struct path *path, struct path *root,
dentry = parent;
}

-out:
if (!error && !slash)
error = prepend(buffer, buflen, "/", 1);

+out:
br_read_unlock(vfsmount_lock);
return error;

@@ -2500,8 +2503,14 @@ global_root:
WARN(1, "Root dentry has weird name <%.*s>\n",
(int) dentry->d_name.len, dentry->d_name.name);
}
- root->mnt = vfsmnt;
- root->dentry = dentry;
+ if (bastard) {
+ bastard->mnt = mntget(vfsmnt);
+ bastard->dentry = dget(dentry);
+ }
+ if (!slash)
+ error = prepend(buffer, buflen, "/", 1);
+ if (!error)
+ error = 1;
goto out;
}

@@ -2521,8 +2530,12 @@ global_root:
*
* If path is not reachable from the supplied root, then the value of
* root is changed (without modifying refcounts).
+ *
+ * XXX: rewrite description
*/
-char *__d_path(const struct path *path, struct path *root,
+char *__d_path(const struct path *path,
+ const struct path *root,
+ struct path *bastard,
char *buf, int buflen)
{
char *res = buf + buflen;
@@ -2530,19 +2543,26 @@ char *__d_path(const struct path *path, struct path *root,

prepend(&res, &buflen, "\0", 1);
write_seqlock(&rename_lock);
- error = prepend_path(path, root, &res, &buflen);
+ error = prepend_path(path, root, bastard, &res, &buflen);
write_sequnlock(&rename_lock);

- if (error)
+ if (error < 0)
return ERR_PTR(error);
+ /*
+ * we'd been asked to stop at specific point, missed it and
+ * had not been asked to tell where we'd ended up.
+ */
+ if (error > 0 && !bastard && root->mnt)
+ return NULL;
return res;
}

/*
* same as __d_path but appends "(deleted)" for unlinked files.
*/
-static int path_with_deleted(const struct path *path, struct path *root,
- char **buf, int *buflen)
+static int path_with_deleted(const struct path *path,
+ const struct path *root,
+ char **buf, int *buflen)
{
prepend(buf, buflen, "\0", 1);
if (d_unlinked(path->dentry)) {
@@ -2551,7 +2571,7 @@ static int path_with_deleted(const struct path *path, struct path *root,
return error;
}

- return prepend_path(path, root, buf, buflen);
+ return prepend_path(path, root, NULL, buf, buflen);
}

static int prepend_unreachable(char **buffer, int *buflen)
@@ -2579,7 +2599,6 @@ char *d_path(const struct path *path, char *buf, int buflen)
{
char *res = buf + buflen;
struct path root;
- struct path tmp;
int error;

/*
@@ -2594,9 +2613,8 @@ char *d_path(const struct path *path, char *buf, int buflen)

get_fs_root(current->fs, &root);
write_seqlock(&rename_lock);
- tmp = root;
- error = path_with_deleted(path, &tmp, &res, &buflen);
- if (error)
+ error = path_with_deleted(path, &root, &res, &buflen);
+ if (error < 0)
res = ERR_PTR(error);
write_sequnlock(&rename_lock);
path_put(&root);
@@ -2617,7 +2635,6 @@ char *d_path_with_unreachable(const struct path *path, char *buf, int buflen)
{
char *res = buf + buflen;
struct path root;
- struct path tmp;
int error;

if (path->dentry->d_op && path->dentry->d_op->d_dname)
@@ -2625,9 +2642,8 @@ char *d_path_with_unreachable(const struct path *path, char *buf, int buflen)

get_fs_root(current->fs, &root);
write_seqlock(&rename_lock);
- tmp = root;
- error = path_with_deleted(path, &tmp, &res, &buflen);
- if (!error && !path_equal(&tmp, &root))
+ error = path_with_deleted(path, &root, &res, &buflen);
+ if (error > 0)
error = prepend_unreachable(&res, &buflen);
write_sequnlock(&rename_lock);
path_put(&root);
@@ -2758,19 +2774,18 @@ SYSCALL_DEFINE2(getcwd, char __user *, buf, unsigned long, size)
write_seqlock(&rename_lock);
if (!d_unlinked(pwd.dentry)) {
unsigned long len;
- struct path tmp = root;
char *cwd = page + PAGE_SIZE;
int buflen = PAGE_SIZE;

prepend(&cwd, &buflen, "\0", 1);
- error = prepend_path(&pwd, &tmp, &cwd, &buflen);
+ error = prepend_path(&pwd, &root, NULL, &cwd, &buflen);
write_sequnlock(&rename_lock);

- if (error)
+ if (error < 0)
goto out;

/* Unreachable from current root */
- if (!path_equal(&tmp, &root)) {
+ if (error > 0) {
error = prepend_unreachable(&cwd, &buflen);
if (error)
goto out;
diff --git a/fs/namespace.c b/fs/namespace.c
index ea1cf5a..58c64c3 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -1023,15 +1023,12 @@ static int show_mountinfo(struct seq_file *m, void *v)
if (err)
goto out;
seq_putc(m, ' ');
- seq_path_root(m, &mnt_path, &root, " \t\n\\");
- if (root.mnt != p->root.mnt || root.dentry != p->root.dentry) {
- /*
- * Mountpoint is outside root, discard that one. Ugly,
- * but less so than trying to do that in iterator in a
- * race-free way (due to renames).
- */
- return SEQ_SKIP;
- }
+
+ /* mountpoints outside of chroot jail will give SEQ_SKIP on this */
+ err = seq_path_root(m, &mnt_path, &root, " \t\n\\");
+ if (err)
+ goto out;
+
seq_puts(m, mnt->mnt_flags & MNT_READONLY ? " ro" : " rw");
show_mnt_opts(m, mnt);

@@ -2761,3 +2758,9 @@ void kern_unmount(struct vfsmount *mnt)
}
}
EXPORT_SYMBOL(kern_unmount);
+
+bool our_mnt(struct vfsmount *mnt)
+{
+ return check_mnt(mnt);
+}
+EXPORT_SYMBOL(our_mnt);
diff --git a/fs/seq_file.c b/fs/seq_file.c
index 05d6b0e..688642d 100644
--- a/fs/seq_file.c
+++ b/fs/seq_file.c
@@ -449,8 +449,6 @@ EXPORT_SYMBOL(seq_path);

/*
* Same as seq_path, but relative to supplied root.
- *
- * root may be changed, see __d_path().
*/
int seq_path_root(struct seq_file *m, struct path *path, struct path *root,
char *esc)
@@ -462,7 +460,9 @@ int seq_path_root(struct seq_file *m, struct path *path, struct path *root,
if (size) {
char *p;

- p = __d_path(path, root, buf, size);
+ p = __d_path(path, root, NULL, buf, size);
+ if (!p)
+ return SEQ_SKIP;
res = PTR_ERR(p);
if (!IS_ERR(p)) {
char *end = mangle_path(buf, p, esc);
@@ -474,7 +474,7 @@ int seq_path_root(struct seq_file *m, struct path *path, struct path *root,
}
seq_commit(m, res);

- return res < 0 ? res : 0;
+ return res < 0 && res != -ENAMETOOLONG ? res : 0;
}

/*
diff --git a/include/linux/dcache.h b/include/linux/dcache.h
index 4df9261..2b6cf1e 100644
--- a/include/linux/dcache.h
+++ b/include/linux/dcache.h
@@ -339,7 +339,8 @@ extern int d_validate(struct dentry *, struct dentry *);
*/
extern char *dynamic_dname(struct dentry *, char *, int, const char *, ...);

-extern char *__d_path(const struct path *path, struct path *root, char *, int);
+extern char *__d_path(const struct path *, const struct path *,
+ struct path *, char *, int);
extern char *d_path(const struct path *, char *, int);
extern char *d_path_with_unreachable(const struct path *, char *, int);
extern char *dentry_path_raw(struct dentry *, char *, int);
diff --git a/include/linux/fs.h b/include/linux/fs.h
index e313022..019dc55 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1942,6 +1942,7 @@ extern int fd_statfs(int, struct kstatfs *);
extern int statfs_by_dentry(struct dentry *, struct kstatfs *);
extern int freeze_super(struct super_block *super);
extern int thaw_super(struct super_block *super);
+extern bool our_mnt(struct vfsmount *mnt);

extern int current_umask(void);

diff --git a/security/apparmor/path.c b/security/apparmor/path.c
index 36cc0cc..ed300cc 100644
--- a/security/apparmor/path.c
+++ b/security/apparmor/path.c
@@ -57,23 +57,16 @@ static int prepend(char **buffer, int buflen, const char *str, int namelen)
static int d_namespace_path(struct path *path, char *buf, int buflen,
char **name, int flags)
{
- struct path root, tmp;
+ struct path root = {};
+ struct path bastard = {};
char *res;
- int connected, error = 0;
+ int error = 0;

- /* Get the root we want to resolve too, released below */
- if (flags & PATH_CHROOT_REL) {
- /* resolve paths relative to chroot */
+ /* resolve paths relative to chroot?*/
+ if (flags & PATH_CHROOT_REL)
get_fs_root(current->fs, &root);
- } else {
- /* resolve paths relative to namespace */
- root.mnt = current->nsproxy->mnt_ns->root;
- root.dentry = root.mnt->mnt_root;
- path_get(&root);
- }

- tmp = root;
- res = __d_path(path, &tmp, buf, buflen);
+ res = __d_path(path, &root, &bastard, buf, buflen);

*name = res;
/* handle error conditions - and still allow a partial path to
@@ -97,10 +90,7 @@ static int d_namespace_path(struct path *path, char *buf, int buflen,
goto out;
}

- /* Determine if the path is connected to the expected root */
- connected = tmp.dentry == root.dentry && tmp.mnt == root.mnt;
-
- /* If the path is not connected,
+ /* If the path is not connected to the expected root,
* check if it is a sysctl and handle specially else remove any
* leading / that __d_path may have returned.
* Unless
@@ -111,9 +101,9 @@ static int d_namespace_path(struct path *path, char *buf, int buflen,
* of chroot) and specifically directed to connect paths to
* namespace root.
*/
- if (!connected) {
+ if (bastard.mnt) {
/* is the disconnect path a sysctl? */
- if (tmp.dentry->d_sb->s_magic == PROC_SUPER_MAGIC &&
+ if (bastard.dentry->d_sb->s_magic == PROC_SUPER_MAGIC &&
strncmp(*name, "/sys/", 5) == 0) {
/* TODO: convert over to using a per namespace
* control instead of hard coded /proc
@@ -121,8 +111,7 @@ static int d_namespace_path(struct path *path, char *buf, int buflen,
error = prepend(name, *name - buf, "/proc", 5);
} else if (!(flags & PATH_CONNECT_PATH) &&
!(((flags & CHROOT_NSCONNECT) == CHROOT_NSCONNECT) &&
- (tmp.mnt == current->nsproxy->mnt_ns->root &&
- tmp.dentry == tmp.mnt->mnt_root))) {
+ our_mnt(bastard.mnt))) {
/* disconnected path, don't return pathname starting
* with '/'
*/
@@ -133,7 +122,10 @@ static int d_namespace_path(struct path *path, char *buf, int buflen,
}

out:
- path_put(&root);
+ if (bastard.mnt)
+ path_put(&bastard);
+ if (flags & PATH_CHROOT_REL)
+ path_put(&root);

return error;
}
diff --git a/security/tomoyo/realpath.c b/security/tomoyo/realpath.c
index e1eb982..d5f469a 100644
--- a/security/tomoyo/realpath.c
+++ b/security/tomoyo/realpath.c
@@ -95,7 +95,7 @@ static char *tomoyo_get_absolute_path(struct path *path, char * const buffer,
if (buflen >= 256) {
struct path ns_root = { };
/* go to whatever namespace root we are under */
- pos = __d_path(path, &ns_root, buffer, buflen - 1);
+ pos = __d_path(path, &ns_root, NULL, buffer, buflen - 1);
if (!IS_ERR(pos) && *pos == '/' && pos[1]) {
struct inode *inode = path->dentry->d_inode;
if (inode && S_ISDIR(inode->i_mode)) {

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