| lists.openwall.net | lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC | |
|
Open Source and information security mailing list archives
| ||
|
Message-ID: <20240806144628.874350-1-mjguzik@gmail.com>
Date: Tue, 6 Aug 2024 16:46:28 +0200
From: Mateusz Guzik <mjguzik@...il.com>
To: brauner@...nel.org
Cc: viro@...iv.linux.org.uk,
jack@...e.cz,
linux-kernel@...r.kernel.org,
linux-fsdevel@...r.kernel.org,
Mateusz Guzik <mjguzik@...il.com>
Subject: [PATCH] vfs: avoid spurious dentry ref/unref cycle on open
Opening a file grabs a reference on the terminal dentry in
__legitimize_path(), then another one in do_dentry_open() and finally
drops the initial reference in terminate_walk().
That's 2 modifications which don't need to be there -- do_dentry_open()
can consume the already held reference instead.
In order to facilitate some debugging a dedicated namei state flag was
added to denote this happened.
When benchmarking on a 20-core vm using will-it-scale's open3_processes
("Same file open/close"), the results are (ops/s):
before: 3087010
after: 4173977 (+35%)
Signed-off-by: Mateusz Guzik <mjguzik@...il.com>
---
The flag thing is optional and can be dropped, but I think the general
direction should be to add *more* asserts and whatnot (even if they are
to land separately). A debug-only variant would not hurt.
The entire "always consume regardless of error" ordeal stems from the
corner case where open succeeds with a fully populated file object and
then returns an error anyway. While odd, it does mean error handling
does not get more complicated at least.
fs/internal.h | 3 ++-
fs/namei.c | 30 +++++++++++++++++++++++-------
fs/open.c | 31 ++++++++++++++++++++++++++++---
3 files changed, 53 insertions(+), 11 deletions(-)
diff --git a/fs/internal.h b/fs/internal.h
index cdd73209eecb..9eeb7e03f81d 100644
--- a/fs/internal.h
+++ b/fs/internal.h
@@ -193,7 +193,8 @@ int chmod_common(const struct path *path, umode_t mode);
int do_fchownat(int dfd, const char __user *filename, uid_t user, gid_t group,
int flag);
int chown_common(const struct path *path, uid_t user, gid_t group);
-extern int vfs_open(const struct path *, struct file *);
+int vfs_open_consume(const struct path *, struct file *);
+int vfs_open(const struct path *, struct file *);
/*
* inode.c
diff --git a/fs/namei.c b/fs/namei.c
index 1bf081959066..20c5823d34dc 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -595,9 +595,10 @@ struct nameidata {
umode_t dir_mode;
} __randomize_layout;
-#define ND_ROOT_PRESET 1
-#define ND_ROOT_GRABBED 2
-#define ND_JUMPED 4
+#define ND_ROOT_PRESET 0x00000001
+#define ND_ROOT_GRABBED 0x00000002
+#define ND_JUMPED 0x00000004
+#define ND_PATH_CONSUMED 0x00000008
static void __set_nameidata(struct nameidata *p, int dfd, struct filename *name)
{
@@ -697,6 +698,7 @@ static void terminate_walk(struct nameidata *nd)
nd->state &= ~ND_ROOT_GRABBED;
}
} else {
+ BUG_ON(nd->state & ND_PATH_CONSUMED);
leave_rcu(nd);
}
nd->depth = 0;
@@ -3683,6 +3685,7 @@ static const char *open_last_lookups(struct nameidata *nd,
static int do_open(struct nameidata *nd,
struct file *file, const struct open_flags *op)
{
+ struct vfsmount *mnt;
struct mnt_idmap *idmap;
int open_flag = op->open_flag;
bool do_truncate;
@@ -3720,11 +3723,22 @@ static int do_open(struct nameidata *nd,
error = mnt_want_write(nd->path.mnt);
if (error)
return error;
+ /*
+ * We grab an additional reference here because vfs_open_consume()
+ * may error out and free the mount from under us, while we need
+ * to undo write access below.
+ */
+ mnt = mntget(nd->path.mnt);
do_truncate = true;
}
error = may_open(idmap, &nd->path, acc_mode, open_flag);
- if (!error && !(file->f_mode & FMODE_OPENED))
- error = vfs_open(&nd->path, file);
+ if (!error && !(file->f_mode & FMODE_OPENED)) {
+ BUG_ON(nd->state & ND_PATH_CONSUMED);
+ error = vfs_open_consume(&nd->path, file);
+ nd->state |= ND_PATH_CONSUMED;
+ nd->path.mnt = NULL;
+ nd->path.dentry = NULL;
+ }
if (!error)
error = security_file_post_open(file, op->acc_mode);
if (!error && do_truncate)
@@ -3733,8 +3747,10 @@ static int do_open(struct nameidata *nd,
WARN_ON(1);
error = -EINVAL;
}
- if (do_truncate)
- mnt_drop_write(nd->path.mnt);
+ if (do_truncate) {
+ mnt_drop_write(mnt);
+ mntput(mnt);
+ }
return error;
}
diff --git a/fs/open.c b/fs/open.c
index 22adbef7ecc2..eb69af3676e3 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -905,6 +905,15 @@ static inline int file_get_write_access(struct file *f)
return error;
}
+/*
+ * Populate struct file
+ *
+ * NOTE: it assumes f_path is populated and consumes the caller's reference.
+ *
+ * The caller must not path_put on it regardless of the error code -- the
+ * routine will either clean it up on its own or rely on fput, which must
+ * be issued anyway.
+ */
static int do_dentry_open(struct file *f,
int (*open)(struct inode *, struct file *))
{
@@ -912,7 +921,6 @@ static int do_dentry_open(struct file *f,
struct inode *inode = f->f_path.dentry->d_inode;
int error;
- path_get(&f->f_path);
f->f_inode = inode;
f->f_mapping = inode->i_mapping;
f->f_wb_err = filemap_sample_wb_err(f->f_mapping);
@@ -1045,6 +1053,7 @@ int finish_open(struct file *file, struct dentry *dentry,
BUG_ON(file->f_mode & FMODE_OPENED); /* once it's opened, it's opened */
file->f_path.dentry = dentry;
+ path_get(&file->f_path);
return do_dentry_open(file, open);
}
EXPORT_SYMBOL(finish_open);
@@ -1077,15 +1086,19 @@ char *file_path(struct file *filp, char *buf, int buflen)
EXPORT_SYMBOL(file_path);
/**
- * vfs_open - open the file at the given path
+ * vfs_open_consume - open the file at the given path and consume the reference
* @path: path to open
* @file: newly allocated file with f_flag initialized
*/
-int vfs_open(const struct path *path, struct file *file)
+int vfs_open_consume(const struct path *path, struct file *file)
{
int ret;
file->f_path = *path;
+ /*
+ * do_dentry_open() does the referene consuming regardless of its return
+ * value
+ */
ret = do_dentry_open(file, NULL);
if (!ret) {
/*
@@ -1098,6 +1111,17 @@ int vfs_open(const struct path *path, struct file *file)
return ret;
}
+/**
+ * vfs_open - open the file at the given path
+ * @path: path to open
+ * @file: newly allocated file with f_flag initialized
+ */
+int vfs_open(const struct path *path, struct file *file)
+{
+ path_get(path);
+ return vfs_open_consume(path, file);
+}
+
struct file *dentry_open(const struct path *path, int flags,
const struct cred *cred)
{
@@ -1183,6 +1207,7 @@ struct file *kernel_file_open(const struct path *path, int flags,
return f;
f->f_path = *path;
+ path_get(&f->f_path);
error = do_dentry_open(f, NULL);
if (error) {
fput(f);
--
2.43.0
Powered by blists - more mailing lists