Re: [PATCH v4 38/68] vfs, cachefiles: Mark a backing file in use with an inode flag

[Jeff Layton <jlayton@kernel.org>]

On Wed, 2021-12-22 at 23:23 +0000, David Howells wrote:
> Use an inode flag, S_KERNEL_FILE, to mark that a backing file is in use by
> the kernel to prevent cachefiles or other kernel services from interfering
> with that file.
> 
> Alter rmdir to reject attempts to remove a directory marked with this flag.
> This is used by cachefiles to prevent cachefilesd from removing them.
> 
> Using S_SWAPFILE instead isn't really viable as that has other effects in
> the I/O paths.
> 
> Changes
> =======
> ver #3:
>  - Check for the object pointer being NULL in the tracepoints rather than
>    the caller.
> 
> Signed-off-by: David Howells <dhowells@redhat.com>
> cc: linux-cachefs@redhat.com
> Link: https://lore.kernel.org/r/163819630256.215744.4815885535039369574.stgit@warthog.procyon.org.uk/ # v1
> Link: https://lore.kernel.org/r/163906931596.143852.8642051223094013028.stgit@warthog.procyon.org.uk/ # v2
> Link: https://lore.kernel.org/r/163967141000.1823006.12920680657559677789.stgit@warthog.procyon.org.uk/ # v3
> ---
> 
>  fs/cachefiles/Makefile            |    1 +
>  fs/cachefiles/namei.c             |   43 +++++++++++++++++++++++++++++++++++++
>  fs/namei.c                        |    3 ++-
>  include/linux/fs.h                |    1 +
>  include/trace/events/cachefiles.h |   42 ++++++++++++++++++++++++++++++++++++
>  5 files changed, 89 insertions(+), 1 deletion(-)
>  create mode 100644 fs/cachefiles/namei.c
> 
> diff --git a/fs/cachefiles/Makefile b/fs/cachefiles/Makefile
> index 463e3d608b75..e0b092ca077f 100644
> --- a/fs/cachefiles/Makefile
> +++ b/fs/cachefiles/Makefile
> @@ -7,6 +7,7 @@ cachefiles-y := \
>  	cache.o \
>  	daemon.o \
>  	main.o \
> +	namei.o \
>  	security.o
>  
>  cachefiles-$(CONFIG_CACHEFILES_ERROR_INJECTION) += error_inject.o
> diff --git a/fs/cachefiles/namei.c b/fs/cachefiles/namei.c
> new file mode 100644
> index 000000000000..913f83f1c900
> --- /dev/null
> +++ b/fs/cachefiles/namei.c
> @@ -0,0 +1,43 @@
> +// SPDX-License-Identifier: GPL-2.0-or-later
> +/* CacheFiles path walking and related routines
> + *
> + * Copyright (C) 2021 Red Hat, Inc. All Rights Reserved.
> + * Written by David Howells (dhowells@redhat.com)
> + */
> +
> +#include <linux/fs.h>
> +#include "internal.h"
> +
> +/*
> + * Mark the backing file as being a cache file if it's not already in use.  The
> + * mark tells the culling request command that it's not allowed to cull the
> + * file or directory.  The caller must hold the inode lock.
> + */
> +static bool __cachefiles_mark_inode_in_use(struct cachefiles_object *object,
> +					   struct dentry *dentry)
> +{
> +	struct inode *inode = d_backing_inode(dentry);
> +	bool can_use = false;
> +
> +	if (!(inode->i_flags & S_KERNEL_FILE)) {

nit: most of the other S_* flags have a corresponding IS_* macro. Should
this be:

    IS_KERNEL_FILE(inode)

?

> +		inode->i_flags |= S_KERNEL_FILE;
> +		trace_cachefiles_mark_active(object, inode);
> +		can_use = true;
> +	} else {
> +		pr_notice("cachefiles: Inode already in use: %pd\n", dentry);
> +	}
> +
> +	return can_use;
> +}
> +
> +/*
> + * Unmark a backing inode.  The caller must hold the inode lock.
> + */
> +static void __cachefiles_unmark_inode_in_use(struct cachefiles_object *object,
> +					     struct dentry *dentry)
> +{
> +	struct inode *inode = d_backing_inode(dentry);
> +
> +	inode->i_flags &= ~S_KERNEL_FILE;
> +	trace_cachefiles_mark_inactive(object, inode);
> +}
> diff --git a/fs/namei.c b/fs/namei.c
> index 1f9d2187c765..d81f04f8d818 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -3958,7 +3958,8 @@ int vfs_rmdir(struct user_namespace *mnt_userns, struct inode *dir,
>  	inode_lock(dentry->d_inode);
>  
>  	error = -EBUSY;
> -	if (is_local_mountpoint(dentry))
> +	if (is_local_mountpoint(dentry) ||
> +	    (dentry->d_inode->i_flags & S_KERNEL_FILE))
>  		goto out;
>  
>  	error = security_inode_rmdir(dir, dentry);
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 2c0b8e77d9ab..bcf1ca430139 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -2249,6 +2249,7 @@ struct super_operations {
>  #define S_ENCRYPTED	(1 << 14) /* Encrypted file (using fs/crypto/) */
>  #define S_CASEFOLD	(1 << 15) /* Casefolded file */
>  #define S_VERITY	(1 << 16) /* Verity file (using fs/verity/) */
> +#define S_KERNEL_FILE	(1 << 17) /* File is in use by the kernel (eg. fs/cachefiles) */
>  
>  /*
>   * Note that nosuid etc flags are inode-specific: setting some file-system
> diff --git a/include/trace/events/cachefiles.h b/include/trace/events/cachefiles.h
> index 9bd5a8a60801..6331cd29880d 100644
> --- a/include/trace/events/cachefiles.h
> +++ b/include/trace/events/cachefiles.h
> @@ -83,6 +83,48 @@ cachefiles_error_traces;
>  #define E_(a, b)	{ a, b }
>  
>  
> +TRACE_EVENT(cachefiles_mark_active,
> +	    TP_PROTO(struct cachefiles_object *obj,
> +		     struct inode *inode),
> +
> +	    TP_ARGS(obj, inode),
> +
> +	    /* Note that obj may be NULL */
> +	    TP_STRUCT__entry(
> +		    __field(unsigned int,		obj		)
> +		    __field(ino_t,			inode		)
> +			     ),
> +
> +	    TP_fast_assign(
> +		    __entry->obj	= obj ? obj->debug_id : 0;
> +		    __entry->inode	= inode->i_ino;
> +			   ),
> +
> +	    TP_printk("o=%08x i=%lx",
> +		      __entry->obj, __entry->inode)
> +	    );
> +
> +TRACE_EVENT(cachefiles_mark_inactive,
> +	    TP_PROTO(struct cachefiles_object *obj,
> +		     struct inode *inode),
> +
> +	    TP_ARGS(obj, inode),
> +
> +	    /* Note that obj may be NULL */
> +	    TP_STRUCT__entry(
> +		    __field(unsigned int,		obj		)
> +		    __field(ino_t,			inode		)
> +			     ),
> +
> +	    TP_fast_assign(
> +		    __entry->obj	= obj ? obj->debug_id : 0;
> +		    __entry->inode	= inode->i_ino;
> +			   ),
> +
> +	    TP_printk("o=%08x i=%lx",
> +		      __entry->obj, __entry->inode)
> +	    );
> +
>  TRACE_EVENT(cachefiles_vfs_error,
>  	    TP_PROTO(struct cachefiles_object *obj, struct inode *backer,
>  		     int error, enum cachefiles_error_trace where),
> 
> 

-- 
Jeff Layton <jlayton@kernel.org>