Date:	Tue, 1 Dec 2015 10:06:09 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Nicolai Stange <nicstange@...il.com>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Jonathan Corbet <corbet@....net>, Jan Kara <jack@...e.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] debugfs: prevent access to removed files' private
 data

On Tue, Dec 01, 2015 at 12:26:41AM +0100, Nicolai Stange wrote:
> Upon return of debugfs_remove()/debugfs_remove_recursive(), it might
> still be attempted to access associated private file data through
> previously opened struct file objects. If that data has been freed by
> the caller of debugfs_remove*() in the meanwhile, the reading/writing
> process would either encounter a fault or, if the memory address in
> question has been reassigned again, unrelated data structures could get
> overwritten.
> 
> However, since debugfs files are seldomly removed, usually from module
> exit handlers only, the impact is very low.
> 
> Since debugfs_remove() and debugfs_remove_recursive() are already
> waiting for a SRCU grace period before returning to their callers,
> enclosing the access to private file data from ->read() and ->write()
> within a SRCU read-side critical section does the trick:
> - Introduce the debugfs_file_use_data_start() and
>   debugfs_file_use_data_finish() helpers which just enter and leave
>   a SRCU read-side critical section. The former also reports whether the
>   file is still alive, that is if d_delete() has _not_ been called on
>   the corresponding dentry.
> - Introduce the DEFINE_DEBUGFS_ATTRIBUTE() macro which is completely
>   equivalent to the DEFINE_SIMPLE_ATTRIBUTE() macro except that
>   ->read() and ->write are set to SRCU protecting wrappers around the
>   original simple_read() and simple_write() helpers.
> - Use that DEFINE_DEBUGFS_ATTRIBUTE() macro for all debugfs_create_*()
>   attribute creation variants where appropriate.
> - Manually introduce SRCU protection to the debugfs-predefined readers
>   and writers not covered by the above DEFINE_SIMPLE_ATTRIBUTE()->
>   DEFINE_DEBUGFS_ATTRIBUTE() replacement.
> 
> Finally, it should be worth to note that in the vast majority of cases
> where debugfs users are handing in a "custom" struct file_operations
> object to debugfs_create_file(), an attribute's associated data's
> lifetime is bound to the one of the containing module and thus,
> taking a reference on ->owner during file opening acts as a proxy here.
> There is no need to do a mass replace of DEFINE_SIMPLE_ATTRIBUTE() to
> DEFINE_DEBUGFS_ATTRIBUTE() outside of debugfs.
> 
> Signed-off-by: Nicolai Stange <nicstange@...il.com>

Also looks good from an RCU perspective.

							Thanx, Paul

> ---
>  Applicable to the Linus tree.
>  The second of the two patches depends on the first one
> 
> Documentation/DocBook/filesystems.tmpl |   1 +
>  fs/debugfs/file.c                      | 185 +++++++++++++++++++++++++--------
>  fs/debugfs/inode.c                     |   1 +
>  include/linux/debugfs.h                |  66 ++++++++++++
>  4 files changed, 209 insertions(+), 44 deletions(-)
> 
> diff --git a/Documentation/DocBook/filesystems.tmpl b/Documentation/DocBook/filesystems.tmpl
> index 6006b63..1f2f6c0 100644
> --- a/Documentation/DocBook/filesystems.tmpl
> +++ b/Documentation/DocBook/filesystems.tmpl
> @@ -99,6 +99,7 @@
>       <sect1 id="debugfs_interface"><title>debugfs interface</title>
>  !Efs/debugfs/inode.c
>  !Efs/debugfs/file.c
> +!Iinclude/linux/debugfs.h
>       </sect1>
>    </chapter>
> 
> diff --git a/fs/debugfs/file.c b/fs/debugfs/file.c
> index 67df2c9..8e10695 100644
> --- a/fs/debugfs/file.c
> +++ b/fs/debugfs/file.c
> @@ -69,6 +69,45 @@ const struct file_operations debugfs_proxy_file_operations = {
>  	.open = proxy_open,
>  };
> 
> +int __debugfs_file_use_data_start(struct file *file, int *srcu_idx)
> +	__acquires(&debugfs_srcu)
> +{
> +	*srcu_idx = srcu_read_lock(&debugfs_srcu);
> +	if (d_unlinked(file->f_path.dentry))
> +		return -EIO;
> +
> +	return 0;
> +}
> +EXPORT_SYMBOL_GPL(__debugfs_file_use_data_start);
> +
> +ssize_t debugfs_attr_read(struct file *file, char __user *buf,
> +			 size_t len, loff_t *ppos)
> +{
> +	ssize_t ret;
> +	int srcu_idx;
> +
> +	ret = debugfs_file_use_data_start(file, &srcu_idx);
> +	if (!ret)
> +		ret = simple_attr_read(file, buf, len, ppos);
> +	debugfs_file_use_data_finish(srcu_idx);
> +	return ret;
> +}
> +EXPORT_SYMBOL_GPL(debugfs_attr_read);
> +
> +ssize_t debugfs_attr_write(struct file *file, const char __user *buf,
> +			  size_t len, loff_t *ppos)
> +{
> +	ssize_t ret;
> +	int srcu_idx;
> +
> +	ret = debugfs_file_use_data_start(file, &srcu_idx);
> +	if (!ret)
> +		ret = simple_attr_write(file, buf, len, ppos);
> +	debugfs_file_use_data_finish(srcu_idx);
> +	return ret;
> +}
> +EXPORT_SYMBOL_GPL(debugfs_attr_write);
> +
>  static struct dentry *debugfs_create_mode(const char *name, umode_t mode,
>  					  struct dentry *parent, void *value,
>  				          const struct file_operations *fops,
> @@ -95,9 +134,9 @@ static int debugfs_u8_get(void *data, u64 *val)
>  	*val = *(u8 *)data;
>  	return 0;
>  }
> -DEFINE_SIMPLE_ATTRIBUTE(fops_u8, debugfs_u8_get, debugfs_u8_set, "%llu\n");
> -DEFINE_SIMPLE_ATTRIBUTE(fops_u8_ro, debugfs_u8_get, NULL, "%llu\n");
> -DEFINE_SIMPLE_ATTRIBUTE(fops_u8_wo, NULL, debugfs_u8_set, "%llu\n");
> +DEFINE_DEBUGFS_ATTRIBUTE(fops_u8, debugfs_u8_get, debugfs_u8_set, "%llu\n");
> +DEFINE_DEBUGFS_ATTRIBUTE(fops_u8_ro, debugfs_u8_get, NULL, "%llu\n");
> +DEFINE_DEBUGFS_ATTRIBUTE(fops_u8_wo, NULL, debugfs_u8_set, "%llu\n");
> 
>  /**
>   * debugfs_create_u8 - create a debugfs file that is used to read and write an unsigned 8-bit value
> @@ -141,9 +180,9 @@ static int debugfs_u16_get(void *data, u64 *val)
>  	*val = *(u16 *)data;
>  	return 0;
>  }
> -DEFINE_SIMPLE_ATTRIBUTE(fops_u16, debugfs_u16_get, debugfs_u16_set, "%llu\n");
> -DEFINE_SIMPLE_ATTRIBUTE(fops_u16_ro, debugfs_u16_get, NULL, "%llu\n");
> -DEFINE_SIMPLE_ATTRIBUTE(fops_u16_wo, NULL, debugfs_u16_set, "%llu\n");
> +DEFINE_DEBUGFS_ATTRIBUTE(fops_u16, debugfs_u16_get, debugfs_u16_set, "%llu\n");
> +DEFINE_DEBUGFS_ATTRIBUTE(fops_u16_ro, debugfs_u16_get, NULL, "%llu\n");
> +DEFINE_DEBUGFS_ATTRIBUTE(fops_u16_wo, NULL, debugfs_u16_set, "%llu\n");
> 
>  /**
>   * debugfs_create_u16 - create a debugfs file that is used to read and write an unsigned 16-bit value
> @@ -187,9 +226,9 @@ static int debugfs_u32_get(void *data, u64 *val)
>  	*val = *(u32 *)data;
>  	return 0;
>  }
> -DEFINE_SIMPLE_ATTRIBUTE(fops_u32, debugfs_u32_get, debugfs_u32_set, "%llu\n");
> -DEFINE_SIMPLE_ATTRIBUTE(fops_u32_ro, debugfs_u32_get, NULL, "%llu\n");
> -DEFINE_SIMPLE_ATTRIBUTE(fops_u32_wo, NULL, debugfs_u32_set, "%llu\n");
> +DEFINE_DEBUGFS_ATTRIBUTE(fops_u32, debugfs_u32_get, debugfs_u32_set, "%llu\n");
> +DEFINE_DEBUGFS_ATTRIBUTE(fops_u32_ro, debugfs_u32_get, NULL, "%llu\n");
> +DEFINE_DEBUGFS_ATTRIBUTE(fops_u32_wo, NULL, debugfs_u32_set, "%llu\n");
> 
>  /**
>   * debugfs_create_u32 - create a debugfs file that is used to read and write an unsigned 32-bit value
> @@ -234,9 +273,9 @@ static int debugfs_u64_get(void *data, u64 *val)
>  	*val = *(u64 *)data;
>  	return 0;
>  }
> -DEFINE_SIMPLE_ATTRIBUTE(fops_u64, debugfs_u64_get, debugfs_u64_set, "%llu\n");
> -DEFINE_SIMPLE_ATTRIBUTE(fops_u64_ro, debugfs_u64_get, NULL, "%llu\n");
> -DEFINE_SIMPLE_ATTRIBUTE(fops_u64_wo, NULL, debugfs_u64_set, "%llu\n");
> +DEFINE_DEBUGFS_ATTRIBUTE(fops_u64, debugfs_u64_get, debugfs_u64_set, "%llu\n");
> +DEFINE_DEBUGFS_ATTRIBUTE(fops_u64_ro, debugfs_u64_get, NULL, "%llu\n");
> +DEFINE_DEBUGFS_ATTRIBUTE(fops_u64_wo, NULL, debugfs_u64_set, "%llu\n");
> 
>  /**
>   * debugfs_create_u64 - create a debugfs file that is used to read and write an unsigned 64-bit value
> @@ -281,9 +320,10 @@ static int debugfs_ulong_get(void *data, u64 *val)
>  	*val = *(unsigned long *)data;
>  	return 0;
>  }
> -DEFINE_SIMPLE_ATTRIBUTE(fops_ulong, debugfs_ulong_get, debugfs_ulong_set, "%llu\n");
> -DEFINE_SIMPLE_ATTRIBUTE(fops_ulong_ro, debugfs_ulong_get, NULL, "%llu\n");
> -DEFINE_SIMPLE_ATTRIBUTE(fops_ulong_wo, NULL, debugfs_ulong_set, "%llu\n");
> +DEFINE_DEBUGFS_ATTRIBUTE(fops_ulong, debugfs_ulong_get, debugfs_ulong_set,
> +			"%llu\n");
> +DEFINE_DEBUGFS_ATTRIBUTE(fops_ulong_ro, debugfs_ulong_get, NULL, "%llu\n");
> +DEFINE_DEBUGFS_ATTRIBUTE(fops_ulong_wo, NULL, debugfs_ulong_set, "%llu\n");
> 
>  /**
>   * debugfs_create_ulong - create a debugfs file that is used to read and write
> @@ -318,21 +358,24 @@ struct dentry *debugfs_create_ulong(const char *name, umode_t mode,
>  }
>  EXPORT_SYMBOL_GPL(debugfs_create_ulong);
> 
> -DEFINE_SIMPLE_ATTRIBUTE(fops_x8, debugfs_u8_get, debugfs_u8_set, "0x%02llx\n");
> -DEFINE_SIMPLE_ATTRIBUTE(fops_x8_ro, debugfs_u8_get, NULL, "0x%02llx\n");
> -DEFINE_SIMPLE_ATTRIBUTE(fops_x8_wo, NULL, debugfs_u8_set, "0x%02llx\n");
> +DEFINE_DEBUGFS_ATTRIBUTE(fops_x8, debugfs_u8_get, debugfs_u8_set, "0x%02llx\n");
> +DEFINE_DEBUGFS_ATTRIBUTE(fops_x8_ro, debugfs_u8_get, NULL, "0x%02llx\n");
> +DEFINE_DEBUGFS_ATTRIBUTE(fops_x8_wo, NULL, debugfs_u8_set, "0x%02llx\n");
> 
> -DEFINE_SIMPLE_ATTRIBUTE(fops_x16, debugfs_u16_get, debugfs_u16_set, "0x%04llx\n");
> -DEFINE_SIMPLE_ATTRIBUTE(fops_x16_ro, debugfs_u16_get, NULL, "0x%04llx\n");
> -DEFINE_SIMPLE_ATTRIBUTE(fops_x16_wo, NULL, debugfs_u16_set, "0x%04llx\n");
> +DEFINE_DEBUGFS_ATTRIBUTE(fops_x16, debugfs_u16_get, debugfs_u16_set,
> +			"0x%04llx\n");
> +DEFINE_DEBUGFS_ATTRIBUTE(fops_x16_ro, debugfs_u16_get, NULL, "0x%04llx\n");
> +DEFINE_DEBUGFS_ATTRIBUTE(fops_x16_wo, NULL, debugfs_u16_set, "0x%04llx\n");
> 
> -DEFINE_SIMPLE_ATTRIBUTE(fops_x32, debugfs_u32_get, debugfs_u32_set, "0x%08llx\n");
> -DEFINE_SIMPLE_ATTRIBUTE(fops_x32_ro, debugfs_u32_get, NULL, "0x%08llx\n");
> -DEFINE_SIMPLE_ATTRIBUTE(fops_x32_wo, NULL, debugfs_u32_set, "0x%08llx\n");
> +DEFINE_DEBUGFS_ATTRIBUTE(fops_x32, debugfs_u32_get, debugfs_u32_set,
> +			"0x%08llx\n");
> +DEFINE_DEBUGFS_ATTRIBUTE(fops_x32_ro, debugfs_u32_get, NULL, "0x%08llx\n");
> +DEFINE_DEBUGFS_ATTRIBUTE(fops_x32_wo, NULL, debugfs_u32_set, "0x%08llx\n");
> 
> -DEFINE_SIMPLE_ATTRIBUTE(fops_x64, debugfs_u64_get, debugfs_u64_set, "0x%016llx\n");
> -DEFINE_SIMPLE_ATTRIBUTE(fops_x64_ro, debugfs_u64_get, NULL, "0x%016llx\n");
> -DEFINE_SIMPLE_ATTRIBUTE(fops_x64_wo, NULL, debugfs_u64_set, "0x%016llx\n");
> +DEFINE_DEBUGFS_ATTRIBUTE(fops_x64, debugfs_u64_get, debugfs_u64_set,
> +			"0x%016llx\n");
> +DEFINE_DEBUGFS_ATTRIBUTE(fops_x64_ro, debugfs_u64_get, NULL, "0x%016llx\n");
> +DEFINE_DEBUGFS_ATTRIBUTE(fops_x64_wo, NULL, debugfs_u64_set, "0x%016llx\n");
> 
>  /*
>   * debugfs_create_x{8,16,32,64} - create a debugfs file that is used to read and write an unsigned {8,16,32,64}-bit value
> @@ -425,10 +468,10 @@ static int debugfs_size_t_get(void *data, u64 *val)
>  	*val = *(size_t *)data;
>  	return 0;
>  }
> -DEFINE_SIMPLE_ATTRIBUTE(fops_size_t, debugfs_size_t_get, debugfs_size_t_set,
> +DEFINE_DEBUGFS_ATTRIBUTE(fops_size_t, debugfs_size_t_get, debugfs_size_t_set,
>  			"%llu\n");	/* %llu and %zu are more or less the same */
> -DEFINE_SIMPLE_ATTRIBUTE(fops_size_t_ro, debugfs_size_t_get, NULL, "%llu\n");
> -DEFINE_SIMPLE_ATTRIBUTE(fops_size_t_wo, NULL, debugfs_size_t_set, "%llu\n");
> +DEFINE_DEBUGFS_ATTRIBUTE(fops_size_t_ro, debugfs_size_t_get, NULL, "%llu\n");
> +DEFINE_DEBUGFS_ATTRIBUTE(fops_size_t_wo, NULL, debugfs_size_t_set, "%llu\n");
> 
>  /**
>   * debugfs_create_size_t - create a debugfs file that is used to read and write an size_t value
> @@ -458,10 +501,12 @@ static int debugfs_atomic_t_get(void *data, u64 *val)
>  	*val = atomic_read((atomic_t *)data);
>  	return 0;
>  }
> -DEFINE_SIMPLE_ATTRIBUTE(fops_atomic_t, debugfs_atomic_t_get,
> +DEFINE_DEBUGFS_ATTRIBUTE(fops_atomic_t, debugfs_atomic_t_get,
>  			debugfs_atomic_t_set, "%lld\n");
> -DEFINE_SIMPLE_ATTRIBUTE(fops_atomic_t_ro, debugfs_atomic_t_get, NULL, "%lld\n");
> -DEFINE_SIMPLE_ATTRIBUTE(fops_atomic_t_wo, NULL, debugfs_atomic_t_set, "%lld\n");
> +DEFINE_DEBUGFS_ATTRIBUTE(fops_atomic_t_ro, debugfs_atomic_t_get, NULL,
> +			"%lld\n");
> +DEFINE_DEBUGFS_ATTRIBUTE(fops_atomic_t_wo, NULL, debugfs_atomic_t_set,
> +			"%lld\n");
> 
>  /**
>   * debugfs_create_atomic_t - create a debugfs file that is used to read and
> @@ -487,11 +532,18 @@ ssize_t debugfs_read_file_bool(struct file *file, char __user *user_buf,
>  {
>  	char buf[3];
>  	bool *val = file->private_data;
> +	int ret, srcu_idx;
> 
> +	ret = debugfs_file_use_data_start(file, &srcu_idx);
> +	if (ret) {
> +		debugfs_file_use_data_finish(srcu_idx);
> +		return ret;
> +	}
>  	if (*val)
>  		buf[0] = 'Y';
>  	else
>  		buf[0] = 'N';
> +	debugfs_file_use_data_finish(srcu_idx);
>  	buf[1] = '\n';
>  	buf[2] = 0x00;
>  	return simple_read_from_buffer(user_buf, count, ppos, buf, 2);
> @@ -505,16 +557,26 @@ ssize_t debugfs_write_file_bool(struct file *file, const char __user *user_buf,
>  	size_t buf_size;
>  	bool bv;
>  	bool *val = file->private_data;
> +	ssize_t ret;
> +	int srcu_idx;
> 
>  	buf_size = min(count, (sizeof(buf)-1));
>  	if (copy_from_user(buf, user_buf, buf_size))
>  		return -EFAULT;
> 
>  	buf[buf_size] = '\0';
> -	if (strtobool(buf, &bv) == 0)
> -		*val = bv;
> +	if (strtobool(buf, &bv) == 0) {
> +		ret = debugfs_file_use_data_start(file, &srcu_idx);
> +		if (!ret) {
> +			*val = bv;
> +			ret = count;
> +		}
> +		debugfs_file_use_data_finish(srcu_idx);
> +	} else {
> +		return -EINVAL;
> +	}
> 
> -	return count;
> +	return ret;
>  }
>  EXPORT_SYMBOL_GPL(debugfs_write_file_bool);
> 
> @@ -572,9 +634,16 @@ EXPORT_SYMBOL_GPL(debugfs_create_bool);
>  static ssize_t read_file_blob(struct file *file, char __user *user_buf,
>  			      size_t count, loff_t *ppos)
>  {
> +	ssize_t ret;
> +	int srcu_idx;
>  	struct debugfs_blob_wrapper *blob = file->private_data;
> -	return simple_read_from_buffer(user_buf, count, ppos, blob->data,
> -			blob->size);
> +
> +	ret = debugfs_file_use_data_start(file, &srcu_idx);
> +	if (!ret)
> +		ret = simple_read_from_buffer(user_buf, count, ppos, blob->data,
> +					blob->size);
> +	debugfs_file_use_data_finish(srcu_idx);
> +	return ret;
>  }
> 
>  static const struct file_operations fops_blob = {
> @@ -643,6 +712,7 @@ static int u32_array_open(struct inode *inode, struct file *file)
>  	struct array_data *data = inode->i_private;
>  	int size, elements = data->elements;
>  	char *buf;
> +	int ret, srcu_idx;
> 
>  	/*
>  	 * Max size:
> @@ -656,9 +726,17 @@ static int u32_array_open(struct inode *inode, struct file *file)
>  	buf[size] = 0;
> 
>  	file->private_data = buf;
> +	ret = debugfs_file_use_data_start(file, &srcu_idx);
> +	if (ret) {
> +		kfree(file->private_data);
> +		goto out;
> +	}
>  	u32_format_array(buf, size, data->array, data->elements);
> +	nonseekable_open(inode, file);
> 
> -	return nonseekable_open(inode, file);
> +out:
> +	debugfs_file_use_data_finish(srcu_idx);
> +	return ret;
>  }
> 
>  static ssize_t u32_array_read(struct file *file, char __user *buf, size_t len,
> @@ -761,15 +839,21 @@ EXPORT_SYMBOL_GPL(debugfs_print_regs32);
> 
>  static int debugfs_show_regset32(struct seq_file *s, void *data)
>  {
> -	struct debugfs_regset32 *regset = s->private;
> +	struct file *f = s->private;
> +	struct debugfs_regset32 *regset = file_inode(f)->i_private;
> +	int ret, srcu_idx;
> 
> -	debugfs_print_regs32(s, regset->regs, regset->nregs, regset->base, "");
> -	return 0;
> +	ret = debugfs_file_use_data_start(f, &srcu_idx);
> +	if (!ret)
> +		debugfs_print_regs32(s, regset->regs, regset->nregs,
> +				regset->base, "");
> +	debugfs_file_use_data_finish(srcu_idx);
> +	return ret;
>  }
> 
>  static int debugfs_open_regset32(struct inode *inode, struct file *file)
>  {
> -	return single_open(file, debugfs_show_regset32, inode->i_private);
> +	return single_open(file, debugfs_show_regset32, file);
>  }
> 
>  static const struct file_operations fops_regset32 = {
> @@ -826,11 +910,24 @@ static int debugfs_devm_entry_open(struct inode *inode, struct file *f)
>  	return single_open(f, entry->read, entry->dev);
>  }
> 
> +static ssize_t debugfs_devm_entry_read(struct file *file, char __user *buf,
> +				size_t size, loff_t *ppos)
> +{
> +	ssize_t ret;
> +	int srcu_idx;
> +
> +	ret = debugfs_file_use_data_start(file, &srcu_idx);
> +	if (!ret)
> +		ret = seq_read(file, buf, size, ppos);
> +	debugfs_file_use_data_finish(srcu_idx);
> +	return ret;
> +}
> +
>  static const struct file_operations debugfs_devm_entry_ops = {
>  	.owner = THIS_MODULE,
>  	.open = debugfs_devm_entry_open,
>  	.release = single_release,
> -	.read = seq_read,
> +	.read = debugfs_devm_entry_read,
>  	.llseek = seq_lseek
>  };
> 
> diff --git a/fs/debugfs/inode.c b/fs/debugfs/inode.c
> index 8ae2e1a..37222c9 100644
> --- a/fs/debugfs/inode.c
> +++ b/fs/debugfs/inode.c
> @@ -31,6 +31,7 @@
>  #define DEBUGFS_DEFAULT_MODE	0700
> 
>  DEFINE_SRCU(debugfs_srcu);
> +EXPORT_SYMBOL_GPL(debugfs_srcu);
> 
>  static struct vfsmount *debugfs_mount;
>  static int debugfs_mount_count;
> diff --git a/include/linux/debugfs.h b/include/linux/debugfs.h
> index f8c7494..ba1a299 100644
> --- a/include/linux/debugfs.h
> +++ b/include/linux/debugfs.h
> @@ -20,6 +20,7 @@
> 
>  #include <linux/types.h>
>  #include <linux/srcu.h>
> +#include <linux/compiler.h>
> 
>  struct device;
>  struct file_operations;
> @@ -128,6 +129,71 @@ ssize_t debugfs_read_file_bool(struct file *file, char __user *user_buf,
>  ssize_t debugfs_write_file_bool(struct file *file, const char __user *user_buf,
>  				size_t count, loff_t *ppos);
> 
> +int __debugfs_file_use_data_start(struct file *file, int *srcu_idx)
> +	__acquires(&debugfs_srcu);
> +
> +/**
> + * debugfs_file_use_data_start - mark the beginning of file data access
> + * @file: the file object whose data is being accessed.
> + * @srcu_idx: a pointer to some memory to store a SRCU index in.
> + *
> + * Up to a matching call to debugfs_file_use_data_finish(), any
> + * successive call into the file removing functions debugfs_remove()
> + * and debugfs_remove_recursive() will block. Since associated private
> + * file data may only get freed after a successful return of any of
> + * the removal functions, you may safely access it after a successful
> + * call to debugfs_file_use_data_start() without worrying about
> + * lifetime issues.
> + *
> + * If -%EIO is returned, the file has already been removed and thus,
> + * it is not safe to access any of its data. If, on the other hand,
> + * it is allowed to access the file data, zero is returned.
> + *
> + * Regardless of the return code, any call to
> + * debugfs_file_use_data_start() must be followed by a matching call
> + * to debugfs_file_use_data_finish().
> + */
> +static inline int debugfs_file_use_data_start(struct file *file, int *srcu_idx)
> +	__acquires(&debugfs_srcu)
> +{
> +	return __debugfs_file_use_data_start(file, srcu_idx);
> +}
> +
> +/**
> + * debugfs_file_use_data_finish - mark the end of file data access
> + * @srcu_idx: the SRCU index "created" by a former call to
> + *            debugfs_file_use_data_start().
> + *
> + * Allow any ongoing concurrent call into debugfs_remove() or
> + * debugfs_remove_recursive() blocked by a former call to
> + * debugfs_file_use_data_start() to proceed and return to its caller.
> + */
> +static inline void debugfs_file_use_data_finish(int srcu_idx)
> +	__releases(&debugfs_srcu)
> +{
> +	srcu_read_unlock(&debugfs_srcu, srcu_idx);
> +}
> +
> +ssize_t debugfs_attr_read(struct file *file, char __user *buf,
> +			 size_t len, loff_t *ppos);
> +ssize_t debugfs_attr_write(struct file *file, const char __user *buf,
> +			  size_t len, loff_t *ppos);
> +
> +#define DEFINE_DEBUGFS_ATTRIBUTE(__fops, __get, __set, __fmt)		\
> +static int __fops ## _open(struct inode *inode, struct file *file)	\
> +{									\
> +	__simple_attr_check_format(__fmt, 0ull);			\
> +	return simple_attr_open(inode, file, __get, __set, __fmt);	\
> +}									\
> +static const struct file_operations __fops = {				\
> +	.owner	 = THIS_MODULE,					\
> +	.open	 = __fops ## _open,					\
> +	.release = simple_attr_release,				\
> +	.read	 = debugfs_attr_read,					\
> +	.write	 = debugfs_attr_write,					\
> +	.llseek	 = generic_file_llseek,			\
> +}
> +
>  #else
> 
>  #include <linux/err.h>
> -- 
> 2.6.3
> 

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