Re: [PATCH v2] vfs: avoid recopying file names in getname_flags

[Boqun Feng]

Ping.

This patch has been tested by 0day test bot.

Thanks,
Boqun Feng


On Thu, Mar 26, 2015 at 02:45:52AM +0800, Boqun Feng wrote:
> In the current implementation of getname_flags, a file name in the
> user-space will be recopied if it takes more space that
> EMBEDDED_NAME_MAX, however, at this moment, EMBEDDED_NAME_MAX bytes of
> the file name are already copied into kernel address space, the only
> reason why the recopy is needed is that "kname", which points to the
> string of the file name, needs to be relocated.
> 
> And the recopy can be avoided if we change the memory layout of the
> `names_cachep` allocation. By putting `struct filename` at the tail of
> the allocation instead of the head, relocation of kname is avoided.
> Once putting the struct at the tail, each byte in the user space will be
> copied only one time, so the recopy is avoided.
> 
> Of course, other functions aware of the layout of the `names_cachep`
> allocation, i.e., getname_kernel and putname also need to be modified to
> adapt to the new layout.
> 
> Since we change the layout of `names_cachep` allocation, in order to
> figure out whether kname and the struct are separate, we now need to
> check whether the file name string starts at the address
> EMBEDDED_NAME_MAX bytes before the address of the struct.  As a result,
> `iname`, which points the end of `struct filename`, is not needed
> anymore.
> 
> Signed-off-by: Boqun Feng <boqun.feng@xxxxxxxxx>
> ---
> v1 --> v2:
> Rebase the patch onto the for-next branch of Al's vfs repo.
> 
> 
>  fs/namei.c         | 45 ++++++++++++++++++++++++++++-----------------
>  include/linux/fs.h |  1 -
>  2 files changed, 28 insertions(+), 18 deletions(-)
> 
> diff --git a/fs/namei.c b/fs/namei.c
> index 7a11ec1..6d04029 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -119,7 +119,7 @@
>   * PATH_MAX includes the nul terminator --RR.
>   */
>  
> -#define EMBEDDED_NAME_MAX	(PATH_MAX - offsetof(struct filename, iname))
> +#define EMBEDDED_NAME_MAX	(PATH_MAX - sizeof(struct filename))
>  
>  struct filename *
>  getname_flags(const char __user *filename, int flags, int *empty)
> @@ -132,44 +132,53 @@ getname_flags(const char __user *filename, int flags, int *empty)
>  	if (result)
>  		return result;
>  
> -	result = __getname();
> -	if (unlikely(!result))
> +	kname = __getname();
> +	if (unlikely(!kname))
>  		return ERR_PTR(-ENOMEM);
>  
>  	/*
>  	 * First, try to embed the struct filename inside the names_cache
>  	 * allocation
>  	 */
> -	kname = (char *)result->iname;
> +	result = (struct filename *)(kname + EMBEDDED_NAME_MAX);
>  	result->name = kname;
>  
>  	len = strncpy_from_user(kname, filename, EMBEDDED_NAME_MAX);
>  	if (unlikely(len < 0)) {
> -		__putname(result);
> +		__putname(kname);
>  		return ERR_PTR(len);
>  	}
>  
>  	/*
>  	 * Uh-oh. We have a name that's approaching PATH_MAX. Allocate a
>  	 * separate struct filename so we can dedicate the entire
> -	 * names_cache allocation for the pathname, and re-do the copy from
> +	 * names_cache allocation for the pathname, and continue the copy from
>  	 * userland.
>  	 */
>  	if (unlikely(len == EMBEDDED_NAME_MAX)) {
> -		kname = (char *)result;
> -
>  		result = kzalloc(sizeof(*result), GFP_KERNEL);
>  		if (unlikely(!result)) {
>  			__putname(kname);
>  			return ERR_PTR(-ENOMEM);
>  		}
>  		result->name = kname;
> -		len = strncpy_from_user(kname, filename, PATH_MAX);
> +		/* we can't simply add the number of rest chars we copy to len,
> +		 * since strncpy_from_user may return negative to indicate
> +		 * something is wrong, so we do the addition later, after
> +		 * strncpy_from_user succeeds, and we know we already copy
> +		 * EMBEDDED_NAME_MAX chars.
> +		 */
> +		len = strncpy_from_user(kname + EMBEDDED_NAME_MAX,
> +				filename + EMBEDDED_NAME_MAX,
> +				PATH_MAX - EMBEDDED_NAME_MAX);
> +
>  		if (unlikely(len < 0)) {
>  			__putname(kname);
>  			kfree(result);
>  			return ERR_PTR(len);
>  		}
> +
> +		len += EMBEDDED_NAME_MAX;
>  		if (unlikely(len == PATH_MAX)) {
>  			__putname(kname);
>  			kfree(result);
> @@ -204,26 +213,28 @@ struct filename *
>  getname_kernel(const char * filename)
>  {
>  	struct filename *result;
> +	char *kname;
>  	int len = strlen(filename) + 1;
>  
> -	result = __getname();
> -	if (unlikely(!result))
> +	kname = __getname();
> +	if (unlikely(!kname))
>  		return ERR_PTR(-ENOMEM);
>  
>  	if (len <= EMBEDDED_NAME_MAX) {
> -		result->name = (char *)result->iname;
> +		result = (struct filename *)(kname + EMBEDDED_NAME_MAX);
> +		result->name = kname;
>  	} else if (len <= PATH_MAX) {
>  		struct filename *tmp;
>  
>  		tmp = kmalloc(sizeof(*tmp), GFP_KERNEL);
>  		if (unlikely(!tmp)) {
> -			__putname(result);
> +			__putname(kname);
>  			return ERR_PTR(-ENOMEM);
>  		}
> -		tmp->name = (char *)result;
> +		tmp->name = kname;
>  		result = tmp;
>  	} else {
> -		__putname(result);
> +		__putname(kname);
>  		return ERR_PTR(-ENAMETOOLONG);
>  	}
>  	memcpy((char *)result->name, filename, len);
> @@ -242,11 +253,11 @@ void putname(struct filename *name)
>  	if (--name->refcnt > 0)
>  		return;
>  
> -	if (name->name != name->iname) {
> +	if (name->name != ((char *)name - EMBEDDED_NAME_MAX)) {
>  		__putname(name->name);
>  		kfree(name);
>  	} else
> -		__putname(name);
> +		__putname(name->name);
>  }
>  
>  static int check_acl(struct inode *inode, int mask)
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index dfbd88a..dd67284 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -2166,7 +2166,6 @@ struct filename {
>  	const __user char	*uptr;	/* original userland pointer */
>  	struct audit_names	*aname;
>  	int			refcnt;
> -	const char		iname[];
>  };
>  
>  extern long vfs_truncate(struct path *, loff_t);
> -- 
> 2.3.3
>

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