Re: [PATCH v5 1/4] fs: move sgid strip operation from inode_init_owner into inode_sgid_strip

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On Thu, Apr 21, 2022 at 03:54:15PM +0800, Yang Xu wrote:
> This has no functional change. Just create and export inode_sgid_strip
> api for the subsequent patch. This function is used to strip inode's
> S_ISGID mode when init a new inode.
> 
> Signed-off-by: Yang Xu <xuyang2018.jy@xxxxxxxxxxx>
> ---

Could you please add the kernel doc I sketched below to the new helper?

Looks good to me,
Reviewed-by: Christian Brauner (Microsoft) <brauner@xxxxxxxxxx>

> v4-v5:
> use umode_t return value instead of mode pointer
>  fs/inode.c         | 23 +++++++++++++++++++----
>  include/linux/fs.h |  2 ++
>  2 files changed, 21 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/inode.c b/fs/inode.c
> index 9d9b422504d1..57130e4ef8b4 100644
> --- a/fs/inode.c
> +++ b/fs/inode.c
> @@ -2246,10 +2246,8 @@ void inode_init_owner(struct user_namespace *mnt_userns, struct inode *inode,
>  		/* Directories are special, and always inherit S_ISGID */
>  		if (S_ISDIR(mode))
>  			mode |= S_ISGID;
> -		else if ((mode & (S_ISGID | S_IXGRP)) == (S_ISGID | S_IXGRP) &&
> -			 !in_group_p(i_gid_into_mnt(mnt_userns, dir)) &&
> -			 !capable_wrt_inode_uidgid(mnt_userns, dir, CAP_FSETID))
> -			mode &= ~S_ISGID;
> +		else
> +			mode = inode_sgid_strip(mnt_userns, dir, mode);
>  	} else
>  		inode_fsgid_set(inode, mnt_userns);
>  	inode->i_mode = mode;
> @@ -2405,3 +2403,20 @@ struct timespec64 current_time(struct inode *inode)
>  	return timestamp_truncate(now, inode);
>  }
>  EXPORT_SYMBOL(current_time);
> +

/**
 * inode_sgid_strip - handle the sgid bit for non-directories
 * @mnt_userns:	idmapping of the mount
 * @dir: parent directory
 * @mode: mode of the file to be created in @dir
 *
 * If the @mode of the new file has both the S_ISGID and S_IXGRP bit
 * raised and @dir has the S_ISGID bit raised ensure that the caller is
 * either in the group of the parent directory or they have CAP_FSETID
 * in their user namespace and are privileged over the parent directory.
 * In all other cases, strip the S_ISGID bit from @mode.
 *
 * Return: the new mode to use for the file
 */
> +umode_t inode_sgid_strip(struct user_namespace *mnt_userns,
> +			 const struct inode *dir, umode_t mode)
> +{
> +	if (S_ISDIR(mode) || !dir || !(dir->i_mode & S_ISGID))
> +		return mode;
> +	if ((mode & (S_ISGID | S_IXGRP)) != (S_ISGID | S_IXGRP))
> +		return mode;
> +	if (in_group_p(i_gid_into_mnt(mnt_userns, dir)))
> +		return mode;
> +	if (capable_wrt_inode_uidgid(mnt_userns, dir, CAP_FSETID))
> +		return mode;
> +
> +	mode &= ~S_ISGID;
> +	return mode;
> +}
> +EXPORT_SYMBOL(inode_sgid_strip);
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index bbde95387a23..532de76c9b91 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -1897,6 +1897,8 @@ extern long compat_ptr_ioctl(struct file *file, unsigned int cmd,
>  void inode_init_owner(struct user_namespace *mnt_userns, struct inode *inode,
>  		      const struct inode *dir, umode_t mode);
>  extern bool may_open_dev(const struct path *path);
> +umode_t inode_sgid_strip(struct user_namespace *mnt_userns,
> +			 const struct inode *dir, umode_t mode);
>  
>  /*
>   * This is the "filldir" function type, used by readdir() to let
> -- 
> 2.27.0
> 



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