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Date:   Tue, 19 Jan 2021 09:37:20 +0000
From:   Christoph Hellwig <hch@infradead.org>
To:     Christian Brauner <christian.brauner@ubuntu.com>
Cc:     Alexander Viro <viro@zeniv.linux.org.uk>,
        Christoph Hellwig <hch@infradead.org>,
        linux-fsdevel@vger.kernel.org,
        John Johansen <john.johansen@canonical.com>,
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        Stephen Smalley <stephen.smalley.work@gmail.com>,
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        Arnd Bergmann <arnd@arndb.de>,
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        OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>,
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        Christoph Hellwig <hch@lst.de>
Subject: Re: [PATCH v5 22/42] open: handle idmapped mounts in do_truncate()
Message-ID: <20210119093720.GG3364550@infradead.org>
References: <20210112220124.837960-1-christian.brauner@ubuntu.com>
 <20210112220124.837960-23-christian.brauner@ubuntu.com>
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On Tue, Jan 12, 2021 at 11:01:04PM +0100, Christian Brauner wrote:
> @@ -930,8 +932,12 @@ void dump_truncate(struct coredump_params *cprm)
>  
>  	if (file->f_op->llseek && file->f_op->llseek != no_llseek) {
>  		offset = file->f_op->llseek(file, 0, SEEK_CUR);
> -		if (i_size_read(file->f_mapping->host) < offset)
> -			do_truncate(file->f_path.dentry, offset, 0, file);
> +		if (i_size_read(file->f_mapping->host) < offset) {
> +			struct user_namespace *mnt_userns;
> +
> +			mnt_userns = file_user_ns(file);
> +			do_truncate(mnt_userns, file->f_path.dentry, offset, 0, file);
> +		}

I think we can skip the local variable here.  In fact for all callers
of do_truncate except vfs_truncate a little file_truncate helper that
takes a struct file would help readability a lot.