linux-kernel - Re: [PATCH v3] erofs: fix file handle encoding for 64-bit NIDs

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Message-ID: <18d272ce-6a80-4fdc-b67b-ddc2ffa522d4@linux.alibaba.com>
Date: Tue, 6 May 2025 23:10:46 +0800
From: Gao Xiang <hsiangkao@...ux.alibaba.com>
To: Hongbo Li <lihongbo22@...wei.com>, xiang@...nel.org, chao@...nel.org,
 zbestahu@...il.com, jefflexu@...ux.alibaba.com
Cc: dhavale@...gle.com, linux-erofs@...ts.ozlabs.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] erofs: fix file handle encoding for 64-bit NIDs

Hi Hongbo,

On 2025/4/29 21:42, Hongbo Li wrote:
> In erofs, the inode number has the location information of
> files. The default encode_fh uses the ino32, this will lack
> of some information when the file is too big. So we need
> the internal helpers to encode filehandle.

EROFS uses NID to indicate the on-disk inode offset, which can
exceed 32 bits. However, the default encode_fh uses the ino32,
thus it doesn't work if the image is large than 128GiB.

Let's introduce our own helpers to encode file handles.

> 
> It is easy to reproduce test:

It's easy to reproduce:

>    1. prepare an erofs image with nid bigger than UINT_MAX

      1. Prepare an EROFS image with NIDs larger than U32_MAX

>    2. mount -t erofs foo.img /mnt/erofs
>    3. set exportfs with configuration: /mnt/erofs *(rw,sync,
>       no_root_squash)
>    4. mount -t nfs $IP:/mnt/erofs /mnt/nfs
>    5. md5sum /mnt/nfs/foo # foo is the file which nid bigger
>       than UINT_MAX.
> For overlayfs case, the under filesystem's file handle is
> encoded in ovl_fb.fid, it is same as NFS's case.
> 
> Fixes: 3e917cc305c6 ("erofs: make filesystem exportable")
> Signed-off-by: Hongbo Li <lihongbo22@...wei.com>
> ---
> v2: https://lore.kernel.org/all/20250429074109.689075-1-lihongbo22@huawei.com/
>    - Assign parent nid with correct value.
> 
> v1: https://lore.kernel.org/all/20250429011139.686847-1-lihongbo22@huawei.com/
>     - Encode generation into file handle and minor clean code.
>     - Update the commiti's title.
> ---
>   fs/erofs/super.c | 54 +++++++++++++++++++++++++++++++++++++++++-------
>   1 file changed, 46 insertions(+), 8 deletions(-)
> 
> diff --git a/fs/erofs/super.c b/fs/erofs/super.c
> index cadec6b1b554..28b3701165cc 100644
> --- a/fs/erofs/super.c
> +++ b/fs/erofs/super.c
> @@ -511,24 +511,62 @@ static int erofs_fc_parse_param(struct fs_context *fc,
>   	return 0;
>   }
>   
> -static struct inode *erofs_nfs_get_inode(struct super_block *sb,
> -					 u64 ino, u32 generation)
> +static int erofs_encode_fh(struct inode *inode, u32 *fh, int *max_len,
> +			   struct inode *parent)
>   {
> -	return erofs_iget(sb, ino);
> +	int len = parent ? 6 : 3;
> +	erofs_nid_t nid;

Just `erofs_nid_t nid = EROFS_I(inode)->nid;`?

I think the compiler will optimize out `if (*max_len < len)`.

> +	u32 generation;

It seems it's unnecessary to introduce `generation` variable here?

> +
> +	if (*max_len < len) {
> +		*max_len = len;
> +		return FILEID_INVALID;
> +	}
> +
> +	nid = EROFS_I(inode)->nid;
> +	generation = inode->i_generation;

So drop these two lines.

> +	fh[0] = (u32)(nid >> 32);
> +	fh[1] = (u32)(nid & 0xffffffff);
> +	fh[2] = generation;
> +
> +	if (parent) {
> +		nid = EROFS_I(parent)->nid;
> +		generation = parent->i_generation;
> +
> +		fh[3] = (u32)(nid >> 32);
> +		fh[4] = (u32)(nid & 0xffffffff);
> +		fh[5] = generation;

		fh[5] = parent->i_generation;

> +	}
> +
> +	*max_len = len;
> +	return parent ? FILEID_INO64_GEN_PARENT : FILEID_INO64_GEN;
>   }
>   
>   static struct dentry *erofs_fh_to_dentry(struct super_block *sb,
>   		struct fid *fid, int fh_len, int fh_type)
>   {
> -	return generic_fh_to_dentry(sb, fid, fh_len, fh_type,
> -				    erofs_nfs_get_inode);
> +	erofs_nid_t nid;
> +
> +	if ((fh_type != FILEID_INO64_GEN &&
> +	     fh_type != FILEID_INO64_GEN_PARENT) || fh_len < 3)
> +		return NULL;
> +
> +	nid = (u64) fid->raw[0] << 32;
> +	nid |= (u64) fid->raw[1];

Unnecessary nid variable.

> +	return d_obtain_alias(erofs_iget(sb, nid));

	return d_obtain_alias(erofs_iget(sb,
			((u64)fid->raw[0] << 32) | fid->raw[1]));

>   }
>   
>   static struct dentry *erofs_fh_to_parent(struct super_block *sb,
>   		struct fid *fid, int fh_len, int fh_type)
>   {
> -	return generic_fh_to_parent(sb, fid, fh_len, fh_type,
> -				    erofs_nfs_get_inode);
> +	erofs_nid_t nid;
> +
> +	if (fh_type != FILEID_INO64_GEN_PARENT || fh_len < 6)
> +		return NULL;
> +
> +	nid = (u64) fid->raw[3] << 32;
> +	nid |= (u64) fid->raw[4];

Same here.

Thanks,
Gao Xiang