Re: [PATCH 2/6] composefs: Add on-disk layout

[Brian Masney]

On Mon, Nov 28, 2022 at 12:16:23PM +0100, Alexander Larsson wrote:
> This commit adds the on-disk layout header file of composefs.
> 
> Signed-off-by: Alexander Larsson <alexl@xxxxxxxxxx>
> Signed-off-by: Giuseppe Scrivano <gscrivan@xxxxxxxxxx>

Add Co-Developed-By: Giuseppe ... ?

Full disclosure: I'm not a file system developer but I'll attempt to
help with the review of this series.

> ---
>  fs/composefs/cfs.h | 242 +++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 242 insertions(+)
>  create mode 100644 fs/composefs/cfs.h
> 
> diff --git a/fs/composefs/cfs.h b/fs/composefs/cfs.h
> new file mode 100644
> index 000000000000..8f001fd28d6b
> --- /dev/null
> +++ b/fs/composefs/cfs.h
> @@ -0,0 +1,242 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +/*
> + * composefs
> + *
> + * Copyright (C) 2021 Giuseppe Scrivano
> + * Copyright (C) 2022 Alexander Larsson
> + *
> + * This file is released under the GPL.
> + */
> +
> +#ifndef _CFS_H
> +#define _CFS_H
> +
> +#include <asm/byteorder.h>
> +#include <crypto/sha2.h>
> +#include <linux/fs.h>
> +#include <linux/stat.h>
> +#include <linux/types.h>
> +
> +#define CFS_VERSION 1
> +
> +#define CFS_MAGIC 0xc078629aU
> +
> +#define CFS_MAX_DIR_CHUNK_SIZE 4096
> +#define CFS_MAX_XATTRS_SIZE 4096
> +
> +static inline u16 cfs_u16_to_file(u16 val)
> +{
> +	return cpu_to_le16(val);
> +}
> +
> +static inline u32 cfs_u32_to_file(u32 val)
> +{
> +	return cpu_to_le32(val);
> +}
> +
> +static inline u64 cfs_u64_to_file(u64 val)
> +{
> +	return cpu_to_le64(val);
> +}
> +
> +static inline u16 cfs_u16_from_file(u16 val)
> +{
> +	return le16_to_cpu(val);
> +}
> +
> +static inline u32 cfs_u32_from_file(u32 val)
> +{
> +	return le32_to_cpu(val);
> +}
> +
> +static inline u64 cfs_u64_from_file(u64 val)
> +{
> +	return le64_to_cpu(val);
> +}

I don't see where the cfs_xxx_{to,from}_file() approach is used in other
filesystems. Instead, move the cpu() functions directly into the code.

> +static inline int cfs_xdigit_value(char c)
> +{
> +	if (c >= '0' && c <= '9')
> +		return c - '0';
> +	if (c >= 'A' && c <= 'F')
> +		return c - 'A' + 10;
> +	if (c >= 'a' && c <= 'f')
> +		return c - 'a' + 10;
> +	return -1;
> +}

There's some utilities in lib/hexdump.c that you can use. hex_to_bin()
will convert a single character and hex2bin() will convert a string for
you.

> +static inline int cfs_digest_from_payload(const char *payload,
> +					  size_t payload_len,
> +					  u8 digest_out[SHA256_DIGEST_SIZE])
> +{
> +	const char *p, *end;
> +	u8 last_digit = 0;
> +	int digit = 0;
> +	size_t n_nibbles = 0;

Put in reverse Christmas tree order.

> +
> +	end = payload + payload_len;
> +	for (p = payload; p != end; p++) {
> +		/* Skip subdir structure */
> +		if (*p == '/')
> +			continue;
> +
> +		/* Break at (and ignore) extension */
> +		if (*p == '.')
> +			break;

A comment would be helpful in this area that shows what the payload is
expected to be.

> +
> +		if (n_nibbles == SHA256_DIGEST_SIZE * 2)
> +			return -1; /* Too long */

return -EINVAL; ?

> +
> +		digit = cfs_xdigit_value(*p);
> +		if (digit == -1)
> +			return -1; /* Not hex digit */

-EINVAL here as well

> +
> +		n_nibbles++;
> +		if ((n_nibbles % 2) == 0) {
> +			digest_out[n_nibbles / 2 - 1] =
> +				(last_digit << 4) | digit;
> +		}
> +		last_digit = digit;
> +	}
> +
> +	if (n_nibbles != SHA256_DIGEST_SIZE * 2)
> +		return -1; /* Too short */

-EINVAL here as well

> +
> +	return 0;
> +}
> +
> +struct cfs_vdata_s {
> +	u64 off;
> +	u32 len;
> +} __packed;
> +
> +struct cfs_header_s {
> +	u8 version;
> +	u8 unused1;
> +	u16 unused2;
> +
> +	u32 magic;

Should the magic number appear first?

> +	u64 data_offset;
> +	u64 root_inode;
> +
> +	u64 unused3[2];
> +} __packed;
> +
> +enum cfs_inode_flags {
> +	CFS_INODE_FLAGS_NONE = 0,
> +	CFS_INODE_FLAGS_PAYLOAD = 1 << 0,
> +	CFS_INODE_FLAGS_MODE = 1 << 1,
> +	CFS_INODE_FLAGS_NLINK = 1 << 2,
> +	CFS_INODE_FLAGS_UIDGID = 1 << 3,
> +	CFS_INODE_FLAGS_RDEV = 1 << 4,
> +	CFS_INODE_FLAGS_TIMES = 1 << 5,
> +	CFS_INODE_FLAGS_TIMES_NSEC = 1 << 6,
> +	CFS_INODE_FLAGS_LOW_SIZE = 1 << 7, /* Low 32bit of st_size */
> +	CFS_INODE_FLAGS_HIGH_SIZE = 1 << 8, /* High 32bit of st_size */
> +	CFS_INODE_FLAGS_XATTRS = 1 << 9,
> +	CFS_INODE_FLAGS_DIGEST = 1
> +				 << 10, /* fs-verity sha256 digest of content */

Include << 10 on line above

Brian