Re: [PATCH 1/2] staging: erofs: add requirements field in superblock

[Chao Yu <yuchao0@huawei.com>]

On 2019/6/10 17:36, Gao Xiang wrote:
> There are some backward incompatible optimizations pending
> for months, mainly due to on-disk format expensions.
> 
> However, we should ensure that it cannot be mounted with
> old kernels. Otherwise, it will causes unexpected behaviors.
> 
> Fixes: ba2b77a82022 ("staging: erofs: add super block operations")
> Cc: <stable@vger.kernel.org> # 4.19+
> Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
> ---
>  drivers/staging/erofs/erofs_fs.h | 11 +++++++++--
>  drivers/staging/erofs/super.c    |  8 ++++++++
>  2 files changed, 17 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/staging/erofs/erofs_fs.h b/drivers/staging/erofs/erofs_fs.h
> index fa52898df006..531821757845 100644
> --- a/drivers/staging/erofs/erofs_fs.h
> +++ b/drivers/staging/erofs/erofs_fs.h
> @@ -17,10 +17,16 @@
>  #define EROFS_SUPER_MAGIC_V1    0xE0F5E1E2
>  #define EROFS_SUPER_OFFSET      1024
>  
> +/*
> + * Any bits that aren't in EROFS_ALL_REQUIREMENTS should be
> + * incompatible with this kernel version.
> + */
> +#define EROFS_ALL_REQUIREMENTS  0
> +
>  struct erofs_super_block {
>  /*  0 */__le32 magic;           /* in the little endian */
>  /*  4 */__le32 checksum;        /* crc32c(super_block) */
> -/*  8 */__le32 features;
> +/*  8 */__le32 features;        /* extra features for the image */
>  /* 12 */__u8 blkszbits;         /* support block_size == PAGE_SIZE only */
>  /* 13 */__u8 reserved;
>  
> @@ -34,8 +40,9 @@ struct erofs_super_block {
>  /* 44 */__le32 xattr_blkaddr;
>  /* 48 */__u8 uuid[16];          /* 128-bit uuid for volume */
>  /* 64 */__u8 volume_name[16];   /* volume name */
> +/* 80 */__le32 requirements;    /* all mandatory minimum requirements */
>  
> -/* 80 */__u8 reserved2[48];     /* 128 bytes */
> +/* 84 */__u8 reserved2[44];     /* 128 bytes */

Xiang,

It needs to update the comment behind reserved2, it's locating at 132 bytes.

>  } __packed;
>  
>  /*
> diff --git a/drivers/staging/erofs/super.c b/drivers/staging/erofs/super.c
> index f580d4ef77a1..815e5825db59 100644
> --- a/drivers/staging/erofs/super.c
> +++ b/drivers/staging/erofs/super.c
> @@ -104,6 +104,14 @@ static int superblock_read(struct super_block *sb)
>  		goto out;
>  	}
>  
> +	/* check if the kernel meets all mandatory requirements */
> +	if (le32_to_cpu(layout->requirements) & (~EROFS_ALL_REQUIREMENTS)) {
> +		errln("too old to meet minimum requirements: %x supported: %x",

It will be better to give a suggestion to user to upgrade kernel version to
match the image with new layout, otherwise it's just a little confused about
above printed message.

Thanks,

> +		      le32_to_cpu(layout->requirements),
> +		      EROFS_ALL_REQUIREMENTS);
> +		goto out;
> +	}
> +
>  	sbi->blocks = le32_to_cpu(layout->blocks);
>  	sbi->meta_blkaddr = le32_to_cpu(layout->meta_blkaddr);
>  #ifdef CONFIG_EROFS_FS_XATTR
>