Re: [PATCH] erofs: code for verifying superblock checksum of an erofs image.

[Gao Xiang]

Hi Pratik,

Some comments as below...

On Mon, Oct 21, 2019 at 12:58:28AM +0530, Pratik Shinde wrote:
> Patch for kernel side changes of checksum feature.I used kernel's
> crc32c library for calculating the checksum.
> 
> Signed-off-by: Pratik Shinde <pratikshinde320@xxxxxxxxx>
> ---
>  fs/erofs/erofs_fs.h |  5 +++--
>  fs/erofs/internal.h |  2 +-
>  fs/erofs/super.c    | 50 ++++++++++++++++++++++++++++++++++++++++++++++----
>  3 files changed, 50 insertions(+), 7 deletions(-)
> 
> diff --git a/fs/erofs/erofs_fs.h b/fs/erofs/erofs_fs.h
> index b1ee565..bab5506 100644
> --- a/fs/erofs/erofs_fs.h
> +++ b/fs/erofs/erofs_fs.h
> @@ -17,6 +17,7 @@
>   */
>  #define EROFS_FEATURE_INCOMPAT_LZ4_0PADDING	0x00000001
>  #define EROFS_ALL_FEATURE_INCOMPAT		EROFS_FEATURE_INCOMPAT_LZ4_0PADDING
> +#define EROFS_FEATURE_SB_CHKSUM 0x0001

How about keeping in sync with erofs-utils?

>  
>  /* 128-byte erofs on-disk super block */
>  struct erofs_super_block {
> @@ -37,8 +38,8 @@ struct erofs_super_block {
>  	__u8 uuid[16];          /* 128-bit uuid for volume */
>  	__u8 volume_name[16];   /* volume name */
>  	__le32 feature_incompat;
> -
> -	__u8 reserved2[44];
> +	__le32 chksum_blocks;	/* number of blocks used for checksum */
> +	__u8 reserved2[40];
>  };
>  
>  /*
> diff --git a/fs/erofs/internal.h b/fs/erofs/internal.h
> index 544a453..cd3af45 100644
> --- a/fs/erofs/internal.h
> +++ b/fs/erofs/internal.h
> @@ -86,7 +86,7 @@ struct erofs_sb_info {
>  	u8 uuid[16];                    /* 128-bit uuid for volume */
>  	u8 volume_name[16];             /* volume name */
>  	u32 feature_incompat;
> -
> +	u32 features;

no use? and 
how about keeping in sync with erofs-utils, naming feature_compat as well? see,

https://lore.kernel.org/r/20191020141102.GA30399@hsiangkao-HP-ZHAN-66-Pro-G1/

>  	unsigned int mount_opt;
>  };
>  
> diff --git a/fs/erofs/super.c b/fs/erofs/super.c
> index 0e36949..94e1d6a 100644
> --- a/fs/erofs/super.c
> +++ b/fs/erofs/super.c
> @@ -9,6 +9,7 @@
>  #include <linux/statfs.h>
>  #include <linux/parser.h>
>  #include <linux/seq_file.h>
> +#include <linux/crc32c.h>
>  #include "xattr.h"
>  
>  #define CREATE_TRACE_POINTS
> @@ -46,6 +47,45 @@ void _erofs_info(struct super_block *sb, const char *function,
>  	va_end(args);
>  }
>  
> +static int erofs_validate_sb_chksum(struct erofs_super_block *dsb,
> +				       struct super_block *sb)

erofs_validate_sb_chksum(struct super_block *sb, void *sbdata)

Can we pass void *sbdata --- first page(block) instead?

> +{
> +	u32 disk_chksum = le32_to_cpu(dsb->checksum);
> +	u32 nblocks = le32_to_cpu(dsb->chksum_blocks);
> +	u32 crc;
> +	struct erofs_super_block *dsb2;
> +	char *buf;
> +	unsigned int off = 0;
> +	void *kaddr;
> +	struct page *page;
> +	int i, ret = -EINVAL;
> +
> +	buf = kmalloc(nblocks * EROFS_BLKSIZ, GFP_KERNEL);

IMO, I think we could just
 1) malloc EROFS_BLKSIZ;
 2) copy sbdata -> buf;
 3) dsb->checksum = 0;
 4) crc32c the first copied block;
 5) kfree.

or some better approach without allocating memory.

> +	if (!buf)
> +		goto out;

how about erroring out with proper return value?


> +	for (i = 0; i < nblocks; i++) {
> +		page = erofs_get_meta_page(sb, i);
> +		if (IS_ERR(page))

ditto.

> +			goto out;
> +		kaddr = kmap_atomic(page);
> +		(void) memcpy(buf + off, kaddr, EROFS_BLKSIZ);

we can call crc32c for multiple times, and no need to
get and calc 0-th block again...

and no memcpy at all.

> +		kunmap_atomic(kaddr);
> +		unlock_page(page);
> +		/* first page will be released by erofs_read_superblock */
> +		if (i != 0)
> +			put_page(page);
> +		off += EROFS_BLKSIZ;
> +	}
> +	dsb2 = (struct erofs_super_block *)(buf + EROFS_SUPER_OFFSET);
> +	dsb2->checksum = 0;
> +	crc = crc32c(0, buf, nblocks * EROFS_BLKSIZ);
> +	if (crc != disk_chksum)
> +		goto out;
> +	ret = 0;
> +out:	kfree(buf);
> +	return ret;
> +}
> +
>  static void erofs_inode_init_once(void *ptr)
>  {
>  	struct erofs_inode *vi = ptr;
> @@ -109,18 +149,20 @@ static int erofs_read_superblock(struct super_block *sb)
>  		erofs_err(sb, "cannot read erofs superblock");
>  		return PTR_ERR(page);
>  	}
> -
>  	sbi = EROFS_SB(sb);
> -
>  	data = kmap_atomic(page);

should use kmap() then, because kmalloc() and get_meta_page() can sleep and
last for long time.

>  	dsb = (struct erofs_super_block *)(data + EROFS_SUPER_OFFSET);
> -
>  	ret = -EINVAL;
>  	if (le32_to_cpu(dsb->magic) != EROFS_SUPER_MAGIC_V1) {
>  		erofs_err(sb, "cannot find valid erofs superblock");
>  		goto out;
>  	}
> -
> +	if (dsb->feature_compat & EROFS_FEATURE_SB_CHKSUM) {
> +		if (erofs_validate_sb_chksum(dsb, sb)) {

		    erofs_validate_sb_chksum(sb, data)

> +			erofs_err(sb, "super block checksum incorrect");

It should be with a proper return value and corresponding messages,
see the related discussion:
https://lore.kernel.org/r/20190822090541.GA193349@architecture4/

Thanks,
Gao Xiang

> +			goto out;
> +		}
> +	}
>  	blkszbits = dsb->blkszbits;
>  	/* 9(512 bytes) + LOG_SECTORS_PER_BLOCK == LOG_BLOCK_SIZE */
>  	if (blkszbits != LOG_BLOCK_SIZE) {
> -- 
> 2.9.3
>