Re: [PATCH U-BOOT v2 3/3] fs: btrfs: Fix LZO false decompression error caused by pending zero

[Marek Behun <marek.behun@nic.cz>]

On Thu, 26 Mar 2020 13:35:56 +0800
Qu Wenruo <wqu@suse.com> wrote:

> For certain btrfs files with compressed file extent, uboot will fail to
> load it:
> 
>   btrfs_read_extent_reg: disk_bytenr=14229504 disk_len=73728 offset=0 nr_bytes=131
>   072
>   decompress_lzo: tot_len=70770
>   decompress_lzo: in_len=1389
>   decompress_lzo: in_len=2400
>   decompress_lzo: in_len=3002
>   decompress_lzo: in_len=1379
>   decompress_lzo: in_len=88539136
>   decompress_lzo: header error, in_len=88539136 clen=65534 tot_len=62580
> 
> NOTE: except the last line, all other lines are debug output.
> 
> Btrfs lzo compression uses its own format to record compressed size
> (segment header, LE32).
> 
> However to make decompression easier, we never put such segment header
> across page boundary.
> 
> In above case, the xxd dump of the lzo compressed data looks like this:
> 
> 00001fe0: 4cdc 02fc 0bfd 02c0 dc02 0d13 0100 0001  L...............
> 00001ff0: 0000 0008 0300 0000 0000 0011 0000|0000  ................
> 00002000: 4705 0000 0001 cc02 0000 0000 0000 1e01  G...............
> 
> '|' is the "expected" segment header start position.
> 
> But in that page, there are only 2 bytes left, can't contain the 4 bytes
> segment header.
> 
> So btrfs compression will skip that 2 bytes, put the segment header in
> next page directly.
> 
> Uboot doesn't have such check, and read the header with 2 bytes offset,
> result 0x05470000 (88539136), other than the expected result
> 0x00000547 (1351), resulting above error.
> 
> Follow the btrfs-progs restore implementation, by introducing tot_in to
> record total processed bytes (including headers), and do proper page
> boundary skip to fix it.
> 
> Please note that, current code base doesn't parse fs_info thus we can't
> grab sector size easily, so it uses PAGE_SIZE, and relying on fs open
> time check to exclude unsupported sector size.
> 
> Signed-off-by: Qu Wenruo <wqu@suse.com>
> Cc: Marek Behun <marek.behun@nic.cz>
> ---
>  fs/btrfs/compression.c | 20 +++++++++++++++++++-
>  1 file changed, 19 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/btrfs/compression.c b/fs/btrfs/compression.c
> index 4ef44ce11485..b1884fc15ee0 100644
> --- a/fs/btrfs/compression.c
> +++ b/fs/btrfs/compression.c
> @@ -9,6 +9,7 @@
>  #include <malloc.h>
>  #include <linux/lzo.h>
>  #include <linux/zstd.h>
> +#include <linux/compat.h>
>  #include <u-boot/zlib.h>
>  #include <asm/unaligned.h>
>  
> @@ -16,7 +17,7 @@
>  #define LZO_LEN		4
>  static u32 decompress_lzo(const u8 *cbuf, u32 clen, u8 *dbuf, u32 dlen)
>  {
> -	u32 tot_len, in_len, res;
> +	u32 tot_len, tot_in, in_len, res;
>  	size_t out_len;
>  	int ret;
>  
> @@ -24,9 +25,11 @@ static u32 decompress_lzo(const u8 *cbuf, u32 clen, u8 *dbuf, u32 dlen)
>  		return -1;
>  
>  	tot_len = le32_to_cpu(get_unaligned((u32 *)cbuf));
> +	tot_in = 0;
>  	cbuf += LZO_LEN;
>  	clen -= LZO_LEN;
>  	tot_len -= LZO_LEN;
> +	tot_in += LZO_LEN;
>  
>  	if (tot_len == 0 && dlen)
>  		return -1;
> @@ -36,6 +39,8 @@ static u32 decompress_lzo(const u8 *cbuf, u32 clen, u8 *dbuf, u32 dlen)
>  	res = 0;
>  
>  	while (tot_len > LZO_LEN) {
> +		u32 rem_page;
> +
>  		in_len = le32_to_cpu(get_unaligned((u32 *)cbuf));
>  		cbuf += LZO_LEN;
>  		clen -= LZO_LEN;
> @@ -44,6 +49,7 @@ static u32 decompress_lzo(const u8 *cbuf, u32 clen, u8 *dbuf, u32 dlen)
>  			return -1;
>  
>  		tot_len -= (LZO_LEN + in_len);
> +		tot_in += (LZO_LEN + in_len);
>  
>  		out_len = dlen;
>  		ret = lzo1x_decompress_safe(cbuf, in_len, dbuf, &out_len);
> @@ -56,6 +62,18 @@ static u32 decompress_lzo(const u8 *cbuf, u32 clen, u8 *dbuf, u32 dlen)
>  		dlen -= out_len;
>  
>  		res += out_len;
> +
> +		/*
> +		 * If the 4 bytes header does not fit to the rest of the page we
> +		 * have to move to next one, or we read some garbage.
> +		 */
> +		rem_page = PAGE_SIZE - (tot_in % PAGE_SIZE);
> +		if (rem_page < LZO_LEN) {
> +			cbuf += rem_page;
> +			tot_in += rem_page;
> +			clen -= rem_page;
> +			tot_len -= rem_page;
> +		}
>  	}
>  
>  	return res;

Reviewed-by: Marek Behún <marek.behun@nic.cz>