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Message-ID: <SG2PR06MB116546DC62F55230D246DC0D8A820@SG2PR06MB1165.apcprd06.prod.outlook.com>
Date: Wed, 16 Aug 2017 18:28:29 +0000
From: Chris Brandt <Chris.Brandt@...esas.com>
To: Nicolas Pitre <nicolas.pitre@...aro.org>,
Alexander Viro <viro@...iv.linux.org.uk>
CC: "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
"linux-embedded@...r.kernel.org" <linux-embedded@...r.kernel.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH v2 3/5] cramfs: implement uncompressed and arbitrary data
block positioning
On Wednesday, August 16, 2017, Nicolas Pitre wrote:
> Two new capabilities are introduced here:
>
> - The ability to store some blocks uncompressed.
>
> - The ability to locate blocks anywhere.
>
> Those capabilities can be used independently, but the combination
> opens the possibility for execute-in-place (XIP) of program text segments
> that must remain uncompressed, and in the MMU case, must have a specific
> alignment. It is even possible to still have the writable data segments
> from the same file compressed as they have to be copied into RAM anyway.
>
> This is achieved by giving special meanings to some unused block pointer
> bits while remaining compatible with legacy cramfs images.
>
> Signed-off-by: Nicolas Pitre <nico@...aro.org>
> ---
> fs/cramfs/README | 31 ++++++++++++++-
> fs/cramfs/inode.c | 87 +++++++++++++++++++++++++++++++++----
> -----
> include/uapi/linux/cramfs_fs.h | 20 +++++++++-
> 3 files changed, 118 insertions(+), 20 deletions(-)
>
> diff --git a/fs/cramfs/README b/fs/cramfs/README
> index 9d4e7ea311..d71b27e0ff 100644
> --- a/fs/cramfs/README
> +++ b/fs/cramfs/README
> @@ -49,17 +49,46 @@ same as the start of the (i+1)'th <block> if there is
> one). The first
> <block> immediately follows the last <block_pointer> for the file.
> <block_pointer>s are each 32 bits long.
>
> +When the CRAMFS_FLAG_EXT_BLOCK_POINTERS capability bit is set, each
> +<block_pointer>'s top bits may contain special flags as follows:
> +
> +CRAMFS_BLK_FLAG_UNCOMPRESSED (bit 31):
> + The block data is not compressed and should be copied verbatim.
> +
> +CRAMFS_BLK_FLAG_DIRECT_PTR (bit 30):
> + The <block_pointer> stores the actual block start offset and not
> + its end, shifted right by 2 bits. The block must therefore be
> + aligned to a 4-byte boundary. The block size is either blksize
> + if CRAMFS_BLK_FLAG_UNCOMPRESSED is also specified, otherwise
> + the compressed data length is included in the first 2 bytes of
> + the block data. This is used to allow discontiguous data layout
> + and specific data block alignments e.g. for XIP applications.
> +
> +
> The order of <file_data>'s is a depth-first descent of the directory
> tree, i.e. the same order as `find -size +0 \( -type f -o -type l \)
> -print'.
>
>
> <block>: The i'th <block> is the output of zlib's compress function
> -applied to the i'th blksize-sized chunk of the input data.
> +applied to the i'th blksize-sized chunk of the input data if the
> +corresponding CRAMFS_BLK_FLAG_UNCOMPRESSED <block_ptr> bit is not set,
> +otherwise it is the input data directly.
> (For the last <block> of the file, the input may of course be smaller.)
> Each <block> may be a different size. (See <block_pointer> above.)
> +
> <block>s are merely byte-aligned, not generally u32-aligned.
>
> +When CRAMFS_BLK_FLAG_DIRECT_PTR is specified then the corresponding
> +<block> may be located anywhere and not necessarily contiguous with
> +the previous/next blocks. In that case it is minimally u32-aligned.
> +If CRAMFS_BLK_FLAG_UNCOMPRESSED is also specified then the size is always
> +blksize except for the last block which is limited by the file length.
> +If CRAMFS_BLK_FLAG_DIRECT_PTR is set and CRAMFS_BLK_FLAG_UNCOMPRESSED
> +is not set then the first 2 bytes of the block contains the size of the
> +remaining block data as this cannot be determined from the placement of
> +logically adjacent blocks.
> +
>
> Holes
> -----
> diff --git a/fs/cramfs/inode.c b/fs/cramfs/inode.c
> index 393eb27ef4..b825ae162c 100644
> --- a/fs/cramfs/inode.c
> +++ b/fs/cramfs/inode.c
> @@ -636,33 +636,84 @@ static int cramfs_readpage(struct file *file, struct
> page *page)
> if (page->index < maxblock) {
> struct super_block *sb = inode->i_sb;
> u32 blkptr_offset = OFFSET(inode) + page->index*4;
> - u32 start_offset, compr_len;
> + u32 block_ptr, block_start, block_len;
> + bool uncompressed, direct;
>
> - start_offset = OFFSET(inode) + maxblock*4;
> mutex_lock(&read_mutex);
> - if (page->index)
> - start_offset = *(u32 *) cramfs_read(sb, blkptr_offset-4,
> - 4);
> - compr_len = (*(u32 *) cramfs_read(sb, blkptr_offset, 4) -
> - start_offset);
> - mutex_unlock(&read_mutex);
> + block_ptr = *(u32 *) cramfs_read(sb, blkptr_offset, 4);
> + uncompressed = (block_ptr & CRAMFS_BLK_FLAG_UNCOMPRESSED);
> + direct = (block_ptr & CRAMFS_BLK_FLAG_DIRECT_PTR);
> + block_ptr &= ~CRAMFS_BLK_FLAGS;
> +
> + if (direct) {
> + /*
> + * The block pointer is an absolute start pointer,
> + * shifted by 2 bits. The size is included in the
> + * first 2 bytes of the data block when compressed,
> + * or PAGE_SIZE otherwise.
> + */
> + block_start = block_ptr << 2;
> + if (uncompressed) {
> + block_len = PAGE_SIZE;
> + /* if last block: cap to file length */
> + if (page->index == maxblock - 1)
> + block_len = offset_in_page(inode->i_size);
> + } else {
> + block_len = *(u16 *)
> + cramfs_read(sb, block_start, 2);
> + block_start += 2;
> + }
> + } else {
> + /*
> + * The block pointer indicates one past the end of
> + * the current block (start of next block). If this
> + * is the first block then it starts where the block
> + * pointer table ends, otherwise its start comes
> + * from the previous block's pointer.
> + */
> + block_start = OFFSET(inode) + maxblock*4;
> + if (page->index)
> + block_start = *(u32 *)
> + cramfs_read(sb, blkptr_offset-4, 4);
> + /* Beware... previous ptr might be a direct ptr */
> + if (unlikely(block_start & CRAMFS_BLK_FLAG_DIRECT_PTR))
> {
> + /* See comments on earlier code. */
> + u32 prev_start = block_start;
> + block_start = prev_start & ~CRAMFS_BLK_FLAGS;
> + block_start <<= 2;
> + if (prev_start & CRAMFS_BLK_FLAG_UNCOMPRESSED) {
> + block_start += PAGE_SIZE;
> + } else {
> + block_len = *(u16 *)
> + cramfs_read(sb, block_start, 2);
> + block_start += 2 + block_len;
> + }
> + }
> + block_start &= ~CRAMFS_BLK_FLAGS;
> + block_len = block_ptr - block_start;
> + }
>
> - if (compr_len == 0)
> + if (block_len == 0)
> ; /* hole */
> - else if (unlikely(compr_len > (PAGE_SIZE << 1))) {
> - pr_err("bad compressed blocksize %u\n",
> - compr_len);
> + else if (unlikely(block_len > 2*PAGE_SIZE ||
> + (uncompressed && block_len > PAGE_SIZE))) {
> + mutex_unlock(&read_mutex);
> + pr_err("bad data blocksize %u\n", block_len);
> goto err;
> + } else if (uncompressed) {
> + memcpy(pgdata,
> + cramfs_read(sb, block_start, block_len),
> + block_len);
> + bytes_filled = block_len;
> } else {
> - mutex_lock(&read_mutex);
> bytes_filled = cramfs_uncompress_block(pgdata,
> PAGE_SIZE,
> - cramfs_read(sb, start_offset, compr_len),
> - compr_len);
> - mutex_unlock(&read_mutex);
> - if (unlikely(bytes_filled < 0))
> - goto err;
> + cramfs_read(sb, block_start, block_len),
> + block_len);
> }
> + mutex_unlock(&read_mutex);
> + if (unlikely(bytes_filled < 0))
> + goto err;
> }
>
> memset(pgdata + bytes_filled, 0, PAGE_SIZE - bytes_filled);
> diff --git a/include/uapi/linux/cramfs_fs.h
> b/include/uapi/linux/cramfs_fs.h
> index e4611a9b92..ed250aa372 100644
> --- a/include/uapi/linux/cramfs_fs.h
> +++ b/include/uapi/linux/cramfs_fs.h
> @@ -73,6 +73,7 @@ struct cramfs_super {
> #define CRAMFS_FLAG_HOLES 0x00000100 /* support for holes */
> #define CRAMFS_FLAG_WRONG_SIGNATURE 0x00000200 /* reserved */
> #define CRAMFS_FLAG_SHIFTED_ROOT_OFFSET 0x00000400 /* shifted root fs
> */
> +#define CRAMFS_FLAG_EXT_BLOCK_POINTERS 0x00000800 /* block pointer
> extensions */
>
> /*
> * Valid values in super.flags. Currently we refuse to mount
> @@ -82,7 +83,24 @@ struct cramfs_super {
> #define CRAMFS_SUPPORTED_FLAGS ( 0x000000ff \
> | CRAMFS_FLAG_HOLES \
> | CRAMFS_FLAG_WRONG_SIGNATURE \
> - | CRAMFS_FLAG_SHIFTED_ROOT_OFFSET )
> + | CRAMFS_FLAG_SHIFTED_ROOT_OFFSET \
> + | CRAMFS_FLAG_EXT_BLOCK_POINTERS )
>
> +/*
> + * Block pointer flags
> + *
> + * The maximum block offset that needs to be represented is roughly:
> + *
trailing whitespace
-Chris
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