[RFC PATCH 2/3] erofs-utils: lib: support on-disk offset for shifted decompression

[Yue Hu]

Add support to uncompressed data layout with on-disk offset for
compressed files.

Signed-off-by: Yue Hu <huy...@coolpad.com>
---
 include/erofs/decompress.h |  3 +++
 lib/data.c                 |  8 +++++++-
 lib/decompress.c           | 10 ++++++++--
 3 files changed, 18 insertions(+), 3 deletions(-)

diff --git a/include/erofs/decompress.h b/include/erofs/decompress.h
index 82bf7b8..b622df5 100644
--- a/include/erofs/decompress.h
+++ b/include/erofs/decompress.h
@@ -23,6 +23,9 @@ struct z_erofs_decompress_req {
        unsigned int decodedskip;
        unsigned int inputsize, decodedlength;
 
+       /* head offset of uncompressed data */
+       unsigned int shiftedhead;
+
        /* indicate the algorithm will be used for decompression */
        unsigned int alg;
        bool partial_decoding;
diff --git a/lib/data.c b/lib/data.c
index c2ecbc9..5e44db9 100644
--- a/lib/data.c
+++ b/lib/data.c
@@ -226,7 +226,7 @@ static int z_erofs_read_data(struct erofs_inode *inode, 
char *buffer,
        };
        struct erofs_map_dev mdev;
        bool partial;
-       unsigned int bufsize = 0;
+       unsigned int bufsize = 0, head;
        char *raw = NULL;
        int ret = 0;
 
@@ -313,10 +313,16 @@ static int z_erofs_read_data(struct erofs_inode *inode, 
char *buffer,
                if (ret < 0)
                        break;
 
+               head = 0;
+               if (erofs_sb_has_fragments() &&
+                   map.m_algorithmformat == Z_EROFS_COMPRESSION_SHIFTED)
+                       head = erofs_blkoff(end);
+
                ret = z_erofs_decompress(&(struct z_erofs_decompress_req) {
                                        .in = raw,
                                        .out = buffer + end - offset,
                                        .decodedskip = skip,
+                                       .shiftedhead = head,
                                        .inputsize = map.m_plen,
                                        .decodedlength = length,
                                        .alg = map.m_algorithmformat,
diff --git a/lib/decompress.c b/lib/decompress.c
index 1661f91..08a0861 100644
--- a/lib/decompress.c
+++ b/lib/decompress.c
@@ -132,14 +132,20 @@ out:
 int z_erofs_decompress(struct z_erofs_decompress_req *rq)
 {
        if (rq->alg == Z_EROFS_COMPRESSION_SHIFTED) {
+               unsigned int count, rightpart;
+
                if (rq->inputsize > EROFS_BLKSIZ)
                        return -EFSCORRUPTED;
 
                DBG_BUGON(rq->decodedlength > EROFS_BLKSIZ);
                DBG_BUGON(rq->decodedlength < rq->decodedskip);
 
-               memcpy(rq->out, rq->in + rq->decodedskip,
-                      rq->decodedlength - rq->decodedskip);
+               count = rq->decodedlength - rq->decodedskip;
+               rightpart = min(EROFS_BLKSIZ - rq->shiftedhead, count);
+
+               memcpy(rq->out, rq->in + (erofs_sb_has_fragments() ?
+                      rq->shiftedhead : rq->decodedskip), rightpart);
+               memcpy(rq->out + rightpart, rq->in, count - rightpart);
                return 0;
        }
 
-- 
2.17.1