From 1fff8f3a1326bd82f8140a61d969994e635834fe Mon Sep 17 00:00:00 2001
From: David Oberhollenzer <david.oberhollenzer@sigma-star.at>
Date: Mon, 5 Aug 2019 23:55:04 +0200
Subject: Add pread(2) like function to data_reader

Signed-off-by: David Oberhollenzer <david.oberhollenzer@sigma-star.at>
---
 include/data_reader.h  | 10 +++++++
 lib/sqfs/data_reader.c | 81 ++++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 91 insertions(+)

diff --git a/include/data_reader.h b/include/data_reader.h
index c6d85f1..51bfff3 100644
--- a/include/data_reader.h
+++ b/include/data_reader.h
@@ -41,4 +41,14 @@ void data_reader_destroy(data_reader_t *data);
 int data_reader_dump_file(data_reader_t *data, file_info_t *fi, int outfd,
 			  bool allow_sparse);
 
+/*
+  Read a chunk of data from a file. Starting from 'offset' into the
+  uncompressed file, read 'size' bytes into 'buffer'.
+
+  Returns the number of bytes read, 0 if EOF, -1 on failure. Prints an
+  error message to stderr on failure.
+ */
+ssize_t data_reader_read(data_reader_t *data, file_info_t *fi,
+			 uint64_t offset, void *buffer, size_t size);
+
 #endif /* DATA_READER_H */
diff --git a/lib/sqfs/data_reader.c b/lib/sqfs/data_reader.c
index e33ffbb..27a8302 100644
--- a/lib/sqfs/data_reader.c
+++ b/lib/sqfs/data_reader.c
@@ -225,3 +225,84 @@ fail_sparse:
 	perror("creating sparse output file");
 	return -1;
 }
+
+ssize_t data_reader_read(data_reader_t *data, file_info_t *fi,
+			 uint64_t offset, void *buffer, size_t size)
+{
+	size_t i, diff, fragsz, count, total = 0;
+	off_t off;
+	char *ptr;
+
+	/* work out block count and fragment size */
+	fragsz = fi->size % data->block_size;
+	count = fi->size / data->block_size;
+
+	if (fragsz != 0 && !(fi->flags & FILE_FLAG_HAS_FRAGMENT)) {
+		fragsz = 0;
+		++count;
+	}
+
+	/* work out block index and on-disk location */
+	off = fi->startblock;
+	i = 0;
+
+	while (offset > data->block_size && i < count) {
+		off += SQFS_ON_DISK_BLOCK_SIZE(fi->blocks[i++].size);
+		offset -= data->block_size;
+	}
+
+	/* copy data from blocks */
+	while (i < count && size > 0) {
+		diff = data->block_size - offset;
+		if (size < diff)
+			size = diff;
+
+		if (SQFS_IS_SPARSE_BLOCK(fi->blocks[i].size)) {
+			memset(buffer, 0, diff);
+		} else {
+			if (precache_data_block(data, off, fi->blocks[i].size))
+				return -1;
+
+			memcpy(buffer, (char *)data->block + offset, diff);
+			off += SQFS_ON_DISK_BLOCK_SIZE(fi->blocks[i].size);
+		}
+
+		++i;
+		offset = 0;
+		size -= diff;
+		total += diff;
+		buffer = (char *)buffer + diff;
+	}
+
+	/* copy from fragment */
+	if (i == count && size > 0 && fragsz > 0) {
+		if (precache_fragment_block(data, fi->fragment))
+			return -1;
+
+		if (fi->fragment_offset >= data->frag_used)
+			goto fail_range;
+
+		if ((fi->fragment_offset + fragsz - 1) >= data->frag_used)
+			goto fail_range;
+
+		ptr = (char *)data->frag_block + fi->fragment_offset;
+		ptr += offset;
+
+		if (offset >= fragsz) {
+			offset = 0;
+			size = 0;
+		}
+
+		if (offset + size > fragsz)
+			size = fragsz - offset;
+
+		if (size > 0) {
+			memcpy(buffer, ptr + offset, size);
+			total += size;
+		}
+	}
+	return total;
+fail_range:
+	fputs("attempted to read past fragment block limits\n", stderr);
+	return -1;
+}
-- 
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