xz: Change io_seek_src and io_pread arguments from off_t to uint64_t.

This helps fixing warnings from -Wsign-conversion and makes the
code look better too.

src/xz/file_io.c

@@ -1170,11 +1170,19 @@ io_read(file_pair *pair, io_buf *buf_union, size_t size)
 
 
 extern bool
-io_seek_src(file_pair *pair, off_t pos)
+io_seek_src(file_pair *pair, uint64_t pos)
 {
-	assert(pos >= 0);
+	// Caller must not attempt to seek past the end of the input file
+	// (seeking to 100 in a 100-byte file is seeking to the end of
+	// the file, not past the end of the file, and thus that is allowed).
+	//
+	// This also validates that pos can be safely cast to off_t.
+	if (pos > (uint64_t)(pair->src_st.st_size))
+		message_bug();
 
-	if (lseek(pair->src_fd, pos, SEEK_SET) != pos) {
+	const off_t offset = (off_t)pos;
+
+	if (lseek(pair->src_fd, offset, SEEK_SET) != offset) {
 		message_error(_("%s: Error seeking the file: %s"),
 				pair->src_name, strerror(errno));
 		return true;
@@ -1187,7 +1195,7 @@ io_seek_src(file_pair *pair, off_t pos)
 
 
 extern bool
-io_pread(file_pair *pair, io_buf *buf, size_t size, off_t pos)
+io_pread(file_pair *pair, io_buf *buf, size_t size, uint64_t pos)
 {
 	// Using lseek() and read() is more portable than pread() and
 	// for us it is as good as real pread().

src/xz/file_io.h

@@ -139,7 +139,7 @@ extern void io_fix_src_pos(file_pair *pair, size_t rewind_size);
 ///
 /// \return     On success, false is returned. On error, error message
 ///             is printed and true is returned.
-extern bool io_seek_src(file_pair *pair, off_t pos);
+extern bool io_seek_src(file_pair *pair, uint64_t pos);
 
 
 /// \brief      Read from source file from given offset to a buffer
@@ -155,7 +155,7 @@ extern bool io_seek_src(file_pair *pair, off_t pos);
 ///
 /// \return     On success, false is returned. On error, error message
 ///             is printed and true is returned.
-extern bool io_pread(file_pair *pair, io_buf *buf, size_t size, off_t pos);
+extern bool io_pread(file_pair *pair, io_buf *buf, size_t size, uint64_t pos);
 
 
 /// \brief      Writes a buffer to the destination file

src/xz/list.c

@@ -382,12 +382,11 @@ parse_indexes(xz_file_info *xfi, file_pair *pair)
 			break;
 
 		case LZMA_SEEK_NEEDED:
-			// The cast is safe because liblzma won't ask us to
+			// liblzma won't ask us to seek past the known size
-			// seek past the known size of the input file which
+			// of the input file.
-			// did fit into off_t.
 			assert(strm.seek_pos
 					<= (uint64_t)(pair->src_st.st_size));
-			if (io_seek_src(pair, (off_t)(strm.seek_pos)))
+			if (io_seek_src(pair, strm.seek_pos))
 				goto error;
 
 			// avail_in must be zero so that we will read new
@@ -589,7 +588,7 @@ parse_check_value(file_pair *pair, const lzma_index_iter *iter)
 
 	// Locate and read the Check field.
 	const uint32_t size = lzma_check_size(iter->stream.flags->check);
-	const off_t offset = iter->block.compressed_file_offset
+	const uint64_t offset = iter->block.compressed_file_offset
 			+ iter->block.total_size - size;
 	io_buf buf;
 	if (io_pread(pair, &buf, size, offset))