[PATCH v2] erofs-utils: fuse: fix random readlink error

[Hu Weiwen <sehuww@mail.scut.edu.cn>]

readlink should fill a **null-terminated** string in the buffer.

To achieve this:
1) memset(0) for unmapped extents;
2) make erofsfuse_read() properly returning the actual bytes read;
3) insert a null character if the path is truncated.

Link: https://lore.kernel.org/r/20210121101233.GC6680@DESKTOP-N4CECTO.huww98.cn
Signed-off-by: Hu Weiwen <sehuww@mail.scut.edu.cn>
---
 fuse/main.c | 8 ++++++++
 lib/data.c  | 6 ++++++
 2 files changed, 14 insertions(+)

diff --git a/fuse/main.c b/fuse/main.c
index c162912..70558b0 100644
--- a/fuse/main.c
+++ b/fuse/main.c
@@ -74,6 +74,10 @@ static int erofsfuse_read(const char *path, char *buffer,
 	ret = erofs_pread(&vi, buffer, size, offset);
 	if (ret)
 		return ret;
+	if (offset + size > vi.i_size)
+		return vi.i_size - offset;
+	if (offset >= vi.i_size)
+		return 0;
 	return size;
 }
 
@@ -83,6 +87,10 @@ static int erofsfuse_readlink(const char *path, char *buffer, size_t size)
 
 	if (ret < 0)
 		return ret;
+	DBG_BUGON(ret > size);
+	if (ret == size)
+		buffer[size - 1] = '\0';
+	erofs_dbg("readlink result %s", buffer);
 	return 0;
 }
 
diff --git a/lib/data.c b/lib/data.c
index 3781846..3641536 100644
--- a/lib/data.c
+++ b/lib/data.c
@@ -29,6 +29,7 @@ static int erofs_map_blocks_flatmode(struct erofs_inode *inode,
 	if (offset >= inode->i_size) {
 		/* leave out-of-bound access unmapped */
 		map->m_flags = 0;
+		map->m_plen = 0;
 		goto out;
 	}
 
@@ -91,9 +92,13 @@ static int erofs_read_raw_data(struct erofs_inode *inode, char *buffer,
 
 		if (!(map.m_flags & EROFS_MAP_MAPPED)) {
 			if (!map.m_llen) {
+				/* reached EOF */
+				memset(buffer + ptr - offset, 0,
+				       offset + size - ptr);
 				ptr = offset + size;
 				continue;
 			}
+			memset(buffer + map.m_la - offset, 0, map.m_llen);
 			ptr = map.m_la + map.m_llen;
 			continue;
 		}
@@ -138,6 +143,7 @@ static int z_erofs_read_data(struct erofs_inode *inode, char *buffer,
 			return ret;
 
 		if (!(map.m_flags & EROFS_MAP_MAPPED)) {
+			memset(buffer + map.m_la - offset, 0, map.m_llen);
 			end = map.m_la;
 			continue;
 		}
-- 
2.25.1