[PATCH v4] erofs-utils: fuse: fix random readlink error

[Gao Xiang via Linux-erofs <linux-erofs@lists.ozlabs.org>]

From: Hu Weiwen <sehuww@mail.scut.edu.cn>

readlink should fill a **null-terminated** string in the buffer [1].

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.

[1] https://lore.kernel.org/r/20210121101233.GC6680@DESKTOP-N4CECTO.huww98.cn
Signed-off-by: Hu Weiwen <sehuww@mail.scut.edu.cn>
Reviewed-by: Li Guifu <bluce.lee@aliyun.com>
Signed-off-by: Gao Xiang <hsiangkao@aol.com>
---
changes since v3:
 - fix z_erofs_read_data() buffer range as well.

 fuse/main.c |  8 ++++++++
 lib/data.c  | 46 +++++++++++++++++++++++++---------------------
 2 files changed, 33 insertions(+), 21 deletions(-)

diff --git a/fuse/main.c b/fuse/main.c
index c16291272e75..37119ea8728d 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(%s): %s", path, buffer);
 	return 0;
 }
 
diff --git a/lib/data.c b/lib/data.c
index 3781846743aa..56de16b3c840 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;
 	}
 
@@ -80,6 +81,7 @@ static int erofs_read_raw_data(struct erofs_inode *inode, char *buffer,
 	erofs_off_t ptr = offset;
 
 	while (ptr < offset + size) {
+		char *const estart = buffer + ptr - offset;
 		erofs_off_t eend;
 
 		map.m_la = ptr;
@@ -89,29 +91,30 @@ static int erofs_read_raw_data(struct erofs_inode *inode, char *buffer,
 
 		DBG_BUGON(map.m_plen != map.m_llen);
 
+		/* trim extent */
+		eend = min(offset + size, map.m_la + map.m_llen);
+		DBG_BUGON(ptr < map.m_la);
+
 		if (!(map.m_flags & EROFS_MAP_MAPPED)) {
 			if (!map.m_llen) {
+				/* reached EOF */
+				memset(estart, 0, offset + size - ptr);
 				ptr = offset + size;
 				continue;
 			}
-			ptr = map.m_la + map.m_llen;
+			memset(estart, 0, eend - ptr);
+			ptr = eend;
 			continue;
 		}
 
-		/* trim extent */
-		eend = min(offset + size, map.m_la + map.m_llen);
-		DBG_BUGON(ptr < map.m_la);
-
 		if (ptr > map.m_la) {
 			map.m_pa += ptr - map.m_la;
 			map.m_la = ptr;
 		}
 
-		ret = dev_read(buffer + ptr - offset,
-			       map.m_pa, eend - map.m_la);
+		ret = dev_read(estart, map.m_pa, eend - map.m_la);
 		if (ret < 0)
 			return -EIO;
-
 		ptr = eend;
 	}
 	return 0;
@@ -137,19 +140,6 @@ static int z_erofs_read_data(struct erofs_inode *inode, char *buffer,
 		if (ret)
 			return ret;
 
-		if (!(map.m_flags & EROFS_MAP_MAPPED)) {
-			end = map.m_la;
-			continue;
-		}
-
-		ret = dev_read(raw, map.m_pa, EROFS_BLKSIZ);
-		if (ret < 0)
-			return -EIO;
-
-		algorithmformat = map.m_flags & EROFS_MAP_ZIPPED ?
-						Z_EROFS_COMPRESSION_LZ4 :
-						Z_EROFS_COMPRESSION_SHIFTED;
-
 		/*
 		 * trim to the needed size if the returned extent is quite
 		 * larger than requested, and set up partial flag as well.
@@ -171,6 +161,20 @@ static int z_erofs_read_data(struct erofs_inode *inode, char *buffer,
 			end = map.m_la;
 		}
 
+		if (!(map.m_flags & EROFS_MAP_MAPPED)) {
+			memset(buffer + end - offset, 0, length);
+			end = map.m_la;
+			continue;
+		}
+
+		ret = dev_read(raw, map.m_pa, EROFS_BLKSIZ);
+		if (ret < 0)
+			return -EIO;
+
+		algorithmformat = map.m_flags & EROFS_MAP_ZIPPED ?
+						Z_EROFS_COMPRESSION_LZ4 :
+						Z_EROFS_COMPRESSION_SHIFTED;
+
 		ret = z_erofs_decompress(&(struct z_erofs_decompress_req) {
 					.in = raw,
 					.out = buffer + end - offset,
-- 
2.24.0