[PATCH] erofs: fix two loop issues when read page beyond EOF

[Chunhai Guo]

When z_erofs_read_folio() reads a page with an offset far beyond EOF, two
issues may occur:
- z_erofs_pcluster_readmore() may take a long time to loop when the offset
  is big enough, which is unnecessary.
    - For example, it will loop 4691368 times and take about 27 seconds
      with following case.
        - offset = 19217289215
        - inode_size = 1442672
- z_erofs_do_read_page() may loop infinitely due to the inappropriate
  truncation in the below statement. Since the offset is 64 bits and
min_t() truncates the result to 32 bits. The solution is to replace
unsigned int with another 64-bit type, such as erofs_off_t.
    cur = end - min_t(unsigned int, offset + end - map->m_la, end);
    - For example:
        - offset = 0x400160000
        - end = 0x370
        - map->m_la = 0x160370
        - offset + end - map->m_la = 0x400000000
        - offset + end - map->m_la = 0x00000000 (truncated as unsigned int)
    - Expected result:
        - cur = 0
    - Actual result:
        - cur = 0x370

Signed-off-by: Chunhai Guo <guochunhai@xxxxxxxx>
---
 fs/erofs/zdata.c | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/fs/erofs/zdata.c b/fs/erofs/zdata.c
index 5f1890e309c6..6abbd4510076 100644
--- a/fs/erofs/zdata.c
+++ b/fs/erofs/zdata.c
@@ -972,7 +972,8 @@ static int z_erofs_do_read_page(struct z_erofs_decompress_frontend *fe,
 	struct erofs_map_blocks *const map = &fe->map;
 	const loff_t offset = page_offset(page);
 	bool tight = true, exclusive;
-	unsigned int cur, end, spiltted;
+	erofs_off_t cur, end;
+	unsigned int spiltted;
 	int err = 0;
 
 	/* register locked file pages as online pages in pack */
@@ -1035,7 +1036,7 @@ static int z_erofs_do_read_page(struct z_erofs_decompress_frontend *fe,
 	 */
 	tight &= (fe->mode > Z_EROFS_PCLUSTER_FOLLOWED_NOINPLACE);
 
-	cur = end - min_t(unsigned int, offset + end - map->m_la, end);
+	cur = end - min_t(erofs_off_t, offset + end - map->m_la, end);
 	if (!(map->m_flags & EROFS_MAP_MAPPED)) {
 		zero_user_segment(page, cur, end);
 		goto next_part;
@@ -1841,7 +1842,7 @@ static void z_erofs_pcluster_readmore(struct z_erofs_decompress_frontend *f,
 	}
 
 	cur = map->m_la + map->m_llen - 1;
-	while (cur >= end) {
+	while ((cur >= end) && (cur < i_size_read(inode))) {
 		pgoff_t index = cur >> PAGE_SHIFT;
 		struct page *page;
 
@@ -1876,6 +1877,12 @@ static int z_erofs_read_folio(struct file *file, struct folio *folio)
 	trace_erofs_readpage(page, false);
 	f.headoffset = (erofs_off_t)page->index << PAGE_SHIFT;
 
+	/* when trying to read beyond EOF, return zero page directly */
+	if (f.headoffset >= i_size_read(inode)) {
+		zero_user_segment(page, 0, PAGE_SIZE);
+		return 0;
+	}
+
 	z_erofs_pcluster_readmore(&f, NULL, true);
 	err = z_erofs_do_read_page(&f, page);
 	z_erofs_pcluster_readmore(&f, NULL, false);
-- 
2.25.1