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

From: Gao Xiang
Date: Sun Jul 09 2023 - 23:37:18 EST




On 2023/7/10 11:32, Chunhai Guo wrote:
Hi Xiang,

On 2023/7/8 17:00, Gao Xiang wrote:
Hi Chunhai,

On 2023/7/8 14:24, Chunhai Guo wrote:
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)

Thanks for the catch!

Could you split these two into two patches?

how about using:
cur = end - min_t(erofs_off_t, offend + end - map->m_la, end)
for this?

since cur and end are all [0, PAGE_SIZE - 1] for now, and
folio_size() later.

OK. I will split the patch.

Sorry that I can not understand what is 'offend' refer to and what do you mean. Could you please describe it more clearly?

Sorry, there is a typo here, I meant 'offset'.

`cur` and `end` both are not exceed 4096 if your page_size
is 4096.

Does
cur = end - min_t(erofs_off_t, offset + end - map->m_la, end)

fix your issue?


      - 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;
+    }
Do we really need to optimize this rare case?
I guess the follow readmore fix is enough, thoughts? >
Thanks,
Gao Xiang

Since the code is constantly being updated and someone may encounter this bug again, I think we had better fix it if possible.

z_erofs_do_read_page() already covers this case so I'm
not sure why we need to add another logic here.

Thanks,
Gao Xiang


Thanks.
Guo Chunhai