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

[Gao Xiang]

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.

     - Expected result:
         - cur = 0
     - Actual result:
         - cur = 0x370

Signed-off-by: Chunhai Guo <guochun...@vivo.com>
---
  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