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Message-ID: <fd738d38-17de-4b61-e4e8-d4f98ef8d1db@linux.alibaba.com>
Date: Mon, 10 Jul 2023 11:37:06 +0800
From: Gao Xiang <hsiangkao@...ux.alibaba.com>
To: Chunhai Guo <guochunhai@...o.com>,
"xiang@...nel.org" <xiang@...nel.org>,
"chao@...nel.org" <chao@...nel.org>
Cc: "huyue2@...lpad.com" <huyue2@...lpad.com>,
"jefflexu@...ux.alibaba.com" <jefflexu@...ux.alibaba.com>,
"linux-erofs@...ts.ozlabs.org" <linux-erofs@...ts.ozlabs.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] erofs: fix two loop issues when read page beyond EOF
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@...o.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
>
> 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
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