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Message-ID: <2d54ca59-9c22-0b75-3087-3718b30b8d11@farpost.com>
Date: Fri, 2 Aug 2019 11:40:02 +1000
From: Sergei Turchanov <turchanov@...post.com>
To: NeilBrown <neilb@...e.com>
Cc: linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
Alexander Viro <viro@...iv.linux.org.uk>
Subject: Re: [BUG] lseek on /proc/meminfo is broken in 4.19.59 maybe due to
commit 1f4aace60b0e ("fs/seq_file.c: simplify seq_file iteration code and
interface")
Hello!
Yes, your patch fixed this bug.
Thank you very much!
With best regards,
Sergei.
On 01.08.2019 19:14, NeilBrown wrote:
> On Thu, Aug 01 2019, Sergei Turchanov wrote:
>
>> Hello!
>>
>> [
>> As suggested in previous discussion this behavior may be caused by your
>> commit 1f4aace60b0e ("fs/seq_file.c: simplify seq_file iteration code and interface")
>> ]
> Yes.... I think I can see what happened.
> removing:
> - if (!m->count) {
> - m->from = 0;
> - m->index++;
> - }
>
> from seq_read meant that ->index didn't get updated in a case that it
> needs to be.
>
> Please confirm that the following patch fixes the problem.
> I think it is correct, but I need to look it over more carefully in the
> morning, and see if I can explain why it is correct.
>
> Thanks for the report.
> NeilBrown
>
> diff --git a/fs/seq_file.c b/fs/seq_file.c
> index 04f09689cd6d..1600034a929b 100644
> --- a/fs/seq_file.c
> +++ b/fs/seq_file.c
> @@ -119,6 +119,7 @@ static int traverse(struct seq_file *m, loff_t offset)
> }
> if (seq_has_overflowed(m))
> goto Eoverflow;
> + p = m->op->next(m, p, &m->index);
> if (pos + m->count > offset) {
> m->from = offset - pos;
> m->count -= m->from;
> @@ -126,7 +127,6 @@ static int traverse(struct seq_file *m, loff_t offset)
> }
> pos += m->count;
> m->count = 0;
> - p = m->op->next(m, p, &m->index);
> if (pos == offset)
> break;
> }
>
>
>> Original bug report:
>>
>> Seeking (to an offset within file size) in /proc/meminfo is broken in 4.19.59. It does seek to a desired position, but reading from that position returns the remainder of file and then a whole copy of file. This doesn't happen with /proc/vmstat or /proc/self/maps for example.
>>
>> Seeking did work correctly in kernel 4.14.47. So it seems something broke in the way.
>>
>> Background: this kind of access pattern (seeking to /proc/meminfo) is used by libvirt-lxc fuse driver for virtualized view of /proc/meminfo. So that /proc/meminfo is broken in guests when running kernel 4.19.x.
>>
>> > On 01.08.2019 17:11, Gao Xiang wrote:
>>> Hi,
>>>
>>> I just took a glance, maybe due to
>>> commit 1f4aace60b0e ("fs/seq_file.c: simplify seq_file iteration code and interface")
>>>
>>> I simply reverted it just now and it seems fine... but I haven't digged into this commit.
>>>
>>> Maybe you could Cc NeilBrown <neilb@...e.com> for some more advice and
>>> I have no idea whether it's an expected behavior or not...
>>>
>>> Thanks,
>>> Gao Xiang
>>>
>>> On 2019/8/1 14:16, Sergei Turchanov wrote:
>>
>> $ ./test /proc/meminfo 0 # Works as expected
>>
>> MemTotal: 394907728 kB
>> MemFree: 173738328 kB
>> ...
>> DirectMap2M: 13062144 kB
>> DirectMap1G: 390070272 kB
>>
>> -----------------------------------------------------------------------
>>
>> $ ./test /proc/meminfo 1024 # returns a copy of file after the remainder
>>
>> Will seek to 1024
>>
>>
>> Data read at offset 1024
>> gePages: 0 kB
>> ShmemHugePages: 0 kB
>> ShmemPmdMapped: 0 kB
>> HugePages_Total: 0
>> HugePages_Free: 0
>> HugePages_Rsvd: 0
>> HugePages_Surp: 0
>> Hugepagesize: 2048 kB
>> Hugetlb: 0 kB
>> DirectMap4k: 245204 kB
>> DirectMap2M: 13062144 kB
>> DirectMap1G: 390070272 kB
>> MemTotal: 394907728 kB
>> MemFree: 173738328 kB
>> MemAvailable: 379989680 kB
>> Buffers: 355812 kB
>> Cached: 207216224 kB
>> ...
>> DirectMap2M: 13062144 kB
>> DirectMap1G: 390070272 kB
>>
>> As you see, after "DirectMap1G:" line, a whole copy of /proc/meminfo returned by "read".
>>
>> Test program:
>>
>> #include <sys/types.h>
>> #include <sys/stat.h>
>> #include <unistd.h>
>> #include <fcntl.h>
>> #include <stdio.h>
>> #include <stdlib.h>
>>
>> #define SIZE 1024
>> char buf[SIZE + 1];
>>
>> int main(int argc, char *argv[]) {
>> int fd;
>> ssize_t rd;
>> off_t ofs = 0;
>>
>> if (argc < 2) {
>> printf("Usage: test <file> [<offset>]\n");
>> exit(1);
>> }
>>
>> if (-1 == (fd = open(argv[1], O_RDONLY))) {
>> perror("open failed");
>> exit(1);
>> }
>>
>> if (argc > 2) {
>> ofs = atol(argv[2]);
>> }
>> printf("Will seek to %ld\n", ofs);
>>
>> if (-1 == (lseek(fd, ofs, SEEK_SET))) {
>> perror("lseek failed");
>> exit(1);
>> }
>>
>> for (;; ofs += rd) {
>> printf("\n\nData read at offset %ld\n", ofs);
>> if (-1 == (rd = read(fd, buf, SIZE))) {
>> perror("read failed");
>> exit(1);
>> }
>> buf[rd] = '\0';
>> printf(buf);
>> if (rd < SIZE) {
>> break;
>> }
>> }
>>
>> return 0;
>> }
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