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")

[NeilBrown <neilb@suse.com>]

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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@suse.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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