[BUG] lseek on /proc/meminfo is broken in 4.19.59

[Sergei Turchanov <turchanov@farpost.com>]

Hello!

(I sent this e-mail two weeks ago with no feedback. Does anyone care? 
Wrong mailing list? Anything....?)

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.

$ ./test /proc/meminfo 0        # Works as expected

MemTotal:       394907728 kB
MemFree:        173738328 kB
...
DirectMap2M:    13062144 kB
DirectMap1G:    390070272 kB

-----------------------------------------------------------------------

$ ./test 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;
}