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

[Sergei Turchanov]

Hello!

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.

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

MemTotal:ÂÂÂÂÂÂ 394907728 kB
MemFree:ÂÂÂÂÂÂÂ 173738328 kB
...
DirectMap2M:ÂÂÂ 13062144 kB
DirectMap1G:ÂÂÂ 390070272 kB

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

$ ./test 1024ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ # returns a copy of file after the remainder

Will seek to 1024


Data read at offset 1024
gePages:ÂÂÂÂÂÂÂÂ 0 kB
ShmemHugePages:ÂÂÂÂÂÂÂ 0 kB
ShmemPmdMapped:ÂÂÂÂÂÂÂ 0 kB
HugePages_Total:ÂÂÂÂÂÂ 0
HugePages_Free:ÂÂÂÂÂÂÂ 0
HugePages_Rsvd:ÂÂÂÂÂÂÂ 0
HugePages_Surp:ÂÂÂÂÂÂÂ 0
Hugepagesize:ÂÂÂÂÂÂ 2048 kB
Hugetlb:ÂÂÂÂÂÂÂÂÂÂÂÂÂÂ 0 kB
DirectMap4k:ÂÂÂÂÂ 245204 kB
DirectMap2M:ÂÂÂ 13062144 kB
DirectMap1G:ÂÂÂ 390070272 kB
MemTotal:ÂÂÂÂÂÂ 394907728 kB
MemFree:ÂÂÂÂÂÂÂ 173738328 kB
MemAvailable:ÂÂ 379989680 kB
Buffers:ÂÂÂÂÂÂÂÂÂ 355812 kB
Cached:ÂÂÂÂÂÂÂÂ 207216224 kB
...
DirectMap2M:ÂÂÂ 13062144 kB
DirectMap1G:ÂÂÂ 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[]) {
ÂÂÂ intÂÂÂÂ fd;
ÂÂÂ ssize_t rd;
ÂÂÂ off_tÂÂ ofs = 0;

ÂÂÂ if (argc < 2) {
ÂÂÂÂÂÂÂ printf("Usage: test <file> [<offset>]\n");
ÂÂÂÂÂÂÂ exit(1);
ÂÂÂ }

ÂÂÂ if (-1 == (fd = open(argv[1], O_RDONLY))) {
ÂÂÂÂÂÂÂ perror("open failed");
ÂÂÂÂÂÂÂ exit(1);
ÂÂÂ }

ÂÂÂ if (argc > 2) {
ÂÂÂÂÂÂÂ ofs = atol(argv[2]);
ÂÂÂ }
ÂÂÂ printf("Will seek to %ld\n", ofs);

ÂÂÂ if (-1 == (lseek(fd, ofs, SEEK_SET))) {
ÂÂÂÂÂÂÂ perror("lseek failed");
ÂÂÂÂÂÂÂ exit(1);
ÂÂÂ }

ÂÂÂ for (;; ofs += rd) {
ÂÂÂÂÂÂÂ printf("\n\nData read at offset %ld\n", ofs);
ÂÂÂÂÂÂÂ if (-1 == (rd = read(fd, buf, SIZE))) {
ÂÂÂÂÂÂÂÂÂÂÂ perror("read failed");
ÂÂÂÂÂÂÂÂÂÂÂ exit(1);
ÂÂÂÂÂÂÂ }
ÂÂÂÂÂÂÂ buf[rd] = '\0';
ÂÂÂÂÂÂÂ printf(buf);
ÂÂÂÂÂÂÂ if (rd < SIZE) {
ÂÂÂÂÂÂÂÂÂÂÂ break;
ÂÂÂÂÂÂÂ }
ÂÂÂ }

ÂÂÂ return 0;
}