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")

[Sergei Turchanov]

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@xxxxxxxx> 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 0ÂÂÂÂÂÂÂ # Works as expected
> >
> > MemTotal:ÂÂÂÂÂÂ 394907728 kB
> > MemFree:ÂÂÂÂÂÂÂ 173738328 kB
> > ...
> > DirectMap2M:ÂÂÂ 13062144 kB
> > DirectMap1G:ÂÂÂ 390070272 kB
> >
> > -----------------------------------------------------------------------
> >
> > $ ./test /proc/meminfo 1024ÂÂÂÂ # 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;
> > }