From: Tvrtko Ursulin <tvrtko.ursulin@sophos.com>
Organization: Sophos Plc
To: Eric Paris <eparis@redhat.com>, Al Viro <viro@zeniv.linux.org.uk>
Subject: Polling(2) /proc/mounts is broken?
Date: Fri, 10 Dec 2010 15:32:48 +0000
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Hi all,

So since we cannot get fanotify global mode we were told to poll(2) on
/proc/mounts and set up new mount marks dynamically.

Is the attached C program roughly how it should work?

Because on 2.6.37-rc5 it busy loops for me (POLLIN always reported), even
though if I remove the seek it gets EOF immediately after POLLIN.

Under 2.6.32 (Ubuntu flavour) I see the same behaviour.

Going further back to 2.6.24 (again Ubuntu flavour) and there it works, but=
 it
set POLLERR instead of POLLIN in revents.

So I guess it regressed somewhere in between. I'll look to provide a fix
unless someone more familiar with this area beats me to it.

Tvrtko

Sophos Limited, The Pentagon, Abingdon Science Park, Abingdon, OX14 3YP, Un=
ited Kingdom.
Company Reg No 2096520. VAT Reg No GB 991 2418 08.

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#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <errno.h>
#include <poll.h>
#include <unistd.h>
#include <stdio.h>

int main()
{
    int fd;
    struct pollfd ev;
    int ret;
    ssize_t bytes;
    char buf[4096];
    long unsigned int cnt = 0;


    fd = open("/proc/mounts", O_RDONLY);

    do {
        ev.events = POLLIN;
        ev.fd = fd;
        ev.revents = 0;

        ret = poll(&ev, 1, -1);
        lseek(fd, 0, SEEK_SET);
        if (ret == 1 && ev.revents == POLLIN) {
            printf("####  %lu ########################################\n", cnt);
            while ((bytes = read(fd, buf, sizeof(buf))) > 0)
                write(1, buf, bytes);
            cnt++;
        }
    } while (ret >= 0);

    return 0;
}

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