Re: [Nbd] [w@...112...: Re: nbd-client started at boot (for root device) is not persistent]

[Victor <vixtor@...17...>]

I have found an idea that seems to work fine - at least for me: if the process cannot find /dev and /sys anymore, it means it was started from initramfs. So i recreate /sys and /dev and mount sysfs and devtmpfs again. The VFS tree of nbd-client will get a duplicate mount of /dev and /sys but i don't think there's any harm in that. What do you think (see attached patch).

On Fri, Nov 25, 2016 at 10:06 AM, Wouter Verhelst <w@...112...> wrote:

Only after I'd sent this did I notice the Cc was gone...

----- Forwarded message from Wouter Verhelst <w@...112...> -----

Date: Fri, 25 Nov 2016 09:04:32 +0100
From: Wouter Verhelst <w@...112...>
To: Victor <vixtor@...17...>
Subject: Re: [Nbd] nbd-client started at boot (for root device) is not persistent
Message-ID: <20161125080432.vgzj7dll3tinydph@...3...>
In-Reply-To: <CAMOh2p+2SFjck=z2C5ynyT0SseKPPJQ7z+RGb2SbocFP65vn-Q@...2782......gmail.com>
Organization: none
User-Agent: NeoMutt/20161104 (1.7.1)

On Fri, Nov 25, 2016 at 09:00:20AM +0200, Victor wrote:
> I am wondering - is there any way for a process to re-read the VFS after /dev
> and /sys were moved to /root/dev and /root/sys and then root was moved with
> MS_MOVE to /, and finally a chroot was performed and init started?
>
> If not, what would you think about the following: create a new run flag for
> nbd-client (let's call it --replace). If this command line flag is used,
> nbd-client first kills an existing nbd-client providing the same resource then
> starts normally.

Can't do that, because then the shutdown protection that systemd provides is
gone (that's only valid for processes which have been started from initramfs).

--
< ron> I mean, the main *practical* problem with C++, is there's like a dozen
       people in the world who think they really understand all of its rules,
       and pretty much all of them are just lying to themselves too.
 -- #debian-devel, OFTC, 2016-02-12

----- End forwarded message -----

--
< ron> I mean, the main *practical* problem with C++, is there's like a dozen
       people in the world who think they really understand all of its rules,
       and pretty much all of them are just lying to themselves too.
 -- #debian-devel, OFTC, 2016-02-12

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--- nbd-3.13/nbd-client.c	2016-01-02 21:34:16.000000000 +0200
+++ nbd-3.13-modified/nbd-client.c	2016-11-25 13:22:11.884886541 +0200
@@ -43,6 +43,8 @@
 #include <stdbool.h>
 #include <time.h>
 
+#include <sys/stat.h>
+
 #include <linux/ioctl.h>
 #define MY_NAME "nbd_client"
 #include "cliserv.h"
@@ -800,8 +802,32 @@
 						sleep (1);
 					}
 					nbd = open(nbddev, O_RDWR);
-					if (nbd < 0)
-						err("Cannot open NBD: %m");
+					if (nbd < 0) {
+						fprintf(stderr, "Maybe we were started from initramfs, trying to fix missing paths\n");
+						struct stat sb;
+						if (stat("/dev", &sb) == 0 && S_ISDIR(sb.st_mode))
+							err("/dev is still there, initramfs workaround will not help\n");
+						else {
+							fprintf(stderr, "/dev is not there anymore, we were in initramfs\n");
+							if(mkdir("/dev", 0755) < 0) 
+								err("Cannot re-create /dev");
+							if(mount("udev", "/dev", "devtmpfs", MS_NOSUID|MS_RELATIME, "size=3772884k,nr_inodes=943221,mode=755") < 0)
+								err("Failed to mount udev");
+						}
+						if (stat("/sys", &sb) == 0 && S_ISDIR(sb.st_mode))
+							err("/sys is still there, initramfs workaround will not help\n");
+						else {
+							fprintf(stderr, "/sys is not there anymore, we were in initramfs\n");
+							if(mkdir("/sys", 0555) < 0)
+								err("Cannot re-create /sys");
+							if(mount("sysfs", "/sys", "sysfs", MS_NOSUID|MS_NODEV|MS_NOEXEC|MS_RELATIME, "") < 0)
+								err("Failed to mount udev");
+						}
+
+						nbd = open(nbddev, O_RDWR);
+						if (nbd < 0)
+							err("Cannot open NBD: %m");
+					}
 					negotiate(sock, &new_size, &new_flags, name, needed_flags, cflags, opts);
 					if (size64 != new_size) {
 						err("Size of the device changed. Bye");