Libguestfs - Apr 2018 - Re: [PATCH] v2v: Give better diagnostics if filesystem falls back to read-only (RHBZ#1567763).

Some filesystems fall back silently to read-only if there are problems
such a dirty filesystem and an unrecoverable journal.  Almost all
conversions involve writing to the root filesystem, so these will
inevitably fail later on with a strange error message.

Test the root filesystem is writable by creating and deleting a
temporary file, and if the creation fails then give better
diagnostics.

Reported-by: Piotr Kliczewski
---
 generator/OCaml.ml    |  1 +
 v2v/inspect_source.ml | 37 +++++++++++++++++++++++++++----------
 2 files changed, 28 insertions(+), 10 deletions(-)

diff --git a/generator/OCaml.ml b/generator/OCaml.ml
index e58f387cc..467a1f0d8 100644
--- a/generator/OCaml.ml
+++ b/generator/OCaml.ml
@@ -40,6 +40,7 @@ let ocaml_errnos = [
   "EPERM";
   "ESRCH";
   "ENOENT";
+  "EROFS";
 ]
 
 (* Generate the OCaml bindings interface. *)
diff --git a/v2v/inspect_source.ml b/v2v/inspect_source.ml
index bd382cb11..c1a7e5737 100644
--- a/v2v/inspect_source.ml
+++ b/v2v/inspect_source.ml
@@ -41,16 +41,33 @@ let rec inspect_source root_choice g    let mps = List.sort
cmp mps in
   List.iter (
     fun (mp, dev) ->
-      try g#mount dev mp
-      with G.Error msg ->
-        if mp = "/" then ( (* RHBZ#1145995 *)
-          if String.find msg "Windows" >= 0 && String.find
msg "NTFS partition is in an unsafe state" >= 0 then
-            error (f_"unable to mount the disk image for writing. This has
probably happened because Windows Hibernation or Fast Restart is being used in
this guest. You have to disable this (in the guest) in order to use
virt-v2v.\n\nOriginal error message: %s") msg
-          else
-            error "%s" msg
-        )
-        else
-          warning (f_"%s (ignored)") msg
+      (try g#mount dev mp
+       with G.Error msg ->
+         if mp = "/" then ( (* RHBZ#1145995 *)
+           if String.find msg "Windows" >= 0 &&
String.find msg "NTFS partition is in an unsafe state" >= 0 then
+             error (f_"unable to mount the disk image for writing. This
has probably happened because Windows Hibernation or Fast Restart is being used
in this guest. You have to disable this (in the guest) in order to use
virt-v2v.\n\nOriginal error message: %s") msg
+           else
+             error "%s" msg
+         )
+         else
+           warning (f_"%s (ignored)") msg
+      );
+
+      (* Some filesystems (hello, ntfs-3g) can silently fall back to
+       * a read-only mount.  Check the root filesystem is really writable.
+       * RHBZ#1567763
+       *)
+      if mp = "/" then (
+        let file = sprintf "/%s" (String.random8 ()) in
+        (try g#touch file
+         with G.Error msg ->
+           if g#last_errno () = G.Errno.errno_EROFS then
+             error (f_"filesystem was mounted read-only, even though we
asked for it to be mounted read-write.  This usually means that the filesystem
was not cleanly unmounted.  Possible causes include trying to convert a guest
which is running, or using Windows Hibernation or Fast Restart.\n\nOriginal
error message: %s") msg
+           else
+             error (f_"could not write to the guest filesystem: %s")
msg
+        );
+        g#rm file
+      )
   ) mps;
 
   (* Get list of applications/packages installed. *)
-- 
2.16.2

On Monday, 16 April 2018 11:48:08 CEST Richard W.M. Jones
wrote:
> Some filesystems fall back silently to read-only if there are problems
> such a dirty filesystem and an unrecoverable journal.  Almost all
> conversions involve writing to the root filesystem, so these will
> inevitably fail later on with a strange error message.
> 
> Test the root filesystem is writable by creating and deleting a
> temporary file, and if the creation fails then give better
> diagnostics.
Another possibility here could be to use the "norecover" mount option.

-- 
Pino Toscano

On Mon, Apr 16, 2018 at 12:28:58PM +0200, Pino Toscano
wrote:
> On Monday, 16 April 2018 11:48:08 CEST Richard W.M. Jones wrote:
> > Some filesystems fall back silently to read-only if there are problems
> > such a dirty filesystem and an unrecoverable journal.  Almost all
> > conversions involve writing to the root filesystem, so these will
> > inevitably fail later on with a strange error message.
> > 
> > Test the root filesystem is writable by creating and deleting a
> > temporary file, and if the creation fails then give better
> > diagnostics.
> 
> Another possibility here could be to use the "norecover" mount
option.
Unfortunately I don't have an actual test case covering this bug.  But
one problem with the norecover option seems to be that it's only
understood by ntfs-3g and other filesystems give an error:

$ guestfish -N fs -m /dev/sda1:/:norecover
libguestfs: error: mount_options: mount exited with status 32: mount: /sysroot:
wrong fs type, bad option, bad superblock on /dev/sda1, missing codepage or
helper program, or other error.
guestfish: ‘/dev/sda1’ could not be mounted.
guestfish: Check mount(8) man page to ensure options ‘norecover’
guestfish: are supported by the filesystem that is being mounted.
guestfish: Did you mean to mount one of these filesystems?
guestfish:     /dev/sda1 (ext2)

So that would entail some more complex logic to determine if the
filesystem is ntfs or not.

It's a tricky call.  You could equally argue that the ‘norecover’
option is important for data integrity, or that what we currently do
(‘recover’, the default) allows us to convert guests with less fuss.

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
virt-p2v converts physical machines to virtual machines.  Boot with a
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http://libguestfs.org/virt-v2v