Libguestfs - Jun 2018 - v2v: docs: Removed vsyscall support in Debian kernels requiring workaround (RHBZ#1592061).

Debian kernels have disabled legacy vsyscall page support meaning they
cannot runs binaries that predate c.2013.  To enable it again you must
add vsyscall=emulate to the kernel command line when booting.  It's
unclear why Debian disabled this as according to the documentation:

  Disabling this option saves about 7K of kernel size and
  possibly 4K of additional runtime pagetable memory.

(presumably the "runtime pagetable memory" is per-process?)  It seems
to me like a rather poor trade-off for not being able to run binaries
which are just 5 years old.

While I was writing this patch I imagined a much more complex
automatic workaround where we would try to detect if the host kernel
had this problem and add the extra vsyscall=emulate parameter
automatically.  ‘./configure’ might carry a binary (for x86_64 only?)
built to use the vsyscall page and try to run it under the host
kernel.  If it failed it would suggest using the workaround.

This solution seems a bit complex, and so this patch simply updates
the documentation which hopefully will make the problem discoverable.

I don't know how widespread this problem is.  We've not seen it in
Fedora/RHEL kernels AFAIK.

Rich.

Thanks: Haigang Li
---
 v2v/virt-v2v.pod | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/v2v/virt-v2v.pod b/v2v/virt-v2v.pod
index 251b4919f..09f505f91 100644
--- a/v2v/virt-v2v.pod
+++ b/v2v/virt-v2v.pod
@@ -977,6 +977,25 @@ A recommended procedure is, before using virt-v2v, to check
that the
 boot kernel is the best kernel available in the guest (for example
 by making sure the guest is up-to-date).
 
+=head2 "vsyscall attempted with vsyscall=none"
+
+When run on a recent Debian host virt-v2v may fail to convert guests
+which were created before 2013.  In the debugging output you will see
+a crash message similar to:
+
+ vsyscall attempted with vsyscall=none ip:...
+ segfault at ...
+
+This is caused because Debian removed support for running old binaries
+which used the legacy vsyscall page to call into the kernel.
+
+You can work around this problem by running this command before
+running virt-v2v:
+
+ export LIBGUESTFS_APPEND="vsyscall=emulate"
+
+For more information, see L<https://bugzilla.redhat.com/1592061>
+
 =head1 WINDOWS
 
 =head2 Windows E<ge> 8 Fast Startup is incompatible with virt-v2v
-- 
2.16.2

On Sunday, 17 June 2018 15:09:51 CEST Richard W.M. Jones
wrote:
> Thanks: Haigang Li
> ---
>  v2v/virt-v2v.pod | 19 +++++++++++++++++++
>  1 file changed, 19 insertions(+)
LGTM.  I guess there is no way to detect that somehow, so libguestfs
can start the appliance with the proper parameter already ...

-- 
Pino Toscano