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Editing a file inside a live VM

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I am currently trying to determine whether it is possible to edit a file inside a running KVM (Proxmox; KVM) that is mounted with a CephFS, using lvm and ext4.

My situation is as follows:

  1. I have physical access to the Proxmox host, but restricted access to the guest VM (non-root user via SSH).
  2. The guest is using rbd/cephfs for its drive, and the FS uses an LVM member that has the same volume group name as the host; i.e. manually mounting it requires changing its name temporarily. It uses ext4.
  3. Rebooting the system to init=/bin/bash is a last resort; the goal here is to not shut down the system at all, but edit a file on the guest OS from the host.

The edited file needs to then be seen from the guest.

I have mounted the CephFS using rbd and am able to view the filesystem, but any changes to the fs will corrupt the fs on the guest requiring fsck to be run on it (I tried already).

One option I have considered it using virsh domfsfreeze on the VM, guestfish to edit the file, and then virsh domfsthaw to thaw the fs. Would this work? What is the risk of it breaking the fs?

Could there be any other way to do this?


Unreplied Posts

Representing $G=text{GL}^+(2,mathbf R)$ as the matrix product $G=TH$. If $H=text{SO}(2)$, what is $T$?


In this paper (Equation 2.6 and 2.7) the author seems to suggest that one can represent the $text{GL}^+(4,mathbf R)$ group using the product of two exponentials: $exp (epsilon cdot T) exp (u cdot J)$, where $T$ are the generators of shears and dilation, and $J$ are the generators of Lorentz transformations.

My take on the subject is that since $T$ and $J$ do not commute, one cannot write $G$ as a product of these two exponentials. One must instead write $G=exp ( epsilon cdot T + u cdot J )$. It appears to me the author is wrong.

Is the author correct, or am I?

How can I represent $text{GL}^+(2,mathbf R)$ as the matrix product $G=TH$ where $H=text{SO}(2)$?


Bounds on the maximum real root of a polynomial with coefficients $-1,0,1$


Suppose I have a polynomial that is given a form
f(x)=x^n – a_{n-1}x^{n-1} – ldots – a_1x – 1

where each $a_k$ can be either $0,1$.

I’ve tried a bunch of examples and found that the maximum real root seems to be between $1,2$, but as for specifics of a polynomial of this structure I am not aware.

Using IVT, we can see pretty simply that $f(1)leq0$ and $f(2)> 0$ so there has to be a root on this interval, but thats a pretty wide range was wondering if this was previously studied


Autoequivalences of $operatorname{Coh}(X)$


Let $X$ be a smooth projective variety over an algebraically closed field $k$ of characteristic zero.

Is there a description of $operatorname{Aut}(operatorname{Coh}(X))$, i.e. the autoequivalences of the category $operatorname{Coh}(X)$?

Clearly it contains $operatorname{Aut}(X)ltimesoperatorname{Pic}(X)$ as a subgroup.


Simple proof for a congruence relation connecting the $p$-adic order of a positive integer and a sum of binomial coefficients


Let $n$ be a positive integer and $p$ be a prime. Let $v_p(n)$ be the $p$-adic order of $n$, i.e., the exponent of the highest power of $p$ that divides $n$. I would like to know if there is a quick and simple proof for the following congruence relation.
sum_{j=1}^{lfloor log_{p} n rfloor} {n-1 choose p^j-1} equiv v_p(n) ;mbox{(mod }pmbox{)}.

Key ideas involved in a ‘not so simple proof’ can be found in

Best wishes.


Continuity of the fractional Laplacian operator


Considering $(-Delta)^s: H^s(Omega) to L^2(Omega)$, is it possible to show that this operator is closed (continuous)?
For instance, taking a sequence $(u_n) subset H^s(Omega)$ with $u_n to u$ in $H^s(Omega)$, we need to show that $(-Delta)^su_n to (-Delta)^su$ in $L^2(Omega)$.


$$lim_{n to +infty}int_Omega |(-Delta^s)u_n(x)|^2 dx = lim_{n to +infty} int_Omega Big(int_Omega frac{u_n(x) – u_n(y)}{|x – y|^{N + 2s}} dyBig)^2 dx.$$

So my idea would be to use some inequality (perhaps something similar to the Poincaré inequality) to obtain the norm of $u_n$ in $H^s(Omega)$ on the right side. But, I don’t know how to get rid of this one on the right side of the expression above.


How to make associated ribbon surface?


I am studying ribbon graphs on surfaces. I am reading this paper (page 5).

But I have confused about how to make surfaces using the ribbon graphs. Actually, we call this surfaces as associated ribbon surfaces.

The definition of ribbon graph is given in definition 1.5 of the above mentioned paper.

Consider the following ribbon graph

enter image description here

To get its associated ribbon surface, first we do its edge refinement by adding each edge a degree two vertex. Then consider the half edges incident to each vertices of the graph.

enter image description here

Then replace the half edges with thin strips with the orientation on boundaries (this orientation of the boundaries follows the orientation of each vertex), which is as follows:

enter image description here

My question is how we connect the strips corresponding to the two half edges following the orientation of their boundaries to form ribbons?

Please help.

enter image description here


Characterization of the Poisson distribution


Let $Z$ be a $[0, infty)$-valued random variable satisfying $lambda E[g(Z+1)] = E[Zg(Z)]$ for all indicator functions $g$ of Borel subsets of $[0, infty)$.

Prove that $mathcal{L}(Z) = Poisson(lambda)$.

Hint: Consider $1_{(n,n+1)}$ for $n in mathbb{N}_0$.


Let $n in mathbb{N}_0$ and g = 1_{(n,n+1)}, then holds:
lambda E[g(Z+1)] = lambda P[Z+1 in (n,n+1)] = lambda (P[Z+1 = n+1] – P[Z+1 = n]) = lambda P[Z = n] – lambda P[Z = n-1]).

But what is
E[Zg(Z)] = E[Z cdot 1_{(n,n+1)}(Z)] = ?

Have I got everything right so far? If so, how should I proceed now?

Thanks for the help!