Hébergeur web | EPFL Graph Search

Un hébergeur web est une entreprise qui fournit l'hébergement sur Internet de systèmes informatiques divers, tels que sites web, stockage d'information, messagerie électronique, etc. à des personnes, associations, projets ou autres entités qui ne désirent pas le faire par leurs propres moyens. Cet hébergement se fait en général au sein d'un parc de serveurs informatiques qui bénéficie 24 heures sur 24 d'une connexion à Internet de haut débit pour offrir un accès rapide aux clients ou à tous les internautes selon le type de service rendu. Activité La principale activité de l'hébergeur web consiste à installer ses serveurs, à les sécuriser (par une alimentation électrique ondulée, secourue par un groupe électrogène, une salle climatisée équipée de dispositifs anti-incendie), à les tenir à jour en installant les mises à jour de sécurité pour éviter les attaques malveillantes, à les réparer en cas de panne, à y installer les technologies logicielles souhaitées par les clie

Improving Scalability of Symbolic Execution for Software with Complex Environment Interfaces

Stefan Bucur

Manual software testing is laborious and prone to human error. Yet, among practitioners, it is the most popular method for quality assurance. Automating the test case generation promises better effectiveness, especially for exposing corner-case bugs. Symbolic execution stands out as an automated testing technique that has no false positives, it eventually enumerates all feasible program executions, and can prioritize executions of interest. However, path explosionâthe fact that the number of program executions is typically at least exponential in the size of the programâhinders the applicability of symbolic execution in the real world, where software commonly reaches millions of lines of code. In practice, large systems can be efficiently executed symbolically by exploiting their modularity and thus symbolically execute the different parts of the system separately. However, a component typically depends on its environment to perform its task. Thus, a symbolic execution engine needs to provide an environment interface that is efficient, while maintaining accuracy and completeness. This conundrum is known as the environment problem. Systematically addressing the environment problem is challenging, as its instantiation depends on the nature of the environment and its interface. This thesis addresses two instances of the environment problem in symbolic execution, which are at opposite ends of the spectrum of interface stability: (1) system software interacting with an operating system with stable and well-documented semantics (e.g., POSIX), and (2) high-level programs written in dynamic languages, such as Python, Ruby, or JavaScript, whose semantics and interfaces are continuously evolving. To address the environment problem for stable operating system interfaces, this thesis introduces the idea of splitting an operating system model into a core set of primitives built into the engine at host level and, on top of it, the full operating system interface emulated inside the guest. As few as two primitives are sufficient to support a complex interface such as POSIX: threads with synchronization and address spaces with shared memory. We prototyped this idea in the Cloud9 symbolic execution platform. Cloud9's accurate and efficient POSIX model exposes hard-to-reproduce bugs in systems such as UNIX utilities, web servers, and distributed systems. Cloud9 is available at http://cloud9.epfl.ch. For programs written in high-level interpreted languages, this thesis introduces the idea of using the language interpreter as an "executable language specification". The interpreter runs inside a low-level (e.g., x86) symbolic execution engine while it executes the target program. The aggregate system acts as a high-level symbolic execution engine for the program. To manage the complexity of symbolically executing the entire interpreter, this thesis introduces Class-Uniform Path Analysis (CUPA), an algorithm for prioritizing paths that groups paths into equivalence classes according to a coverage goal. We built a prototype of these ideas in the form of Chef, a symbolic execution platform for interpreted languages that generates up to 1000 times more tests in popular Python and Lua packages compared to a plain execution of the interpreters. Chef is available at http://dslab.epfl.ch/proj/chef/.

EPFL2015

Space lower bounds for low-stretch greedy embeddings

Christos Kalaitzis

Greedy (geometric) routing is an important paradigm for routing in communication networks. It uses an embedding of the nodes of a network into points of a space (e.g., R-d) equipped with a distance function (e.g., the Euclidean distance l(2)) and uses as address of each node the coordinates of the corresponding point. The embedding has particular properties so that the routing decision for a packet is taken greedily based only on the addresses of the current node, its neighbors, and the destination node and the distances of the corresponding points. In this way, there is no need to keep extensive routing information (e.g., routing tables) at the nodes. Embeddings that allow for this functionality are called greedy embeddings. Even though greedy embeddings do exist for several spaces and distance functions, they usually result in paths of high stretch, i.e., significantly longer than the corresponding shortest paths. In this paper, we show that greedy embeddings in low-dimensional (Euclidean) spaces necessarily have high stretch. In particular, greedy embeddings of n-node graphs with optimal stretch require at least Omega(n) dimensions for distance l(2). This result disproves a conjecture by Maymounkov (2006) [14] stating that greedy embeddings of optimal stretch are possible in Euclidean spaces with polylogarithmic number of dimensions. Furthermore, we present trade-offs between the stretch and the number of dimensions of the host space. Our results imply that every greedy embedding into a space with polylogarithmic number of dimensions (and Euclidean distance) has stretch Omega(logn/log log n). We extend this result for a distance function used by an o(log n)-stretch greedy embedding presented by Flury et al. (2009) [9]. Our lower bound implies that their embedding has almost best possible stretch. (C) 2015 Elsevier B.V. All rights reserved.

Elsevier Science Bv2016

Host shift and cospeciation rate estimation from co-phylogenies

Séverine Vuilleumier Varisco

Host shifts can cause novel infectious diseases, and is a key process in diversification. Disentangling the effects of host shift vs. those of cospeciation is non-trivial as both can result in phylogenic congruence. We develop a new framework based on network analysis and Approximate Bayesian Computation to quantify host shift and cospeciation rates in host-parasite systems. Our method enables estimation of the expected time to the next host shift or cospeciation event. We then apply it to avian haemosporidian parasite systems and to the pocket gophers-chewing lice system, and demonstrate that both host shift and cospeciation can be reliably estimated by our method. We confirm that host shifts have shaped the evolutionary history of avian haemosporidian parasites and have played a minor role in the gopher-chewing lice system. Our method is promising for predicting the rate of potential host shifts and thus the emergence of novel infectious diseases.

Wiley2017

Improving Scalability of Symbolic Execution for Software with Complex Environment Interfaces

Stefan Bucur

EPFL2015