1 – Articles du blog. Licence Creative Commons Les articles de ce blog sont fournis selon les termes de la Licence Creative Commons CC-BY-ND. 8 novembre. Shells Linux et Unix par la pratique (French Edition) Feb 06, by CHRISTOPHE BLAESS · Paperback. $$ More Buying Choices. $ (2 Used. Langages de scripts sous Linux [Christophe Blaess] on *FREE* shipping on qualifying offers.

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A program concerning the system security shouldn’t work relying on the exlusive access to a file content. Here, we bleass focus on system applications and we’ll consider that the concerned resources are filesytem nodes. The method allowing to benefit blaeds a security hole based on race condition is therefore boring and repetitive, but it really is usable!

We can see fcntl can lock limited portions of the file, but it’s able to do much more compared to flock. The second type of lock comes from System V, and relies on the fcntl system call which invocation is a bit complicated. The simplest cases don’t require filename change, neither transmission to another process, but only storage and data re-reading in a temporary area.

The directory having a full write access, each user can put his files in it, being sure they are protected – at least till the next clean up managed by the sysadmin. We then talk about cooperative lock, what shows the application liability towards data access.

cjristophe The Sticky-Bit represented by the letter t at the end or the octal mode, has a particular meaning blass applied to a directory: Of course, this program was very “helpful” waiting for 20 seconds we finish to modify the files behind its back. Langages de scripts sous Linux.


Nevertheless, using the temporary storage directory may cause a few problems. The ln command -f option forces the creation, even if that name already exists:.

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Let’s have a look at a small program asking for a lock for reading concerning files which names are given as an argument, and waiting for the user to press the Enter key before finishing and thus releasing the locks.

This file is automatically deleted chgistophe closing time.

As a matter of fact, the system is more clever than that: The tempnam function returns a pointer to a string. The third argument is a pointer to a variable of struct flock type, describing the lock. However, unlike previously discussed security holes buffer overflow, format strings Opening a temporary file, if not done christope, is often the starting point of race condition situations for an ill-intentioned user.

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This is also true for writing into a file i. Shopbop Designer Fashion Brands.

The most usual case is inserting a record in the middle of a sequential ordered file, what implies to make a copy of the original file in a temporary file, while adding new information.

In a directory belonging to us, we create a new link to a system file. Usually, the temporary file creation is done in three steps: How get a temporary file?

The dir directory must be “suitable” the man page describes the right meaning christpohe “suitable”. East Dane Designer Men’s Fashion. Very often a program needs to temporarily store data in an external file.

There’s a library function called lockf close to the system call but not so performing. To avoid a program being sensitive to this kind of situation, it’s necessary to institute an exclusive access mechanism to the file data. Scripts shell, linux et unix: It also will check that the file is not a symlink to a system file.


The kernel itself automatically manages this association when it provides us with a file descriptor. Thus, enough to take the example of a physical link to check this association is not at all a permanent one. It checks what it has to, sleeps for a while before really accessing the file. This lapse of time is often extremely short but it isn’t null, then an attacker can benefit from it to change the file’s characteristics.

Let’s change the line 30 previously empty and insert:. When a process wants to write into a file, it asks the kernel to lock that file – or a part of it. These two operations have to be done in an atomic way, without any manipulation able to take place between them.

To get next an IO flow around the descriptor we’ll use the fdopen function working like fopen while relying on a descriptor rather than on a filename. Let’s remind a few principles to avoid this sort of trouble. Most of the time, an attack aiming to system security is done against Set-UID applications, since the attacker can run the program till he can benefit from the privileges given christoohe the executable file’s owner.

The flock structure important members are the following:.