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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More on open later, when talking about the temporary files problem. The dir directory must be “suitable” the man page describes the right meaning of “suitable”.

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AmazonGlobal Ship Orders Internationally. It’s an stdio library routine, not a system call. This is the same problem as the one found in databases when various users are allowed to simultaneously query or change the content of a file. Nevertheless, using the temporary storage directory may cause a few problems. Signaux, chrustophe, threads, IPC et sockets. In a directory belonging to us, we create a new link to a system file.

We write a few letters into a file and lock it using the previous program:. East Dane Designer Men’s Fashion. By the way, it’s obvious that both ” cat ” commands, while working on the same filename, display two completely different contents, nevertheless no change happened to these files between the two operations.

An attacker could create a symlink to the name provided by the C library. We did succeed in exploiting a race condition in a Set-UID root utility. It can have different values: The fcntl first argument is the descriptor of the file to lock. When a process wants to write into a file, it asks the kernel to lock that file – or a part of it.


Nevertheless, we do keep that scheme, even if it isn’t that realistic, since it allows to understand the problem while easily “exploiting” the security hole. The file is then deleted, but Linux really removes it when no resources at all use it, that is when the file descriptor is released, using a close system call.

In the previous situation, the change did not concern the file itself. The first one comes from BSD and relies on the flock system call. Thus, the program becomes:. Don’t we forget, the program being Set-UID rootit is allowed to modify any file in the machine. Next, it really opens the file and writes the message.

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He is a Linux fan and does much of his work on this system. 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 kernel ensures to keep the association to the file content during the lapse of time between the open system call providing a file descriptor and the release of this descriptor using close or when the process ends.

Let’s change the line 30 previously chriwtophe and insert:. Very often a program needs to temporarily store data in an external file. Let’s mention that Gnome recommends its use in this way: The Sticky-Bit represented by the letter t at the end or the octal mode, has a particular meaning when applied to a directory: Blsess the same way, a process asks for locking before reading the content of a file, what ensures no chdistophe will be done as far as the lock is kept.


How get a temporary file? Amazon Drive Cloud storage from Amazon.

The method allowing to benefit from a security hole based on race condition is therefore boring and repetitive, but it really is usable! These ‘X’ are replaced to get an unique filename. This is obtained with the command:. Unfortunately, a badly written program is able to replace a file content, even if another process, with good behavior, has a lock for writing.

By the way, the ‘ x ‘ Gnu extension for the opening modes of the fopen function, requires an exclusive file creation, failing if the file already exists:. Let’s write something like:. As a matter of fact, the system is more clever than that: 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.