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9. Process Credentials > File-System User ID and File-System Group ID

File-System User ID and File-System Group ID

On Linux, it is the file-system user and group IDs, rather than the effective user and group IDs, that are used (in conjunction with the supplementary group IDs) to determine permissions when performing file-system operations such as opening files, changing file ownership, and modifying file permissions. (The effective IDs are still used, as on other UNIX implementations, for the other purposes described earlier.)

Normally, the file-system user and group IDs have the same values as the corresponding effective IDs (and thus typically are the same as the corresponding real IDs). Furthermore, whenever the effective user or group ID is changed, either by a system call or by execution of a set-user-ID or set-group-ID program, the corresponding file-system ID is also changed to the same value. Since the file-system IDs follow the effective IDs in this way, this means that Linux effectively behaves just like any other UNIX implementation when privileges and permissions are being checked. The file-system IDs differ from the corresponding ....setfsuid()setfsgid()


  

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