1© David Morgan 2008How users access resourcesHow users access resources(answer: processes)(answer: processes)David Morgan© David Morgan 2008controlling resource accesscontrolling resource access a major foundation of security to understand it, must understand its elements– users– processes– permissions2© David Morgan 2008user access to resourcesuser access to resources typical resource: a file controlling access is a central aspect of local security differentiated access given to different users but... users themselves don’t actually access resources huh? sure they do! So, if users don’t what does?© David Morgan 2008#include <unistd.h>#include <sys/stat.h>#include <fcntl.h>int main(){char c; int in, out;in = open("file.in", O_RDONLY); out = open("file.out", O_WRONLY|O_CREAT, S_IRUSR|S_IWUSR);while(read(in,&c,1) == 1)write(out,&c,1);exit(0);}processes access resourcesprocesses access resourcesa process is a running programa process (not a user)is the “actor/agent” that actually accesses resourcesvia system callssee?note system calls “open” “read” “write”They do the file accessuser? isn’t even mentioned in the calls3© David Morgan 2008Processes are userProcesses are user--associatedassociated some particular user account identity is always embedded in each process operating systems provide, enforce, and track this user-to-process hard-wire the process operates on behalf of the user users access resources, but only indirectly through processes that “belong” to them© David Morgan 2008How we think of itHow we think of ituser filereads4© David Morgan 2008How it actually worksHow it actually worksuser filereadsprocessruns© David Morgan 2008Who can access what?Who can access what? determined by permissions permissions belong to resources (e.g. files) which have different ones for different users processes run as those users are accordingly permitted (or denied) access therby achieving differentiated per-user access5© David Morgan 2008Separate discussionsSeparate discussions users and groups processes
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