Why have almost all the shared libraries in /usr/lib/ have the executable permission bit set? I don't see any use case for executing them. Some do manage to hook-up some form of main function to print a short copyright and version note, but many don't do even that and segfault upon execution.
So, what's the point of setting this x? Must all library packagers do that? What will happen if I dlopen() a shared library that has 0644 permissions?
Best Answer
Under HP-UX, shared libraries are mapped into memory using mmap(), and all memory pages in the system have protection bits which are coupled with the kernel and processor hardware's memory page protection mechanisms. In order to execute the contents of any page of memory on the system, that page must have PROT_EXEC set - a useful feature to prevent data execution exploits.
The mmap() call uses the permission bits on the file it is about to map to define the protection bits of the mapped memory pages which are to contain it: rwx -> PROT_READ|PROT_WRITE|PROT_EXEC (from sys/mman.h). so in order for a shared library to be usable on HP-UX, the file containing the shared library must have execute permissions to insure that the mapped library also has execute permission.
A shared library with mode 644 on an HP-UX system will cause core dumps.