While examining Linux shared libraries, you might notice something interesting: most .so files in /usr/lib have execute permissions (+x) set, yet they appear to function fine with just read permissions (+r). This leads to the natural question - why enforce execute permissions on libraries that aren't directly executed?

# Typical permission set for shared libraries
-rwxr-xr-x 1 root root 1.2M Jan 15  libssl.so.1.1

The answer lies in how the Linux kernel and dynamic linker interact with ELF (Executable and Linkable Format) files:

1. Memory Mapping: When a program loads a shared library, the system uses mmap() with PROT_EXEC flag to load executable segments into memory
2. Security Constraints: The kernel enforces that files being mapped as executable must have filesystem execute permissions
3. Dynamic Linking Process: ld.so performs additional verification during runtime linking

While some basic operations might work with just read permissions, several critical functions will fail:

# Example of what breaks without +x
$ gcc -o test test.c -L. -lmylibrary
/usr/bin/ld: cannot open ./libmylibrary.so: Permission denied

The dynamic linker (ld.so) specifically checks execute permissions when:

• Loading dependencies during program startup
• Performing dlopen() operations
• Handling lazy binding

Here's a simple demonstration of the permission check in action:

#include <dlfcn.h>
#include <stdio.h>

int main() {
    void* handle = dlopen("./libsample.so", RTLD_LAZY);
    if (!handle) {
        fprintf(stderr, "%s\n", dlerror());
        return 1;
    }
    dlclose(handle);
    return 0;
}

With incorrect permissions, you'll see errors like:

./libsample.so: failed to map segment from shared object: Operation not permitted

The execute permission requirement serves multiple security purposes:

• Prevents accidental execution of data files
• Forms part of the overall Linux security model (DAC)
• Works in conjunction with SELinux/AppArmor policies
• Helps prevent certain privilege escalation vectors

For system administrators and package maintainers:

# Correct way to set permissions during installation
install -m 0755 libmylibrary.so /usr/local/lib/

Common pitfalls to avoid:

• Using cp without preserving permissions
• Extracting archives with umask 077
• NFS mounts with root_squash and restrictive permissions