When you need to check which options were compiled into a running Linux kernel but don't have access to the /boot/config-* file, there are several reliable alternatives:

Many modern kernels compress their configuration into a special file:

zcat /proc/config.gz | grep CONFIG_OPTION_NAME

This requires the kernel to be built with CONFIG_IKCONFIG_PROC=y. To check if this is available:

if [ -f /proc/config.gz ]; then
    echo "Config available in /proc/config.gz";
else
    echo "Alternative methods required";
fi

For kernels without /proc/config.gz, you can try extracting from the binary:

scripts/extract-ikconfig /boot/vmlinuz-$(uname -r)

This requires kernel source scripts. If you don't have them, install via:

sudo apt install linux-source   # Debian/Ubuntu
sudo dnf install kernel-devel   # RHEL/Fedora

Many module-related options appear in sysfs:

ls /sys/module/
grep -r CONFIG_ /sys/module/*/parameters/

Some configurations appear in boot parameters:

cat /proc/cmdline

To verify if KASLR is enabled:

if grep -q "nokaslr" /proc/cmdline; then
    echo "KASLR disabled";
elif [ -f /proc/config.gz ]; then
    zcat /proc/config.gz | grep CONFIG_RANDOMIZE_BASE;
else
    echo "Check via dmesg:";
    dmesg | grep "KASLR";
fi

Here's a bash function to check multiple options:

check_kernel_config() {
    local option=$1
    if [ -f /proc/config.gz ]; then
        zcat /proc/config.gz | grep "^${option}="
    elif [ -x "$(command -v extract-ikconfig)" ]; then
        extract-ikconfig /boot/vmlinuz-$(uname -r) | grep "^${option}="
    else
        echo "No config extraction method available" >&2
        return 1
    fi
}

# Usage:
check_kernel_config CONFIG_KVM

When working on systems where the original /boot/config-* file is unavailable, you still have several reliable methods to examine the kernel's compile-time configuration:

Modern Linux kernels often provide compressed configuration through /proc:

zcat /proc/config.gz | grep CONFIG_OPTION_NAME
# Example for checking SMP support:
zcat /proc/config.gz | grep CONFIG_SMP

Note: This requires kernel built with CONFIG_IKCONFIG_PROC=y.

For kernels with configuration embedded (CONFIG_IKCONFIG=y):

scripts/extract-ikconfig /boot/vmlinuz-$(uname -r)
# Alternative for some distributions:
sudo grep -a "CONFIG_" /boot/vmlinuz-$(uname -r)

For specific feature verification without full config:

# Check if module is built-in (returns nothing if not built-in)
grep MODULE /lib/modules/$(uname -r)/modules.builtin

# Check for specific filesystem support
grep -qw ext4 /proc/filesystems && echo "EXT4 supported"

# Verify security features
grep "NX protection:" /var/log/dmesg

Examine /proc/cmdline for active kernel parameters:

cat /proc/cmdline
# Compare with possible parameters from:
# https://www.kernel.org/doc/html/latest/admin-guide/kernel-parameters.html

On Debian-based systems when the package is installed:

dpkg -L linux-image-$(uname -r) | grep config
# Extract from deb package directly:
ar p /var/cache/apt/archives/linux-image-*.deb data.tar.xz | tar -xOJ ./boot/config-*

Here's how I recently debugged a kernel without config file:

# First try proc method
if [ -f /proc/config.gz ]; then
    zcat /proc/config.gz > /tmp/kernel_config
else
    # Fallback to module inspection
    find /lib/modules/$(uname -r) -name "*.ko" | xargs modinfo | grep depends
fi

This revealed that certain cryptographic modules were compiled as loadable rather than built-in.