Changes between Initial Version and Version 1 of linux/kernel/modules

Timestamp:

04/25/2025 03:31:49 PM (7 months ago)

Author:

Tim Harvey

Comment:

initial page

linux/kernel/modules

@@ +1 @@
+[[PageOutline]]
+
+= Linux Kernel modules
+If your kernel is built with loadable module support (CONFIG_MODULES=y) you can load and unload pre-built kernel modules to add, remove, and alter functionality of a running kernel.
+
+The userspace support for doing this comes from a set of commands typically packaged in a 'kmod' package name consisting of:
+- {{{lsmod}}} - show the status of modules currently loaded in the kernel (uses info from /proc/modules)
+- {{{insmod}}} - manually insert a module into the kernel
+- {{{rmmod}}} - manually remove a module from the kernel
+- {{{modinfo}}} - show info about a module
+- {{{depmod}}} - create a modules.dep and map file describing module dependencies
+- {{{modprobe}}} - add or remove a module from the kernel (handling dependencies and finding modules in the /lib/modules/$(uname -r) directory) taking into account module parameters from optional configuration files in /etc/modprobe.d
+
+Examples:
+ * load a module:
+{{{#!bash
+insmod ./mymod.ko # by path
+modprobe mymod # from /lib/modules using dependencies
+}}}
+ * remove a module:
+{{{#!bash
+rmmod mymod
+}}}
+ * show loaded modules:
+{{{#!bash
+lsmod
+}}}
+ * show kernel params for a given module:
+{{{#!bash
+$ modinfo g_ether | grep parm
+parm:           idVendor:USB Vendor ID (ushort)
+parm:           idProduct:USB Product ID (ushort)
+parm:           bcdDevice:USB Device version (BCD) (ushort)
+parm:           iSerialNumber:SerialNumber string (charp)
+parm:           iManufacturer:USB Manufacturer string (charp)
+parm:           iProduct:USB Product string (charp)
+parm:           qmult:queue length multiplier at high/super speed (uint)
+parm:           dev_addr:Device Ethernet Address (charp)
+parm:           host_addr:Host Ethernet Address (charp)
+parm:           use_eem:use CDC EEM mode (bool)
+}}}
+ * show vermagic for a given module:
+{{{#!bash
+$ modinfo g_ether | grep vermagic
+vermagic:       6.12.0-g530fc2d305a3 SMP mod_unload aarch64
+}}}
+
+== Building
+
+== Linux Kernel vermagic
+Vermagic is a magic string present in the Linux Kernel and added into the .modinfo section of the Linux Kernel Modules.
+
+This is used to verify whether the kernel module was compiled for the particular kernel version or not.
+
+This string is generated by the kernel configuration via the following in include/linux/vermagic.h:
+{{{#!bash
+#define VERMAGIC_STRING                                                 \
+        UTS_RELEASE " "                                                 \
+        MODULE_VERMAGIC_SMP MODULE_VERMAGIC_PREEMPT                     \
+        MODULE_VERMAGIC_MODULE_UNLOAD MODULE_VERMAGIC_MODVERSIONS       \
+        MODULE_ARCH_VERMAGIC                                            \
+        MODULE_RANDSTRUCT
+}}}
+
+A typical VERMAGIC_STRING for an aarch64 kernel would look something like:{{{6.12.0-g530fc2d305a3 SMP mod_unload aarch64}}}
+
+While you can easily find the kernel version and arch of a running kenrel, perhaps the best way to see this exact string would be to use modinfo command on an existing module:
+{{{#!bash
+# modinfo $(find /lib/modules/$(uname -r)/ -name "*.ko" | head -n1) | grep vermagic
+vermagic:       6.12.0-g530fc2d305a3 SMP mod_unload aarch64
+}}}
+
+The vermagic string is checked upon module loading and if the string in the module does not match the string in the kernel the module loading will fail by design:
+{{{#!bash
+# insmod mymod.ko
+mymod: version magic '6.12.0-00017-g530fc2d305a3 SMP mod_unload aarch64' should be '6.12.0-g530fc2d305a3 SMP mod_unload aarch64'
+insmod: ERROR: could not insert module ./mymod.ko: Invalid module format
+}}}
+
+There is no easy way to disable the kernel module loading vermagic string unless you disable a lot of security measures and the vermagic check is there for a very important reason: to disallow loading of modules that are not compatible with the specific version of kernel running.
+
+There are times however as a developer building an out-of-tree kernel module that you may wish to avoid this check. For example, if your running kernel version varies only slightly in ways you know are compatible. In such a case where you assume full responsibility for breaking something you could use the [https://dev.gateworks.com/sources/setvermagic.sh setvermagic.sh] script to modify the vermagic string:
+{{{#!bash
+$ wget https://dev.gateworks.com/sources/setvermagic.sh
+$ chmod +x setvermagic.sh
+$ modinfo mymod.ko | grep vermagic
+vermagic:       6.6.8-00035-gc6ef8b5e47a0 SMP mod_unload aarch64
+Show quoted text
+$ ./setvermagic.sh mymod.ko "6.6.8-gc6ef8b5e47a0 SMP mod_unload aarch64"
+Current vermagic: 6.6.8-00035-gc6ef8b5e47a0 SMP mod_unload aarch64
+New vermagic    : 6.6.8-gc6ef8b5e47a0 SMP mod_unload aarch64
+Modified module saved as: mymod-modified.ko
+You can load it with: insmod mymod-modified.ko
+}}}