Modulesymvers

Module.symvers is a file produced by the Linux kernel build system that records the version information of exported symbols for modules built against a particular kernel. When symbol versioning is enabled (CONFIG_MODVERSIONS), the build process assigns a version CRC to each exported symbol and writes these entries into Module.symvers. This file is used by the kbuild system to verify that external or out-of-tree modules are built against a kernel with matching symbol versions.

Contents and purpose: Each entry in Module.symvers associates a symbol name with its version hash. The primary purpose is to enable strict compatibility checks between the running kernel and loadable modules. If a module asks for a symbol whose version does not match the kernel’s, the module will fail to load, preventing potential instability due to incompatible interfaces.

Generation and location: Module.symvers is generated during the kernel build, typically in the build output directory, by the modpost step. It is not usually committed into the kernel source tree, but a corresponding copy is often kept to aid building external modules against a specific kernel version. When rebuilding the kernel with the same configuration, a new Module.symvers will reflect any changes to exported symbols.

Usage and implications: Module developers and administrators use Module.symvers to ensure that external modules are compiled against the correct symbol versions. If the file is missing or out of date, module compilation may succeed but loading can fail due to version mismatches. It is distinct from vermagic, which encodes kernel version, compiler version, and other build flags used to determine overall module compatibility.

See also: Kernel modules, symbol versioning, modpost, EXPORT_SYMBOL, and CONFIG_MODVERSIONS.