Where does the systemd builtin 'kmod' gets the module aliases from

Question

udev is loading the necessary driver modules when plugging in a hotplug-capable device. This is done by the following udev-rule

DRIVER!="?*", ENV{MODALIAS}=="?*", RUN{builtin}="kmod load $env{MODALIAS}"

Therefore kmod gets invoked by e.g. kmod load hid:b0005g0001v0000045Ep000002E0

The source of the builtin kmod is documented here

hid:b0005g0001v0000045Ep000002E0 is just an alias for a module like hid_xpadneo.

I know that there is generated a file called modules.alias in /usr/lib/modules/$(uname -r)/modules.alias where those aliases are listed like this:

alias hid:b0005g*v0000045Ep000002E0 hid_xpadneo
alias hid:b0005g*v0000045Ep000002FD hid_xpadneo

This file is generated out of all drivers and their MODULE_DEVICE_TABLEs while compile-time.

Unfortunately I cannot find where exactly kmod does read in that file or where it gets the alias information from.

Therefore: How does kmod know which alias stands for which module?

Answer 1

From systemd/udev-builtin-kmod.c:

static int builtin_kmod_init(void) {
        ...
        kmod_load_resources(ctx);
        ...
}

From kmod/libkmod.c:

static struct _index_files {
    const char *fn;
    const char *prefix;
} index_files[] = {
    [KMOD_INDEX_MODULES_DEP] = { .fn = "modules.dep", .prefix = "" },
    [KMOD_INDEX_MODULES_ALIAS] = { .fn = "modules.alias", .prefix = "alias " },
    [KMOD_INDEX_MODULES_SYMBOL] = { .fn = "modules.symbols", .prefix = "alias "},
    [KMOD_INDEX_MODULES_BUILTIN] = { .fn = "modules.builtin", .prefix = ""},
};

...

KMOD_EXPORT int kmod_load_resources(struct kmod_ctx *ctx)
{
    for (i = 0; i < _KMOD_INDEX_MODULES_SIZE; i++) {
        ...
        snprintf(path, sizeof(path), "%s/%s.bin", ctx->dirname,
                            index_files[i].fn);
        ctx->indexes[i] = index_mm_open(ctx, path,
                        &ctx->indexes_stamp[i]);
        ...
    }
}

From kmon/libkmod-index.c:

struct index_mm *index_mm_open(struct kmod_ctx *ctx, const char *filename,
                        unsigned long long *stamp)
{
    ...
    if ((fd = open(filename, O_RDONLY|O_CLOEXEC)) < 0) {
    ...
}

kmod reads and indexes all the possible files modules.dep modules.alias modules.symbols and modules.builtin and it get's it's alias information from that files.

Then later in udev-buildin-kmod.c:

static int builtin_kmod(sd_device *dev, int argc, char *argv[], bool test) {
        ...
        for (i = 2; argv[i]; i++)
                (void) module_load_and_warn(ctx, argv[i], false);
        ...
}

And from systemd/module-util.c:

int module_load_and_warn(struct kmod_ctx *ctx, const char *module, bool verbose) {
    ...
    r = kmod_module_new_from_lookup(ctx, module, &modlist);
    if (r < 0)
            return log_full_errno(verbose ? LOG_ERR : LOG_DEBUG, r,
                                  "Failed to lookup module alias '%s': %m", module);

    ...
}

And inside kmod_module_new_from_lookup the lookup for module alias is performed, ie. the data read into kmod object is searched through.