[LTP] [PATCH 1/1] device-drivers/zram: Fix false-judgement on zram's presence

[Petr Vorel <pvorel@suse.cz>]

Hi Leo,

> Date: Thu, 14 Jan 2021 15:27:34 +0800
> From: Leo Yu-Chi Liang <ycliang@andestech.com>
> Subject: [LTP][PATCH 1/1] device-drivers/zram: Fix false-judgement on zram's presence

> zram_lib.sh uses the return value of modinfo to check if zram module exists,
> but the behavior of modinfo implemented by busybox is different.

> The busybox-implemented modinfo would also return true (code: 0)
> even if zram module is not present,
> so grep the info that only shows when the module exists.

> -modinfo zram > /dev/null 2>&1 ||
> +modinfo zram | grep "filename" > /dev/null 2>&1 ||
nit: 
modinfo zram | grep -q "filename" ||

>  	tst_brk TCONF "zram not configured in kernel"

Thank you for a report. Actually, we have a helper for it:
TST_NEEDS_DRIVERS="zram"

But this helper is broken for BusyBox, which means it's broken for many tests.

The helper calls tst_check_driver() C function (lib/tst_kernel.c):

int tst_check_driver(const char *name)
{
#ifndef __ANDROID__
	const char * const argv[] = { "modprobe", "-n", name, NULL };
	int res = tst_cmd_(NULL, argv, "/dev/null", "/dev/null",
			       TST_CMD_PASS_RETVAL);

	/* 255 - it looks like modprobe not available */
	return (res == 255) ? 0 : res;
#else
	/* Android modprobe may not have '-n', or properly installed
	 * module.*.bin files to determine built-in drivers. Assume
	 * all drivers are available.
	 */
	return 0;
#endif
}

and the problem is that modprobe from busybox does not support -n.
It does support -D, which could be used, *but* unless is busybox binary
configured with CONFIG_MODPROBE_SMALL=y (IMHO the default) => not suitable
for us.

IMHO we have only 2 options:
* write something on our own which would look into /lib/modules and
/system/lib/modules (Android). That's what BusyBox implementation does
(also kmod implementation looks into /lib/modules). BusyBox has this path in
defined in build time configuration (CONFIG_DEFAULT_MODULES_DIR), but I'd be
surprised if any system had both directories.
pros: no external dependency
cons: more code

* use modinfo, but grep for output: for 'filename:' (turn Leo's suggestion into
C code in the API):
cons: module not checked, when modprobe missing (we check for 255 exit code).

BTW not sure whether bother about android support anyway. On Android phone I
have available (Android 8), there is empty /system/lib/modules directory and no
/proc/modules:, thus nor BusyBox neither even toybox modprobe/modinfo
implementations work.

Kind regards,
Petr