[android-kernel] Android NDK can't run fanotify: fanotify_mark: Invalid argument (kernel problem?)

['Fabian B' via Android Linux Kernel Development]

I'm stuck with the fanotify
<http://man7.org/linux/man-pages/man7/fanotify.7.html>systemcall to get
access control for file-access events, when a file on the device gets
opened. My specs:

- Samsung i9300 ARM with LineageOS 14.1
<https://wiki.lineageos.org/devices/i9300/>
- Linux kernel 3.0.101 (fanotify is available in kernel > 2.6.36 /
android 5)
- NDK r14b
- rooted


I have build an Android native application (written in C) based on fsmon
<https://github.com/nowsecure/fsmon>to handle the file access. After
compiling I pushed the executable armeabi-v7 file with ADB on the device
(/data/local/tmp/). Then I tried to execute (as su) the file and got
following error message:


fanotify_mark: Invalid argument.

I had changed the kernel config by editing the
/lineage/kernel/samsung/smdk4412/arch/arm/configs/lineageos_i9300_defconfig
to:

CONFIG_FANOTIFY=y
CONFIG_FANOTIFY_ACCESS_PERMISSIONS=y


Also after flashing the new kernel and running

cat /proc/config.gz | gunzip > running.config

from the device to get the kernel config, I can see that both config-params
are activated. Following is the code (the calling function, if more is
necessary, I will extend it) which crashes:

static bool fm_begin (FileMonitor *fm) {
// Define event mask which fanotify should signal.
// Important: the FAN_ALL_PERM_EVENTS flag to enable access control
uint64_t fan_mask = FAN_OPEN | FAN_CLOSE | FAN_ACCESS | FAN_MODIFY |
FAN_ALL_PERM_EVENTS;
// Set which type of operation it should do on fanotify_mark() - adding
the flaggs
unsigned int mark_flags = FAN_MARK_ADD, init_flags = 0;
struct sigaction sa;
int res = 0;


// Set function to close the file descriptor
fm->control_c = fm_control_c;
sa.sa_flags = SA_SIGINFO | SA_RESTART;
sigemptyset (&sa.sa_mask);
sa.sa_sigaction = usr1_handler;
if (sigaction (SIGUSR1, &sa, NULL) == -1) {
printf ("Cannot set SIGUSR1 signal handler\n");
return false;
}
fan_mask |= FAN_ONDIR;
fan_mask |= FAN_EVENT_ON_CHILD;
// Walk into subdirectories
mark_flags |= FAN_MARK_MOUNT;

// Important: Set FAN_CLASS_CONTENT to allow/deny access to files
init_flags |= (fan_mask & FAN_ALL_PERM_EVENTS)
? FAN_CLASS_CONTENT
: FAN_CLASS_NOTIF;
if (!fm->root) {
fm->root = "/";
}


// Call the fanotify_init systemcall with read only and enable for large
files
fan_fd = fanotify_init (init_flags, O_RDONLY | O_LARGEFILE);
if (fan_fd < 0) {
perror ("fanotify_init");
return false;
}

// Call the fanotify_mark systemcall with flags
// CRASHES HERE!!!!!!!!!!!!!!! res = -1
res = fanotify_mark (fan_fd, mark_flags, fan_mask, AT_FDCWD, fm->root);
if (res != 0) {
printf("Result: %i\n", res); // == -1
perror ("fanotify_mark");
return false;
}

FD_ZERO (&rfds);
FD_SET (fan_fd, &rfds);
return true;
}

I logged the flags and they seemed to be valid, no null value or something
else. I also tried different combinations of the mask, no result...

The fanotify_init() function works properly and doesn't throw an error
(before I set the CONFIG in the kernel config-file, the application crashed
here).

I tried the same code on Ubuntu and on a x86 Stock Android emulator
(compiled AOSP) with success - the application get's started and I can
control the access. But on the emulator the goldfish kernel is running.

Running "adb shell logcat" and "adb shell dmesg" doesn't show me an error
on execution...

It's a little bit strange because the return value 1 (fanotify negate the
value) of the fanotify_mark() call is on the errno.h list:

#define EPERM 1 /* Operation not permitted */

I think it's a kernel problem (not LineageOs) so this could be the right
place to ask...

Some suggestions?...