Re: rcutorture’s init segfaults in ppc64le VM

["Paul E. McKenney" <paulmck@kernel.org>]

On Mon, Feb 07, 2022 at 05:44:47PM +0100, Paul Menzel wrote:
> Dear Linux folks,
> 
> 
> On the POWER8 server IBM S822LC running Ubuntu 21.10, building Linux
> 5.17-rc2+ with rcutorture tests
> 
>     $ tools/testing/selftests/rcutorture/bin/torture.sh --duration 10
> 
> the built init
> 
>     $ file tools/testing/selftests/rcutorture/initrd/init
>     tools/testing/selftests/rcutorture/initrd/init: ELF 64-bit LSB
> executable, 64-bit PowerPC or cisco 7500, version 1 (SYSV), statically
> linked, BuildID[sha1]=0ded0e45649184a296f30d611f7a03cc51ecb616, for
> GNU/Linux 3.10.0, stripped
> 
> segfaults in QEMU. From one of the log files
> 
> 
> /dev/shm/linux/tools/testing/selftests/rcutorture/res/2022.02.01-21.52.37-torture/results-rcutorture/TREE03/console.log
> 
>     [    1.119803][    T1] Run /init as init process
>     [    1.122011][    T1] init[1]: segfault (11) at f0656d90 nip 10000a18
> lr 0 code 1 in init[10000000+d0000]
>     [    1.124863][    T1] init[1]: code: 2c2903e7 f9210030 4081ff84
> 4bffff58 00000000 01000000 00000580 3c40100f
>     [    1.128823][    T1] init[1]: code: 38427c00 7c290b78 782106e4
> 38000000 <f821ff81> 7c0803a6 f8010000 e9028010
> 
> Executing the init, which just seems to be an endless loop, from userspace
> work:
> 
>     $ strace ./tools/testing/selftests/rcutorture/initrd/init
>     execve("./tools/testing/selftests/rcutorture/initrd/init",
> ["./tools/testing/selftests/rcutor"...], 0x7ffffdb9e860 /* 31 vars */) = 0
>     brk(NULL)                               = 0x1001d940000
>     brk(0x1001d940b98)                      = 0x1001d940b98
>     set_tid_address(0x1001d9400d0)          = 2890832
>     set_robust_list(0x1001d9400e0, 24)      = 0
>     uname({sysname="Linux",
> nodename="flughafenberlinbrandenburgwillybrandt.molgen.mpg.de", ...}) = 0
>     prlimit64(0, RLIMIT_STACK, NULL, {rlim_cur=8192*1024,
> rlim_max=RLIM64_INFINITY}) = 0
>     readlink("/proc/self/exe", "/dev/shm/linux/tools/testing/sel"..., 4096)
> = 61
>     getrandom("\xf1\x30\x4c\x9e\x82\x8d\x26\xd7", 8, GRND_NONBLOCK) = 8
>     brk(0x1001d970b98)                      = 0x1001d970b98
>     brk(0x1001d980000)                      = 0x1001d980000
>     mprotect(0x100e0000, 65536, PROT_READ)  = 0
>     clock_nanosleep(CLOCK_REALTIME, 0, {tv_sec=1, tv_nsec=0},
> 0x7ffffb22c8a8) = 0
>     clock_nanosleep(CLOCK_REALTIME, 0, {tv_sec=1, tv_nsec=0},
> 0x7ffffb22c8a8) = 0
>     clock_nanosleep(CLOCK_REALTIME, 0, {tv_sec=1, tv_nsec=0}, ^C{tv_sec=0,
> tv_nsec=872674044}) = ? ERESTART_RESTARTBLOCK (Interrupted by signal)
>     strace: Process 2890832 detached

Huh.  In PowerPC, is there some difference between system calls
executed in initrd and those same system calls executed in userspace?

And just to make sure, the above strace was from exactly the same
binary "init" file that is included in initrd, correct?

Adding Willy Tarreau for his thoughts.

							Thanx, Paul

> Any ideas, what `mkinitrd.sh` [2] should do differently?
> 
> ```
> cat > init.c << '___EOF___'
> #ifndef NOLIBC
> #include <unistd.h>
> #include <sys/time.h>
> #endif
> 
> volatile unsigned long delaycount;
> 
> int main(int argc, int argv[])
> {
> 	int i;
> 	struct timeval tv;
> 	struct timeval tvb;
> 
> 	for (;;) {
> 		sleep(1);
> 		/* Need some userspace time. */
> 		if (gettimeofday(&tvb, NULL))
> 			continue;
> 		do {
> 			for (i = 0; i < 1000 * 100; i++)
> 				delaycount = i * i;
> 			if (gettimeofday(&tv, NULL))
> 				break;
> 			tv.tv_sec -= tvb.tv_sec;
> 			if (tv.tv_sec > 1)
> 				break;
> 			tv.tv_usec += tv.tv_sec * 1000 * 1000;
> 			tv.tv_usec -= tvb.tv_usec;
> 		} while (tv.tv_usec < 1000);
> 	}
> 	return 0;
> }
> ___EOF___
> 
> # build using nolibc on supported archs (smaller executable) and fall
> # back to regular glibc on other ones.
> if echo -e "#if __x86_64__||__i386__||__i486__||__i586__||__i686__" \
>            "||__ARM_EABI__||__aarch64__\nyes\n#endif" \
>    | ${CROSS_COMPILE}gcc -E -nostdlib -xc - \
>    | grep -q '^yes'; then
> 	# architecture supported by nolibc
>         ${CROSS_COMPILE}gcc -fno-asynchronous-unwind-tables -fno-ident \
> 		-nostdlib -include ../../../../include/nolibc/nolibc.h \
> 		-s -static -Os -o init init.c -lgcc
> else
> 	${CROSS_COMPILE}gcc -s -static -Os -o init init.c
> fi
> ```
> 
> 
> Kind regards,
> 
> Paul
> 
> 
> [1]: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/testing/selftests/rcutorture/doc/initrd.txt
> [2]: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/testing/selftests/rcutorture/bin/mkinitrd.sh