[QEMU] Question regarding user mode support for ARM syscalls

[Lukasz Majewski]

Dear Qemu Community,

I would like to ask you for some advice regarding the usage of
arm-linux-user/qemu-arm user space program simulation.

Background:
-----------

I'm looking for a way to efficiently test y2038 in-glibc solution for
32 bit architectures (e.g. ARM).

For now I do use qemu-system-arm (part of Yocto/OE), which I'm using to
run Linux kernel 5.1, glibc under test and Y2038 tests. It works [1].

Problem:
--------

I would like to test cross-compiled tests (which are built from glibc
sources) without the need to run the emulated system with
qemu-system-arm.

I've come across the "QEMU user mode", which would execute the
cross-compiled test (with already cross-compiled glibc via -L switch)
and just return exit status code. This sounds appealing.

As fair as I've read - QEMU user mode emulates ARM syscalls.

During test execution (single qemu user mode process) I would need to
adjust date with clock_settime64 syscall and then execute other
syscalls if needed.


Please correct me if I'm wrong:
- It looks like qemu-arm doesn't have switch which would allow it to
  set time offset (to e.g. year 2039 - something similar to
  qemu-system-arm -rtc=).

- As of 5.1 qemu version there is no support for syscalls supporting 64
  bit time on 32 bit architectures (e.g. clock_settime64 and friends
  from [2]).

For my example program [3] statically build for testing (it works with
qemu-system-arm):

~/work/qemu-arm-tests-program$
../qemu-5.1.0-arm/arm-linux-user/qemu-arm -L
~/work/yocto/y2038/build/tmp/armv7at2hf-neon-poky-linux-gnueabi/y2038-glibc/2.30+git999-r0/image/opt
-strace ./cst

17746 brk(NULL) = 0x00074000
17746 brk(0x000748a8) = 0x000748a8
17746 uname(0x40800370) = 0
17746 readlink("/proc/self/exe",0x407ff488,4096) = 43
17746 brk(0x000958a8) = 0x000958a8 
17746 brk(0x00096000) = 0x00096000
17746 mprotect(0x00070000,8192,PROT_READ) = 0
17746statx(1,"",AT_EMPTY_PATH|AT_NO_AUTOMOUNT,STATX_BASIC_STATS,0x407ffd70)
= 0 
17746 Unknown syscall 404 --> is the syscall number of clock_settime64

17746 dup(2) = 3
17746 fcntl64(3,F_GETFL) = 2
17746statx(3,"",AT_EMPTY_PATH|AT_NO_AUTOMOUNT,STATX_BASIC_STATS,0x407ff8e8)
= 0 ERR

Questions:
----------

1. Is there any plan to add support for emulating syscalls supporting
64 bit time on 32 bit architectures [2]?

2. Provide QEMU user space switch to adjust its time (i.e. add some
offset to in-fly emulated time syscalls - like clock_settime64) when it
is started?


Thanks in advance for your help and reply.


Links:
[1] - https://github.com/lmajewski/meta-y2038/
[2] -
https://elixir.bootlin.com/linux/latest/source/arch/arm/tools/syscall.tbl#L419

[3]:
Example program:
cat <<- EOF >> clock_settime_test.c
#include <stdio.h>
#include <time.h>

int main (int argc, char **argv)
{
        struct timespec tv;
        int ret;

        tv.tv_sec = 0x7FFFFFFF;
        tv.tv_sec += 61;
        tv.tv_nsec = 0;

        printf("clock_settime test program: ");
        ret = clock_settime(CLOCK_REALTIME, &tv);
        if (!ret)
                printf("OK\n");
        else
                perror("ERR\n");

        return 0;
}
EOF

Build the test program:
gcc -Wall -ggdb -D_TIME_BITS=64 -D_FILE_OFFSET_BITS=64 -I/opt/include
-I/opt/usr/include -L/opt/usr/lib \ -Wl,-rpath=/opt/lib
-Wl,--dynamic-linker=/opt/lib/ld-linux.so.2 clock_settime_test.c -o cst
-static



Best regards,

Lukasz Majewski

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