[Bug]: Qt6 QProcess fails to create process for Linux kernel >= 5

[knyipab]

Most things in this PR shocut app works well, except that the export job does not work.,, stuck at finite time "--:--:--". You may try the builds in the PR #20353 or this action: https://github.com/knyipab/termux-packages/actions/runs/9290613878

After use the program to export file, the only useful log from terminal is below.

[Debug  ] <MeltJob::start> "/data/data/com.termux/files/usr/bin/melt-7 -verbose -progress2 -abort xml:%2Fdata%2Fdata%2Fcom.termux%2Ffiles%2Fusr%2Ftmp%2Fshotcut-jnXVRh.mlt" 

Indeed, if I run that command /data/data/com.termux/files/usr/bin/melt-7 -verbose -progress2 -abort xml:%2Fdata%2Fdata%2Fcom.termux%2Ffiles%2Fusr%2Ftmp%2Fshotcut-jnXVRh.mlt, it works well and output the intended video.

That line of log corresponds to this line of code in MeltJob::start(). That method should ultimately call QProcess::start() at this line. So I suspect that QProcess does not function. I have no proof and nothing more in the log is helpful.

[Biswa96]

Does strace provide any hint?

[knyipab]

Don't have experience with strace but it seems to fail to create child process.

clone(child_stack=0x7ffdb65530, flags=CLONE_VM|CLONE_PIDFD|CLONE_VFORK|SIGCHLD, parent_tid=NULL) = -1 EFAULT (Bad address)

strace log: shortcut_excerpt.log

[knyipab]

A simple Qt6 example (generated by ChatGPT) fails to execute command with QProcess. strace show similar error in clone syscall.

g++ -o qprocess qprocess.cpp -lQt6Core -I"$PREFIX"/include/qt6 -I"$PREFIX"/include/qt6/QtCore

qprocess.cpp

#include <QCoreApplication>
#include <QProcess>
#include <QDebug>

int main(int argc, char *argv[])
{
    QCoreApplication app(argc, argv);

    QProcess process;
    process.start("ls", QStringList() << "-l");

    if (process.waitForStarted() && process.waitForFinished()) {
        QByteArray output = process.readAllStandardOutput();
        qDebug() << output;
    } else {
        qDebug() << "Failed to start or execute command.";
    }

    return app.exec();
}

Qt5

This same example compiled with qt5 works

g++ -o qprocess qprocess.cpp -lQt5Core -I"$PREFIX"/include -I"$PREFIX"/include/QtCore

[Biswa96]

That example program worked in my PC (x86_64) and in my phone (aarch64) without any error.

execve("/data/data/com.termux/files/usr/bin/test", ["test"], 0x7fd1e78830 /* 54 vars */) = 0

[knyipab]

When it is without strace, does that example program print something similar to ls -l on your device? On my phone, the qt6 build prints Failed to start or execute command. while qt5 build prints something similar to ls -l.

As for strace syscall clone results,

• qt6 build shows clone(child_stack=0x7fed797470, flags=CLONE_VM|CLONE_PIDFD|CLONE_VFORK|SIGCHLD, parent_tid=NULL) = -1 EFAULT (Bad address)
• qt5 build shows clone(child_stack=NULL, flags=CLONE_PIDFD|SIGCHLD, parent_tid=[12]) = 19309

Do you see similar thing for qt6 build? If not, it might be vendor-specific.

[Biswa96]

When it is without strace, does that example program print something similar to ls -l on your device?

Yes.

[knyipab]

It seems to be system_vforkfd introduced in Qt6.4 (this commit). I think for us, we should patch it to use the old system_forkfd instead.

FYI, this problem seems to occur in runtime depending on the kernel version:

Device	Arch	Android	Linux	Build	strace clone flags	success?
Emulated	x86_64	14	6.1.23	Qt6	CLONE_VM|CLONE_PIDFD|CLONE_VFORK|SIGCHLD	no
Emulated	x86_64	14	6.1.23	Qt5	CLONE_PIDFD|SIGCHLD	yes
Emulated	x86_64	13	5.15.41	Qt6	CLONE_VM|CLONE_PIDFD|CLONE_VFORK|SIGCHLD	no
Emulated	x86_64	13	5.15.41	Qt5	CLONE_PIDFD|SIGCHLD	yes
Samsung Fold5	aarch64	13	5.15.41	Qt6	CLONE_VM|CLONE_PIDFD|CLONE_VFORK|SIGCHLD	no
Samsung Fold5	aarch64	13	5.15.41	Qt5	CLONE_PIDFD|SIGCHLD	yes
Emulated	x86	11	5.4.61	Qt6	CLONE_VM|CLONE_PIDFD|CLONE_VFORK|SIGCHLD	no
Emulated	x86	11	5.4.61	Qt5	CLONE_PIDFD|SIGCHLD	yes
Pixel 4a	aarch64	13	4.14.302	Qt6	CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD	yes
Pixel 4a	aarch64	13	4.14.302	Qt5	CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD	yes
Emulated	x86	9	4.4.124	Qt6	CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD	yes
Emulated	x86	9	4.4.124	Qt5	CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD	yes