Forward application exit status to host

[edubart]

Context

An application running inside the virtual machine can exit with an error status when it fails or crashes. We should start exposing this exit status to be readable by the host after the machine is halted, so the host is able to determine if the guest machine finished its application with success or failure.

Possible solutions

We need to edit our init script so it captures the exit status of the application and saves this exit status in a machine CSR before halting. The first obvious solution would be creating a new poweroff utility that halts the machine while saving the exit status.

• relates feat: forward application exit status #131
• relates Ports/cartesi opensbi#1
• relates Update yield ecall to OpenSBI ABI 0.3+ linux#2
• relates Forward application exit status via reboot syscall linux#3
• relates Feature/forward application exit status machine-emulator-tools#20
• relates Bump kernel and replace BBL with OpenSBI image-kernel#22
• relates Exit code should be non zero for kernel panics #144

[mpolitzer]

Retrieved from Chat archives:

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Regarding the exit status / return code for applications:

OpenSBI would require a patch to pass one of the registers into sbi_system_reset

https://github.com/cartesi/opensbi/blob/5ef1484038f287106fe9706c02d0f45f31df6914/lib/sbi/sbi_ecall_legacy.c#L110

Linux accepts a string when the reboot syscall is invoked with: LINUX_REBOOT_CMD_RESTART2, could be a way to extract the status code of applications.
It would need a patch to arch/riscv/kernel/reset.c:machine_restart to remove the infinite loop and pass something useful down to sbi_shutdown.

https://github.com/cartesi/linux/blob/d40b7543be1650576bc0672f868218a8a73ade30/arch/riscv/kernel/sbi.c#L101C6-L101C18
https://github.com/cartesi/linux/blob/d40b7543be1650576bc0672f868218a8a73ade30/arch/riscv/kernel/reset.c#L18

Then we could do something like this to get the string into the kernel:

#include<unistd.h>
#include<syscall.h>
#include<linux/reboot.h>

int main(int argc, char *argv[]) {
    syscall(SYS_reboot, LINUX_REBOOT_MAGIC1, LINUX_REBOOT_MAGIC2, LINUX_REBOOT_CMD_RESTART2, argv[1]);
}

[   18.488384] reboot: Restarting system with command 'bye'

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