Re: [PATCH printk v2 5/6] printk: Add panic+kexec flush point

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential
issue to consider:
- [Medium] crash: printk_trigger_flush() in __crash_kexec() called with
  wrong panic_cpu
--

commit fef7b57cf45508b52c2322cc5354dee7d0cedbc1
Author: John Ogness <john.ogness@linutronix.de>

printk: Add panic+kexec flush point

This commit adds a call to printk_trigger_flush() in __crash_kexec() to
allow consoles with the panic flush policy to flush before booting into
the crash kernel.

> diff --git a/kernel/crash_core.c b/kernel/crash_core.c
> index d0bd2d0cf8990..064b0af903c38 100644
> --- a/kernel/crash_core.c
> +++ b/kernel/crash_core.c
> @@ -140,6 +140,7 @@ void __noclone __crash_kexec(struct pt_regs *regs)
>  
>  			crash_setup_regs(&fixed_regs, regs);
>  			crash_save_vmcoreinfo();
> +			printk_trigger_flush(PR_FLUSH_RESTART);

[Severity: Medium]
Can printk_trigger_flush() be called here on CPU0 while panic_cpu still
holds a non-zero CPU number or its invalid initial value?

On Hyper-V root partitions, crash_nmi_callback() parks all non-zero CPUs
when a panic occurs on another CPU (or when the hypervisor itself crashes
with panic_cpu unset), while CPU0 disables the hypervisor and calls
hv_crash_handle() -> __crash_kexec() without updating panic_cpu:

arch/x86/hyperv/hv_crash.c:hv_crash_handle() {
    ...
	/* we are now fully in devirtualized normal kernel mode */
	__crash_kexec(NULL);
}

Because panic_on_this_cpu() is false on CPU0, nbcon_get_default_prio()
returns normal priority instead of panic priority.  printk_trigger_flush()
then skips atomic panic flushing and takes the offload path via
defer_console_output() -> irq_work_queue().  When the local tick is
stopped, raising the self-interrupt calls hv_send_ipi_self(), which
invokes a hypercall after the hypervisor has already been disabled,
faulting and aborting machine_kexec().

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261111181942.593695-1-john.ogness@linutronix.de?part=5