Re: [PATCH v5 17/17] arm64: crash: Add crash hotplug support

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] kexec: arch_crash_get_elfcorehdr_size() exports undersized buffer when MEMORY_HOTPLUG is disabled
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commit fd293f9cfd1a12554d9352f4ad58fc9f85721fa3
Author: Jinjie Ruan <ruanjinjie@xxxxxxxxxx>
arm64: crash: Add crash hotplug support

This commit introduces kernel mechanisms to update only the elfcorehdr
on memory hotplug events for arm64. It adds ARCH_SUPPORTS_CRASH_HOTPLUG 
and implements arch-specific functions to handle memory hotplug, avoiding
full kdump image reloads from userspace.

Link: https://lore.kernel.org/all/20240529133446.28446-1-Jonathan.Cameron@xxxxxxxxxx/

> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index b5a51b0ef9440..96de2660b1cbb 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -1720,6 +1720,9 @@ config ARCH_DEFAULT_CRASH_DUMP
>  config ARCH_HAS_GENERIC_CRASHKERNEL_RESERVATION
>  	def_bool CRASH_RESERVE
>  
> +config ARCH_SUPPORTS_CRASH_HOTPLUG
> +	def_bool y
> +

[Severity: High]
Does unconditionally enabling ARCH_SUPPORTS_CRASH_HOTPLUG here expose an
undersized buffer to userspace when CONFIG_MEMORY_HOTPLUG is disabled?

Since CONFIG_HOTPLUG_CPU natively selects CONFIG_CRASH_HOTPLUG on
architectures that support it, having this default to 'y' allows
CRASH_HOTPLUG to be active even without MEMORY_HOTPLUG. This leads to the
size calculation issue below.

[ ... ]
> diff --git a/arch/arm64/kernel/crash.c b/arch/arm64/kernel/crash.c
> new file mode 100644
> index 0000000000000..08dc9b952498e
> --- /dev/null
> +++ b/arch/arm64/kernel/crash.c
[ ... ]
> +unsigned int arch_crash_get_elfcorehdr_size(void)
> +{
> +	unsigned long phdr_cnt;
> +
> +	/* A program header for possible CPUs, vmcoreinfo and kernel_map */
> +	phdr_cnt = 2 + num_possible_cpus();
> +	if (IS_ENABLED(CONFIG_MEMORY_HOTPLUG))
> +		phdr_cnt += CONFIG_CRASH_MAX_MEMORY_RANGES;
> +

[Severity: High]
Is this calculation missing the static memory ranges when
CONFIG_MEMORY_HOTPLUG is disabled?

If compiled with CONFIG_HOTPLUG_CPU=y (which natively selects
CONFIG_CRASH_HOTPLUG) and CONFIG_MEMORY_HOTPLUG=n, phdr_cnt is computed as
exactly 2 + num_possible_cpus(). This does not account for the mandatory
memblock ranges from arch_get_system_nr_ranges(). 

Unlike x86 or powerpc which might survive this due to heavily padded
CONFIG_NR_CPUS_DEFAULT values, arm64's precise num_possible_cpus() leaves
zero headroom. As a result, the exported maximum elfcorehdr size is strictly
smaller than the required size to hold static memory ranges, and any attempt
to arm kdump (kexec -p) will fail because kexec-tools cannot load the crash
kernel into the undersized segment.

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260918100442.3841135-1-ruanjinjie@xxxxxxxxxx?part=17