Re: [PATCH v2 1/2] x86/crash: reserve elfcorehdr for CONFIG_NR_CPUS, not CONFIG_NR_CPUS_DEFAULT

[Baoquan He <baoquan.he@linux.dev>]

On 08/25/26 at 10:50am, Ionut Nechita (Wind River) wrote:
> From: Ionut Nechita <ionut.nechita@windriver.com>
> 
> kexec_file_load(2) fails with -EINVAL when loading a crash kernel on a
> machine whose number of possible CPUs exceeds CONFIG_NR_CPUS_DEFAULT,
> even though the classic kexec_load(2) path succeeds on the same machine.
> 
> With CONFIG_CRASH_HOTPLUG=y the elfcorehdr segment is over-allocated so
> it can be updated in place on CPU/memory hotplug.  On the
> !CONFIG_MEMORY_HOTPLUG path, crash_load_segments() sizes that
> reservation as:
> 
> 	ret = crash_prepare_headers(..., &kbuf.bufsz, &pnum);
> 	...
> 	pnum += 2 + CONFIG_NR_CPUS_DEFAULT;
> 
> The value that lands in @pnum is crash_prepare_headers()'s
> @nr_mem_ranges out parameter, i.e. cmem->nr_ranges - the number of
> memory ranges only, not a phdr count.  The header that
> crash_prepare_elf64_headers() actually builds adds one phdr per
> *possible* CPU on top of those ranges:
> 
> 	nr_phdr = nr_cpus + 1;		/* + vmcoreinfo */
> 	nr_phdr += mem->nr_ranges;
> 	nr_phdr++;			/* + kernel text map */
> 
> So the reservation covers
> 
> 	nr_ranges + 2 + CONFIG_NR_CPUS_DEFAULT
> 
> phdrs while the buffer holds
> 
> 	nr_ranges + 2 + num_possible_cpus()
> 
> phdrs, and the buffer exceeds the reservation by
> 
> 	(num_possible_cpus() - CONFIG_NR_CPUS_DEFAULT) * sizeof(Elf64_Phdr)
> 
> bytes as soon as num_possible_cpus() grows past CONFIG_NR_CPUS_DEFAULT.
> num_possible_cpus() is bounded by CONFIG_NR_CPUS, not by
> CONFIG_NR_CPUS_DEFAULT, so this is reachable on any config that raises
> CONFIG_NR_CPUS above the arch default without CONFIG_MAXSMP.
> 
> crash_prepare_elf64_headers() rounds bufsz up to ELF_CORE_HEADER_ALIGN
> and kexec_add_buffer() rounds memsz up to PAGE_SIZE (both 4096), so the
> excess is invisible until it outgrows that padding.  Once it does,
> sanity_check_segment_list() rejects the image:
> 
> 	if (image->segment[i].bufsz > image->segment[i].memsz)
> 		return -EINVAL;
> 
> kexec_load(2) is unaffected because user space builds the elfcorehdr
> without the hotplug over-allocation.
> 
> Observed on a single-socket Xeon 6776P (144 possible CPUs) running a
> PREEMPT_RT kernel with:
> 
> 	# CONFIG_MAXSMP is not set
> 	# CONFIG_MEMORY_HOTPLUG is not set
> 	CONFIG_NR_CPUS_RANGE_BEGIN=2
> 	CONFIG_NR_CPUS_RANGE_END=512
> 	CONFIG_NR_CPUS_DEFAULT=64
> 	CONFIG_NR_CPUS=256
> 
> At 144 possible CPUs the buffer exceeds the reservation by
> (144 - 64) * 56 = 4480 bytes.  That is more than the 4096 bytes of page
> padding, so the overflow is guaranteed and kexec -p -s fails with
> "kexec_file_load failed: Invalid argument".  Reducing the possible CPU
> count to 72 leaves an excess of (72 - 64) * 56 = 448 bytes, which the
> page rounding still absorbs, and the load succeeds - confirming the
> reservation is the limiting factor.
> 
> Reserve the elfcorehdr for CONFIG_NR_CPUS, the compile-time upper bound
> of num_possible_cpus(), so the reservation always covers the header that
> is actually generated.
> 
> The CONFIG_MEMORY_HOTPLUG=y path discards @pnum and reserves
> 2 + CONFIG_NR_CPUS_DEFAULT + CONFIG_CRASH_MAX_MEMORY_RANGES phdrs
> instead.  With the default CONFIG_CRASH_MAX_MEMORY_RANGES=8192 the
> memory range allowance dwarfs the CPU shortfall, so that path does not
> fail in practice; it is switched to CONFIG_NR_CPUS as well for
> consistency and to stay correct for small CONFIG_CRASH_MAX_MEMORY_RANGES
> values.
> 
> This does not change the reservation for defconfig-like builds, since
> CONFIG_NR_CPUS defaults to CONFIG_NR_CPUS_DEFAULT.  Only configs that
> raise CONFIG_NR_CPUS reserve more, and the worst case is bounded by the
> top of the range (CONFIG_NR_CPUS=8192 with CONFIG_CPUMASK_OFFSTACK=y),
> which is exactly what CONFIG_MAXSMP already reserves today.
> 
> Fixes: ea53ad9cf73b ("x86/crash: add x86 crash hotplug support")
> Signed-off-by: Ionut Nechita <ionut.nechita@windriver.com>
> Reviewed-by: Jinjie Ruan <ruanjinjie@huawei.com>
> Reviewed-by: Bradley Morgan <include@grrlz.net>
> ---
>  arch/x86/kernel/crash.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kernel/crash.c b/arch/x86/kernel/crash.c
> index e681ec9cf1dc..e6f23933a6df 100644
> --- a/arch/x86/kernel/crash.c
> +++ b/arch/x86/kernel/crash.c
> @@ -369,9 +369,9 @@ int crash_load_segments(struct kimage *image)
>  	 * maximum CPUs and maximum memory ranges.
>  	 */
>  	if (IS_ENABLED(CONFIG_MEMORY_HOTPLUG))
> -		pnum = 2 + CONFIG_NR_CPUS_DEFAULT + CONFIG_CRASH_MAX_MEMORY_RANGES;
> +		pnum = 2 + CONFIG_NR_CPUS + CONFIG_CRASH_MAX_MEMORY_RANGES;
>  	else
> -		pnum += 2 + CONFIG_NR_CPUS_DEFAULT;
> +		pnum += 2 + CONFIG_NR_CPUS;
>  
>  	if (pnum < (unsigned long)PN_XNUM) {
>  		kbuf.memsz = pnum * sizeof(Elf64_Phdr);
> @@ -430,7 +430,7 @@ unsigned int arch_crash_get_elfcorehdr_size(void)
>  	unsigned int sz;
>  
>  	/* kernel_map, VMCOREINFO and maximum CPUs */
> -	sz = 2 + CONFIG_NR_CPUS_DEFAULT;
> +	sz = 2 + CONFIG_NR_CPUS;
>  	if (IS_ENABLED(CONFIG_MEMORY_HOTPLUG))
>  		sz += CONFIG_CRASH_MAX_MEMORY_RANGES;
>  	sz *= sizeof(Elf64_Phdr);

I didn't dare to read the commit log, but judging from the code, it looks
like a good fix.

Acked-by: Baoquan He <baoquan.he@linux.dev>

> 
> base-commit: 4b18edbd8e70f7e6860d56370f13244896d0f95c
> -- 
> 2.55.0
>