linux-kernel - Re: [PATCH v5 8/8] x86/crash: Add x86 crash hotplug support for kexec

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Message-ID: <YkWMFzxJpkiTjKvL@MiWiFi-R3L-srv>
Date:   Thu, 31 Mar 2022 19:10:15 +0800
From:   Baoquan He <bhe@...hat.com>
To:     Eric DeVolder <eric.devolder@...cle.com>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        kexec@...ts.infradead.org, ebiederm@...ssion.com,
        dyoung@...hat.com, vgoyal@...hat.com, tglx@...utronix.de,
        mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com,
        hpa@...or.com, nramas@...ux.microsoft.com, thomas.lendacky@....com,
        robh@...nel.org, efault@....de, rppt@...nel.org, david@...hat.com,
        konrad.wilk@...cle.com, boris.ostrovsky@...cle.com
Subject: Re: [PATCH v5 8/8] x86/crash: Add x86 crash hotplug support for
 kexec_load

On 03/03/22 at 11:27am, Eric DeVolder wrote:
> For kexec_file_load support, the loading of the crash kernel occurs
> entirely within the kernel, and as such the elfcorehdr is readily
> identified (so that it can be modified upon hotplug events).
> 
> This change enables support for kexec_load by identifying the
> elfcorehdr segment in the arch_crash_hotplug_handler(), if it has
> not already been identified.
> 
> In general, support for kexec_load requires corresponding changes
> to the userspace kexec-tools utility. It is the responsibility of
> the userspace kexec utility to ensure that:
>  - the elfcorehdr segment is sufficiently large enough to accommodate
>    hotplug changes, ala CRASH_HOTPLUG_ELFCOREHDR_SZ.
>  - provides a purgatory that excludes the elfcorehdr from its list of
>    run-time segments to check.
> These changes to the userspace kexec utility are available, but not
> yet accepted upstream.

These backgroud information should be put in cover letter, or in patch
where they won't be grabbed into the final commit log. 

> 
> Signed-off-by: Eric DeVolder <eric.devolder@...cle.com>
> ---
Here, if you want to put some status or backgroud information and expect
they are not got into log, this is a appropriate place.


>  arch/x86/kernel/crash.c | 26 ++++++++++++++++++++++++++
>  1 file changed, 26 insertions(+)
> 
> diff --git a/arch/x86/kernel/crash.c b/arch/x86/kernel/crash.c
> index 5da30e2bc780..45cc6e3af63e 100644
> --- a/arch/x86/kernel/crash.c
> +++ b/arch/x86/kernel/crash.c
> @@ -486,6 +486,32 @@ void arch_crash_hotplug_handler(struct kimage *image,
>  	void *elfbuf = NULL;
>  	unsigned long mem, memsz;
>  
> +	/*
> +	 * When the struct kimage is alloced, it is wiped to zero, so
> +	 * the elf_index_valid defaults to false. It is set on the
> +	 * kexec_file_load path, or here for kexec_load.

         I would make the last line as:
	 kexec_file_load path, or here for kexec_load if not already
         identified..

> +	 */
> +	if (!image->elf_index_valid) {
> +		unsigned int n;
> +
> +		for (n = 0; n < image->nr_segments; n++) {
> +			mem = image->segment[n].mem;
> +			memsz = image->segment[n].memsz;
> +			ptr = map_crash_pages(mem, memsz);
> +			if (ptr) {
> +				/* The segment containing elfcorehdr */
> +				if ((ptr[0] == 0x7F) &&
> +					(ptr[1] == 'E') &&
> +					(ptr[2] == 'L') &&
> +					(ptr[3] == 'F')) {

Can it be like memcmp(ptr, ELFMAG, SELFMAG) as we have done in
parse_crash_elf_headers()? With that, one line is taken.

> +					image->elf_index = (int)n;
> +					image->elf_index_valid = true;
> +				}
> +			}
> +			unmap_crash_pages((void **)&ptr);
> +		}
> +	}
> +
>  	/* Must have valid elfcorehdr index */
>  	if (!image->elf_index_valid) {
>  		pr_err("crash hp: unable to locate elfcorehdr segment");
> -- 
> 2.27.0
>