Re: [PATCH v14 7/7] x86/crash: add x86 crash hotplug support

Eric DeVolder
2022-12-08T20:36:37Z

On 12/7/22 04:19, Borislav Petkov wrote:
> On Wed, Nov 16, 2022 at 04:46:43PM -0500, Eric DeVolder wrote:
>> When CPU or memory is hot un/plugged, the crash elfcorehdr, which
>> describes the CPUs and memory in the system, must also be updated.
>>
>> A new elfcorehdr is generated from the available CPUs and memory
>> into a buffer, and then installed over the top of the existing
>> elfcorehdr. The segment containing the elfcorehdr is identified
>> at run time in crash_core:handle_hotplug_event(), which works for
>> both the kexec_load() and kexec_file_load() syscalls.
>>
>> In the patch 'kexec: exclude elfcorehdr from the segment digest'
>> the need to update purgatory due to the change in elfcorehdr was
>> eliminated. As a result, no changes to purgatory or boot_params
>> (as the elfcorehdr= kernel command line parameter pointer
>> remains unchanged and correct) are needed, just elfcorehdr.
>>
>> To accommodate a growing number of resources via hotplug, the
>> elfcorehdr segment must be sufficiently large enough to accommodate
>> changes, see the CRASH_MAX_MEMORY_RANGES description.
>>
>> Signed-off-by: Eric DeVolder <eric.devolder@oracle.com>
>> ---
>> arch/x86/Kconfig | 9 +++
>> arch/x86/include/asm/kexec.h | 15 +++++
>> arch/x86/kernel/crash.c | 106 ++++++++++++++++++++++++++++++++++-
>> 3 files changed, 127 insertions(+), 3 deletions(-)
>
> Some quick cleanups ontop, there's potential for more:
>
> ---
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index ffee99046942..486509030d3a 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -2091,13 +2091,16 @@ config CRASH_DUMP
> For more details see Documentation/admin-guide/kdump/kdump.rst
>
> config CRASH_HOTPLUG
> - bool "kernel updates of crash elfcorehdr"
> + bool "Update the crash elfcorehdr on system configuration changes"
> default n
> depends on CRASH_DUMP && (HOTPLUG_CPU || MEMORY_HOTPLUG)
> help
> - Enable the kernel to directly update the crash elfcorehdr (which
> - contains the list of CPUs and memory regions to be dumped upon
> - a crash) in response to hot plug/unplug of CPUs or memory.
> + Enable direct updates to the crash elfcorehdr (which contains
> + the list of CPUs and memory regions to be dumped upon a crash)
> + in response to hot plug/unplug of CPUs or memory. This is a much
> + more advanced approach than userspace attempting that.
> +
> + If unsure, say Y.
Done!

>
> config KEXEC_JUMP
> bool "kexec jump"
> diff --git a/arch/x86/kernel/crash.c b/arch/x86/kernel/crash.c
> index d2238bcf8106..d26f208e582e 100644
> --- a/arch/x86/kernel/crash.c
> +++ b/arch/x86/kernel/crash.c
> @@ -413,25 +413,32 @@ int crash_load_segments(struct kimage *image)
> image->elf_headers_sz = kbuf.bufsz;
> kbuf.memsz = kbuf.bufsz;
>
> -#ifdef CONFIG_CRASH_HOTPLUG
> - /* Ensure elfcorehdr segment large enough for hotplug changes */
> - unsigned long pnum = 2; /* VMCOREINFO and kernel_map */
> -
> - if (IS_ENABLED(CONFIG_HOTPLUG_CPU))
> - pnum += CONFIG_NR_CPUS_DEFAULT;
> - if (IS_ENABLED(CONFIG_MEMORY_HOTPLUG))
> - pnum += CRASH_MAX_MEMORY_RANGES;
> - if (pnum < (unsigned long)PN_XNUM) {
> - kbuf.memsz = pnum * sizeof(Elf64_Phdr);
> - kbuf.memsz += sizeof(Elf64_Ehdr);
> - image->elfcorehdr_index = image->nr_segments;
> - image->elfcorehdr_index_valid = true;
> - /* Mark as usable to crash kernel, else crash kernel fails on boot */
> - image->elf_headers_sz = kbuf.memsz;
> - } else {
> - pr_err("number of Phdrs %lu exceeds max\n", pnum);
> + if (IS_ENABLED(CONFIG_CRASH_HOTPLUG)) {
> + /*
> + * Ensure the elfcorehdr segment large enough for hotplug changes.
> + * Start with VMCOREINFO and kernel_map:
> + */
> + unsigned long pnum = 2;
> +
> + if (IS_ENABLED(CONFIG_HOTPLUG_CPU))
> + pnum += CONFIG_NR_CPUS_DEFAULT;
> +
> + if (IS_ENABLED(CONFIG_MEMORY_HOTPLUG))
> + pnum += CRASH_MAX_MEMORY_RANGES;
> +
> + if (pnum < (unsigned long)PN_XNUM) {
> + kbuf.memsz = pnum * sizeof(Elf64_Phdr);
> + kbuf.memsz += sizeof(Elf64_Ehdr);
> +
> + image->elfcorehdr_index = image->nr_segments;
> + image->elfcorehdr_index_valid = true;
> +
> + /* Mark as usable to crash kernel, else crash kernel fails on boot */
> + image->elf_headers_sz = kbuf.memsz;
> + } else {
> + pr_err("number of Phdrs %lu exceeds max\n", pnum);
> + }
Done, converted this block to an IS_ENABLED(CONFIG_CRASH_HOTPLUG).

> }
> -#endif
>
> kbuf.buf_align = ELF_CORE_HEADER_ALIGN;
> kbuf.mem = KEXEC_BUF_MEM_UNKNOWN;
> @@ -463,10 +470,9 @@ int crash_load_segments(struct kimage *image)
> */
> void arch_crash_handle_hotplug_event(struct kimage *image)
> {
> + void *elfbuf, *old_elfcorehdr;
> unsigned long mem, memsz;
> - unsigned long elfsz = 0;
> - void *elfbuf = NULL;
> - void *ptr;
> + unsigned long elfsz;
Done, note that I leave elfbuf initialized to NULL as its use in prepare_elf_headers() does not
necessarily write it, so it could otherwise have been used uninitialized.

>
> /*
> * Create the new elfcorehdr reflecting the changes to CPU and/or
> @@ -489,26 +495,24 @@ void arch_crash_handle_hotplug_event(struct kimage *image)
> goto out;
> }
>
> - /*
> - * Copy new elfcorehdr over the old elfcorehdr at destination.
> - */
> - ptr = arch_map_crash_pages(mem, memsz);
> - if (ptr) {
> - /*
> - * Temporarily invalidate the crash image while the
> - * elfcorehdr is updated.
> - */
> - xchg(&kexec_crash_image, NULL);
> - memcpy_flushcache(ptr, elfbuf, elfsz);
> - xchg(&kexec_crash_image, image);
> - arch_unmap_crash_pages(ptr);
> - pr_debug("updated elfcorehdr\n");
> - } else {
> + /* Copy new elfcorehdr over the old elfcorehdr at destination. */
> + old_elfcorehdr = arch_map_crash_pages(mem, memsz);
> + if (!old_elfcorehdr) {
> pr_err("updating elfcorehdr failed\n");
> + goto out;
> }
Done, changed to old_elfcorehdr and error/early-out.

>
> + /*
> + * Temporarily invalidate the crash image while the elfcorehdr
> + * is updated.
> + * */
> + xchg(&kexec_crash_image, NULL);
> + memcpy_flushcache(old_elfcorehdr, elfbuf, elfsz);
> + xchg(&kexec_crash_image, image);
> + arch_unmap_crash_pages(old_elfcorehdr);
> + pr_debug("updated elfcorehdr\n");
> +
> out:
> - if (elfbuf)
> - vfree(elfbuf);
> + vfree(elfbuf);
> }
> #endif
>
Done.

Thanks!
eric