linux-kernel - Re: [PATCH v15 1/7] crash: move crash_prepare_elf64

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Date:   Thu, 12 Jan 2023 11:23:18 -0600
From:   Eric DeVolder <eric.devolder@...cle.com>
To:     Sourabh Jain <sourabhjain@...ux.ibm.com>,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        kexec@...ts.infradead.org, ebiederm@...ssion.com,
        dyoung@...hat.com, bhe@...hat.com, vgoyal@...hat.com
Cc:     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 v15 1/7] crash: move crash_prepare_elf64_headers()



On 1/8/23 23:05, Sourabh Jain wrote:
> 
> On 09/12/22 21:06, Eric DeVolder wrote:
>> At the outcome of this patch set, the crash_prepare_elf64_headers()
>> is utilized on both the kexec_file_load() and kexec_load() paths. As
>> such, need to move this function out of kexec_file.c and into a
>> common location crash_core.c.
>>
>> No functionality change.
>>
>> Signed-off-by: Eric DeVolder <eric.devolder@...cle.com>
>> Acked-by: Baoquan He <bhe@...hat.com>
>> ---
>>   kernel/crash_core.c | 100 ++++++++++++++++++++++++++++++++++++++++++++
>>   kernel/kexec_file.c |  99 -------------------------------------------
>>   2 files changed, 100 insertions(+), 99 deletions(-)
>>
>> diff --git a/kernel/crash_core.c b/kernel/crash_core.c
>> index a0eb4d5cf557..46c160d14045 100644
>> --- a/kernel/crash_core.c
>> +++ b/kernel/crash_core.c
>> @@ -10,6 +10,7 @@
>>   #include <linux/utsname.h>
>>   #include <linux/vmalloc.h>
>>   #include <linux/sizes.h>
>> +#include <linux/kexec.h>
>>   #include <asm/page.h>
>>   #include <asm/sections.h>
>> @@ -314,6 +315,105 @@ static int __init parse_crashkernel_dummy(char *arg)
>>   }
>>   early_param("crashkernel", parse_crashkernel_dummy);
>> +int crash_prepare_elf64_headers(struct crash_mem *mem, int need_kernel_map,
>> +              void **addr, unsigned long *sz)
>> +{
>> +    Elf64_Ehdr *ehdr;
>> +    Elf64_Phdr *phdr;
>> +    unsigned long nr_cpus = num_possible_cpus(), nr_phdr, elf_sz;
>> +    unsigned char *buf;
>> +    unsigned int cpu, i;
>> +    unsigned long long notes_addr;
>> +    unsigned long mstart, mend;
>> +
>> +    /* extra phdr for vmcoreinfo ELF note */
>> +    nr_phdr = nr_cpus + 1;
>> +    nr_phdr += mem->nr_ranges;
>> +
>> +    /*
>> +     * kexec-tools creates an extra PT_LOAD phdr for kernel text mapping
>> +     * area (for example, ffffffff80000000 - ffffffffa0000000 on x86_64).
>> +     * I think this is required by tools like gdb. So same physical
>> +     * memory will be mapped in two ELF headers. One will contain kernel
>> +     * text virtual addresses and other will have __va(physical) addresses.
>> +     */
>> +
>> +    nr_phdr++;
>> +    elf_sz = sizeof(Elf64_Ehdr) + nr_phdr * sizeof(Elf64_Phdr);
>> +    elf_sz = ALIGN(elf_sz, ELF_CORE_HEADER_ALIGN);
> Seems like above function is out of CONFIG_KEXEC_FILE but some of the
> structure/attributes like crash_mem and ELF_CORE_HEADER_ALIGN are
> still defined under CONFIG_KEXEC_FILE (look for include/linux/kexec.h).
> 
> This leads to kernel build issue when CONFIG_KEXEC_FILE is disabled.
> 
> Thanks,
> Sourabh Jain

After looking into this for a bit, to allow hotplug without kexec_file would require quite a bit of 
code movement. Why? Because hotplug is basically built on top of (part of) the infrastructure that 
was needed for kexec_file.

I'd be inclined to suggest that KEXEC_FILE be a required dependency for CRASH_HOTPLUG, ie:

  config CRASH_HOTPLUG
         bool "Update the crash elfcorehdr on system configuration changes"
         default n
-       depends on CRASH_DUMP && (HOTPLUG_CPU || MEMORY_HOTPLUG)
+       depends on CRASH_DUMP && KEXEC_FILE && (HOTPLUG_CPU || MEMORY_HOTPLUG)


If that isn't feasible, then it would appear quite a bit of surgery is needed to properly separate 
out the items hotplug needs from kexec_file.

Thoughts?
eric