From: ebiederm@xmission.com (Eric W. Biederman)
To: Xunlei Pang <xlpang@redhat.com>
Cc: linux-kernel@vger.kernel.org, kexec@lists.infradead.org,
        akpm@linux-foundation.org, Dave Young <dyoung@redhat.com>,
        Baoquan He <bhe@redhat.com>, Petr Tesarik <ptesarik@suse.cz>,
        Mahesh Salgaonkar <mahesh@linux.vnet.ibm.com>
References: <1489989033-1179-1-git-send-email-xlpang@redhat.com>
Date: Mon, 20 Mar 2017 22:33:07 -0500
In-Reply-To: <1489989033-1179-1-git-send-email-xlpang@redhat.com> (Xunlei
        Pang's message of "Mon, 20 Mar 2017 13:50:31 +0800")
Message-ID: <87pohbz4lo.fsf@xmission.com>
User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/25.1 (gnu/linux)
MIME-Version: 1.0
Content-Type: text/plain
Subject: Re: [PATCH v3 1/3] kexec: Move vmcoreinfo out of the kernel's .bss section
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 5438
Lines: 148

Xunlei Pang <xlpang@redhat.com> writes:

> As Eric said,
> "what we need to do is move the variable vmcoreinfo_note out
> of the kernel's .bss section.  And modify the code to regenerate
> and keep this information in something like the control page.
>
> Definitely something like this needs a page all to itself, and ideally
> far away from any other kernel data structures.  I clearly was not
> watching closely the data someone decided to keep this silly thing
> in the kernel's .bss section."
>
> This patch allocates extra pages for these vmcoreinfo_XXX variables,
> one advantage is that it enhances some safety of vmcoreinfo, because
> vmcoreinfo now is kept far away from other kernel data structures.

Can you preceed this patch with a patch that removes CRASHTIME from
vmcoreinfo?  If someone actually cares we can add a separate note that holds
a 64bit crashtime in the per cpu notes.  

As we are looking at reliability concerns removing CRASHTIME should make
everything in vmcoreinfo a boot time constant.  Which should simplify
everything considerably.

Which means we only need to worry abou the per-cpu notes being written
at the time of a crash.

> Suggested-by: Eric Biederman <ebiederm@xmission.com>
> Signed-off-by: Xunlei Pang <xlpang@redhat.com>
> ---
>  arch/ia64/kernel/machine_kexec.c |  5 -----
>  arch/x86/kernel/crash.c          |  2 +-
>  include/linux/kexec.h            |  2 +-
>  kernel/kexec_core.c              | 29 ++++++++++++++++++++++++-----
>  kernel/ksysfs.c                  |  2 +-
>  5 files changed, 27 insertions(+), 13 deletions(-)
>
> diff --git a/arch/ia64/kernel/machine_kexec.c b/arch/ia64/kernel/machine_kexec.c
> index 599507b..c14815d 100644
> --- a/arch/ia64/kernel/machine_kexec.c
> +++ b/arch/ia64/kernel/machine_kexec.c
> @@ -163,8 +163,3 @@ void arch_crash_save_vmcoreinfo(void)
>  #endif
>  }
>  
> -phys_addr_t paddr_vmcoreinfo_note(void)
> -{
> -	return ia64_tpa((unsigned long)(char *)&vmcoreinfo_note);
> -}
> -
> diff --git a/arch/x86/kernel/crash.c b/arch/x86/kernel/crash.c
> index 3741461..4d35fbb 100644
> --- a/arch/x86/kernel/crash.c
> +++ b/arch/x86/kernel/crash.c
> @@ -456,7 +456,7 @@ static int prepare_elf64_headers(struct crash_elf_data *ced,
>  	bufp += sizeof(Elf64_Phdr);
>  	phdr->p_type = PT_NOTE;
>  	phdr->p_offset = phdr->p_paddr = paddr_vmcoreinfo_note();
> -	phdr->p_filesz = phdr->p_memsz = sizeof(vmcoreinfo_note);
> +	phdr->p_filesz = phdr->p_memsz = VMCOREINFO_NOTE_SIZE;
>  	(ehdr->e_phnum)++;
>  
>  #ifdef CONFIG_X86_64
> diff --git a/include/linux/kexec.h b/include/linux/kexec.h
> index e98e546..f1c601b 100644
> --- a/include/linux/kexec.h
> +++ b/include/linux/kexec.h
> @@ -317,7 +317,7 @@ extern void *kexec_purgatory_get_symbol_addr(struct kimage *image,
>  extern struct resource crashk_low_res;
>  typedef u32 note_buf_t[KEXEC_NOTE_BYTES/4];
>  extern note_buf_t __percpu *crash_notes;
> -extern u32 vmcoreinfo_note[VMCOREINFO_NOTE_SIZE/4];
> +extern u32 *vmcoreinfo_note;
>  extern size_t vmcoreinfo_size;
>  extern size_t vmcoreinfo_max_size;
>  
> diff --git a/kernel/kexec_core.c b/kernel/kexec_core.c
> index bfe62d5..e3a4bda 100644
> --- a/kernel/kexec_core.c
> +++ b/kernel/kexec_core.c
> @@ -52,10 +52,10 @@
>  note_buf_t __percpu *crash_notes;
>  
>  /* vmcoreinfo stuff */
> -static unsigned char vmcoreinfo_data[VMCOREINFO_BYTES];
> -u32 vmcoreinfo_note[VMCOREINFO_NOTE_SIZE/4];
> +static unsigned char *vmcoreinfo_data;
>  size_t vmcoreinfo_size;
> -size_t vmcoreinfo_max_size = sizeof(vmcoreinfo_data);
> +size_t vmcoreinfo_max_size = VMCOREINFO_BYTES;
> +u32 *vmcoreinfo_note;
>  
>  /* Flag to indicate we are going to kexec a new kernel */
>  bool kexec_in_progress = false;
> @@ -1369,6 +1369,9 @@ static void update_vmcoreinfo_note(void)
>  
>  void crash_save_vmcoreinfo(void)
>  {
> +	if (!vmcoreinfo_note)
> +		return;
> +
>  	vmcoreinfo_append_str("CRASHTIME=%ld\n", get_seconds());
>  	update_vmcoreinfo_note();
>  }
> @@ -1397,13 +1400,29 @@ void vmcoreinfo_append_str(const char *fmt, ...)
>  void __weak arch_crash_save_vmcoreinfo(void)
>  {}
>  
> -phys_addr_t __weak paddr_vmcoreinfo_note(void)
> +phys_addr_t paddr_vmcoreinfo_note(void)
>  {
> -	return __pa_symbol((unsigned long)(char *)&vmcoreinfo_note);
> +	return __pa(vmcoreinfo_note);
>  }
>  
>  static int __init crash_save_vmcoreinfo_init(void)
>  {
> +	/* One page should be enough for VMCOREINFO_BYTES under all archs */
> +	vmcoreinfo_data = (unsigned char *)get_zeroed_page(GFP_KERNEL);
> +	if (!vmcoreinfo_data) {
> +		pr_warn("Memory allocation for vmcoreinfo_data failed\n");
> +		return -ENOMEM;
> +	}
> +
> +	vmcoreinfo_note = alloc_pages_exact(VMCOREINFO_NOTE_SIZE,
> +						GFP_KERNEL | __GFP_ZERO);
> +	if (!vmcoreinfo_note) {
> +		free_page((unsigned long)vmcoreinfo_data);
> +		vmcoreinfo_data = NULL;
> +		pr_warn("Memory allocation for vmcoreinfo_note failed\n");
> +		return -ENOMEM;
> +	}
> +
>  	VMCOREINFO_OSRELEASE(init_uts_ns.name.release);
>  	VMCOREINFO_PAGESIZE(PAGE_SIZE);
>  
> diff --git a/kernel/ksysfs.c b/kernel/ksysfs.c
> index ee1bc1b..9de6fcc 100644
> --- a/kernel/ksysfs.c
> +++ b/kernel/ksysfs.c
> @@ -130,7 +130,7 @@ static ssize_t vmcoreinfo_show(struct kobject *kobj,
>  {
>  	phys_addr_t vmcore_base = paddr_vmcoreinfo_note();
>  	return sprintf(buf, "%pa %x\n", &vmcore_base,
> -		       (unsigned int)sizeof(vmcoreinfo_note));
> +			(unsigned int)VMCOREINFO_NOTE_SIZE);
>  }
>  KERNEL_ATTR_RO(vmcoreinfo);