Re: [PATCH v3 1/3] kexec: Move vmcoreinfo out of the kernel's .bss section

[Dave Young]

On 03/20/17 at 10:33pm, Eric W. Biederman wrote:
> Xunlei Pang <xlpang@xxxxxxxxxx> 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.  

I think makedumpfile is using it, but I also vote to remove the
CRASHTIME. It is better not to do this while crashing and a makedumpfile
userspace patch is needed to drop the use of it.

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

It is a nice improvement..

> 
> 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@xxxxxxxxxxxx>
> > Signed-off-by: Xunlei Pang <xlpang@xxxxxxxxxx>
> > ---
> >  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);
> 
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Thanks
Dave