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

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