Re: [PATCH] kexec: Update vmcoreinfo after crash happened

Xunlei Pang
2017-03-16T13:38:18Z

On 03/16/2017 at 09:18 PM, Baoquan He wrote:
> On 03/16/17 at 08:36pm, Xunlei Pang wrote:
>> On 03/16/2017 at 08:27 PM, Baoquan He wrote:
>>> Hi Xunlei,
>>>
>>> Did you really see this ever happened? Because the vmcore size estimate
>>> feature, namely --mem-usage option of makedumpfile, depends on the
>>> vmcoreinfo in 1st kernel, your change will break it.
>> Hi Baoquan,
>>
>> I can reproduce it using a kernel module which modifies the vmcoreinfo,
>> so it's a problem can actually happen.
>>
>>> If not, it could be not good to change that.
>> That's a good point, then I guess we can keep the crash_save_vmcoreinfo_init(),
>> and store again all the vmcoreinfo after crash. What do you think?
> Well, then it will make makedumpfile segfault happen too when execute
> below command in 1st kernel if it existed:
> makedumpfile --mem-usage /proc/kcore

Yes, if the initial vmcoreinfo data was modified before "makedumpfile --mem-usage", it might happen,
after all the system is going something wrong. And that's why we deploy kdump service at the very
beginning when the system has a low possibility of going wrong.

But we have to guarantee kdump vmcore can be generated correctly as possible as it can.

>
> So we still need to face that problem and need fix it. vmcoreinfo_note
> is in kernel data area, how does module intrude into this area? And can
> we fix the module code?
>

Bugs always exist in products, we can't know what will happen and fix all the errors,
that's why we need kdump.

I think the following update should guarantee the correct vmcoreinfo for kdump.

---
kernel/kexec_core.c | 22 +++++++++++++++-------
1 file changed, 15 insertions(+), 7 deletions(-)

diff --git a/kernel/kexec_core.c b/kernel/kexec_core.c
index bfe62d5..0f7b328 100644
--- a/kernel/kexec_core.c
+++ b/kernel/kexec_core.c
@@ -1367,12 +1367,6 @@ static void update_vmcoreinfo_note(void)
final_note(buf);
}

-void crash_save_vmcoreinfo(void)
-{
- vmcoreinfo_append_str("CRASHTIME=%ld\n", get_seconds());
- update_vmcoreinfo_note();
-}
-
void vmcoreinfo_append_str(const char *fmt, ...)
{
va_list args;
@@ -1402,7 +1396,7 @@ phys_addr_t __weak paddr_vmcoreinfo_note(void)
return __pa_symbol((unsigned long)(char *)&vmcoreinfo_note);
}

-static int __init crash_save_vmcoreinfo_init(void)
+static void do_crash_save_vmcoreinfo_init(void)
{
VMCOREINFO_OSRELEASE(init_uts_ns.name.release);
VMCOREINFO_PAGESIZE(PAGE_SIZE);
@@ -1474,6 +1468,20 @@ static int __init crash_save_vmcoreinfo_init(void)
#endif

arch_crash_save_vmcoreinfo();
+}
+
+void crash_save_vmcoreinfo(void)
+{
+ /* Save again to protect vmcoreinfo from being modified */
+ vmcoreinfo_size = 0;
+ do_crash_save_vmcoreinfo_init();
+ vmcoreinfo_append_str("CRASHTIME=%ld\n", get_seconds());
+ update_vmcoreinfo_note();
+}
+
+static int __init crash_save_vmcoreinfo_init(void)
+{
+ do_crash_save_vmcoreinfo_init();
update_vmcoreinfo_note();

return 0;
--
1.8.3.1