kdump: Fix exported size of vmcoreinfo note

Right now we seem to be exporting the max data size contained inside vmcoreinfo note. But this does not include the size of meta data around vmcore info data. Like name of the note and starting and ending elf_note.

I think user space expects total size and that size is put in PT_NOTE elf header. Things seem to be fine so far because we are not using vmcoreinfo note to the maximum capacity. But as it starts filling up, to capacity, at some point of time, problem will be visible.

---
 kernel/ksysfs.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Index: linux-2.6/kernel/ksysfs.c
===================================================================
@@ -126,7 +126,7 @@ static ssize_t vmcoreinfo_show(struct ko
 {
 	return sprintf(buf, "%lx %x\n",
 		       paddr_vmcoreinfo_note(),
-		       (unsigned int)vmcoreinfo_max_size);
+		       (unsigned int)sizeof(vmcoreinfo_note));
 }
 KERNEL_ATTR_RO(vmcoreinfo);
 

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CCed to old mail id of Greg. Fixing it.

urgh. This is what we get for adding undocumented interfaces.

Looking through the fd59d231f81cb0287 changelog and the various email threads it points to I can find no mention of what vmcoreinfo is *supposed* to contain. It was just kinda silently tossed in there.

So as a remedial step, could we please get this and any associated interfaces written down in a way which people can very belatedly review?

Phrases like "I think user space expects" and "Things seem to be fine so far" don't inspire a ton of confidence. What are the chances of userspace breakage here? Would it be safer/saner to leave vmcoreinfo alone and add a new vmcoreinfo2 with the altered behaviour?

Sure. I will send another patch to add some documentation for vmcoreinfo in Documentation/ABI/testing/* in a separate patch.

I don't think user space will be broken with this change. So there is no need to introduce vmcoreinfo2. This change is safe and backward compatible. More explanation on why this change is safe is below.

vmcoreinfo contains information about kernel which user space needs to know to do things like filtering. For example, various kernel config options or information about size or offset of some data structures etc. All this information is commmunicated to user space with an ELF note present in ELF /proc/vmcore file.

Currently vmcoreinfo data size is 4096. With some elf note meta data around it, actual size is 4132 bytes. But we are using barely 25% of that size. Rest is empty. So even if we tell user space that size of ELf note is 4096 and not 4132, nothing will be broken becase after around 1000 bytes, everything is zero anyway.

But once we start filling up the note to the capacity, and not report the full size of note, bad things will start happening. Either some data will be lost or tools will be confused that they did not fine the zero note at the end.

So I think this change is safe and should not break existing tools.

Thanks
Vivek

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