Hi Bhupesh,

(CC: +mips list, looks like mips is missing vmcore's KERNELOFFSET too.
Start of this thread: https://lkml.org/lkml/2018/7/18/951 )

On 19/07/18 15:55, Bhupesh Sharma wrote:

On Thu, Jul 19, 2018 at 5:01 PM, James Morse [off-list ref] wrote:

quoted

On 18/07/18 22:37, Bhupesh Sharma wrote:

quoted

Include KASLR offset in VMCOREINFO ELF notes to assist in debugging.

makedumpfile user-space utility will need fixup to use this KASLR offset
to work with cases where we need to find a way to translate symbol
address from vmlinux to kernel run time address in case of KASLR boot on
arm64.

You need the kernel VA for a symbol. Isn't this what kallsyms is for?
| root at frikadeller:~# cat /proc/kallsyms | grep swapper_pg_dir
| ffff5404610d0000 B swapper_pg_dir

Its already used by other archs like x86_64.
Its just that we are enabling this feature for arm64 now that the
KASLR boot cases are more widely seen on arm64 boards (as newer EFI
firmware implementations are available which support EFI_RNG_PROTOCOL
and hence KASLR boot).

And we want existing user-space application(s) to work similarly on
arm64 distributions as they work historically on other archs like
x86_64 (in most cases the user-space user is not even aware, if he is
developing for or using an underlying hardware which is arm64 or
x86_64)

Aha, so its ABI. This is the information that should be in the commit message as
it describes why this patch should be merged.

... Ideally core code would do this, that way this information won't be missed
when an architecture adds KASLR support.

But mips has CONFIG_RANDOMIZE_BASE, and doesn't provide kaslr_offset(),
and x86 always provides this value, not just if CONFIG_RANDOMIZE_BASE is
selected. I can't see a way to do this without touching all three architectures.
(we can try and tidy it up once its clear whether mips needs this too)


I think the patch is fine, but could you re-post it with a commit message that
describes that vmcore parsing in user-space already expects this value in the
notes. We're providing it for portability of those existing tools with x86.

quoted

I picked swapper_pg_dir, but you could use any of the vmcore:SYMBOL() addresses
to work out this offset. (you should expect the kernel to rename these symbols
at a whim).

Perhaps you missed what the above makedumpfile command was doing, so
let me summarize again:

Yes, I glossed over it, all that seemed relevant is you are trying to find the
kernel-va of a symbol from the value in the vmlinux.

as it breaks the
existing user-space use-cases and the .conf file can be user-defined
(and hence he can pick any symbol/functions

My suggestion was you can calculate the offset between the link-time and
run-time address from information you already have. You just need one of each.
This would be better as its independent of how the kernel does the relocation.

But, this is irrelevant as 'KERNELOFFSET=' is already an ABI string.

which might not even be present in 'kallsyms').

Eh? How can that happen? I thought even modules had their symbols added to kallsyms.



Thanks,

James