Re: [PATCH v2 2/2] ftrace: recordmcount: Handle sections with no non-weak symbols

From: Naveen N. Rao
Date: Fri Apr 29 2022 - 13:39:47 EST

Steven Rostedt wrote:
On Thu, 28 Apr 2022 22:49:52 +0530
"Naveen N. Rao" <naveen.n.rao@xxxxxxxxxxxxxxxxxx> wrote:

But, with ppc64 elf abi v1 which only supports the old -pg flag, mcount
location can differ between the weak and non-weak variants of a
function. In such scenarios, one of the two mcount entries will be
invalid. Such architectures need to validate mcount locations by
ensuring that the instruction(s) at those locations are as expected. On
powerpc, this can be a simple check to ensure that the instruction is a
'bl'. This check can be further tightened as necessary.

I was thinking about this more, and I was thinking that we could create
another section; Perhaps __mcount_loc_weak. And place these in that
section. That way, we could check if these symbols to see if there's
already a symbol for it, and if there is, then drop it.

If I'm understanding your suggestion right:
- we now create a new section in each object file: __mcount_loc_weak, and capture such relocations using weak symbols there.
- we then ask the linker to put these separately between, say, __start_mcount_loc_weak and __stop_mcount_loc_weak
- on ftrace init, we go through entries in this range, but discard those that belong to functions that also have an entry between __start_mcount_loc and __stop_mcount loc.

The primary issue I see here is that the mcount locations within the new weak section will end up being offsets from a different function in vmlinux, since the linker does not create a symbol for the weak functions that were over-ridden.

As an example, in the issue described in this patch set, if powerpc starts over-riding kexec_arch_apply_relocations(), then the current weak implementation in kexec_file.o gets carried over to the final vmlinux, but the instructions will instead appear under the previous function in kexec_file.o: crash_prepare_elf64_headers(). This function may or may not be traced to begin with, so we won't be able to figure out if this is valid or not.


- Naveen