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

From: "Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: llvm@lists.linux.dev, Nick Desaulniers <ndesaulniers@google.com>,
	linux-kernel@vger.kernel.org,
	Nathan Chancellor <nathan@kernel.org>,
	linuxppc-dev@lists.ozlabs.org
Subject: Re: [PATCH v2 2/2] ftrace: recordmcount: Handle sections with no non-weak symbols
Date: Fri, 29 Apr 2022 23:09:19 +0530	[thread overview]
Message-ID: <1651252324.js9790ngjg.naveen@linux.ibm.com> (raw)
In-Reply-To: <20220428184212.18fbf438@gandalf.local.home>

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