linux-kernel - Re: [PATCH] ftrace: Do not reference symbols in sections without size

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Date:   Mon, 15 Feb 2021 21:00:57 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH] ftrace: Do not reference symbols in sections without
 size

On Mon, 15 Feb 2021 20:06:39 -0500
Steven Rostedt <rostedt@...dmis.org> wrote:

> Strange, that when I applied this to the latest kernel on a my build
> system (binutils 2.35), it still created all the necessary mcount
> locations??

I know why it worked. If you are using the latest gcc on the latest
mainline, it will not even bother with recordmcount, and will just
create the __mcount_loc sections, as latest gcc knows about ftrace.

(this is what I get for working on a company holiday)

Since this is a toolchain issue, perhaps the correct thing to do is to
backport to stable the changes to have it build with -mrecord-mcount if
the build system enables it.

If you are using the lastest compilers to build stable releases, and
that's causing issues, then you should have the stable releases use the
latest kernel compiler options.

Greg,

Can you test the following two backports. It does change the semantics
of what is built, but then again if you are using a newer compiler to
build stable kernels, that can change things too.

96f60dfa5819a ("trace: Use -mcount-record for dynamic ftrace")
07d0408120216 ("tracing: Avoid calling cc-option -mrecord-mcount for every Makefile")

I attached the backports to 4.4. (just compiled tested, I'll test them more tomorrow)

Thanks!

-- Steve

View attachment "0001-trace-Use-mcount-record-for-dynamic-ftrace.patch" of type "text/x-patch" (1597 bytes)

View attachment "0002-tracing-Avoid-calling-cc-option-mrecord-mcount-for-e.patch" of type "text/x-patch" (2342 bytes)