Re: [PATCH v7 0/2] ARM: Implement MODULE_PLT support in FTRACE

[Linus Walleij <linus.walleij@linaro.org>]

On Mon, Feb 15, 2021 at 7:32 PM Ard Biesheuvel <ardb@kernel.org> wrote:
>
> (+ Linus)
>
> On Wed, 3 Feb 2021 at 19:24, Florian Fainelli <f.fainelli@gmail.com> wrote:
> >
> > On 1/27/21 3:09 AM, Alexander A Sverdlin wrote:
> > > From: Alexander Sverdlin <alexander.sverdlin@nokia.com>
> > >
> > > FTRACE's function tracer currently doesn't always work on ARM with
> > > MODULE_PLT option enabled. If the module is loaded too far, FTRACE's
> > > code modifier cannot cope with introduced veneers and turns the
> > > function tracer off globally.
> > >
> > > ARM64 already has a solution for the problem, refer to the following
> > > patches:
> > >
> > > arm64: ftrace: emit ftrace-mod.o contents through code
> > > arm64: module-plts: factor out PLT generation code for ftrace
> > > arm64: ftrace: fix !CONFIG_ARM64_MODULE_PLTS kernels
> > > arm64: ftrace: fix building without CONFIG_MODULES
> > > arm64: ftrace: add support for far branches to dynamic ftrace
> > > arm64: ftrace: don't validate branch via PLT in ftrace_make_nop()
> > >
> > > But the presented ARM variant has just a half of the footprint in terms of
> > > the changed LoCs. It also retains the code validation-before-modification
> > > instead of switching it off.
> >
> > Ard, Russell should this be sent to the patch tracker?
>
> Apologies for the delay. Unfortunately, I don't have time to review this.
>
> Linus?

I can look at them, I just need some starting strip because I honestly
almost never use tracing, so I need to figure out how to provoke the
error before the patches and then how to test that it is gone after.

Any suggestions on a quick use case that illustrates how the problem
manifest and how to test it is gone? The errors in patch 2, what do
I need to configure in to get them? Does it manifest at modprobe?

Yours,
Linus Walleij