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From:   Florian Fainelli <f.fainelli@gmail.com>
Subject: Re: [PATCH v8 0/3] ARM: Implement MODULE_PLT support in FTRACE
To:     Alexander A Sverdlin <alexander.sverdlin@nokia.com>,
        linux-arm-kernel@lists.infradead.org
Cc:     Steven Rostedt <rostedt@goodmis.org>,
        Ingo Molnar <mingo@redhat.com>,
        Russell King <linux@armlinux.org.uk>,
        linux-kernel@vger.kernel.org,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Qais Yousef <qais.yousef@arm.com>
References: <20210330114035.18575-1-alexander.sverdlin@nokia.com>
Message-ID: <a8b8dabd-6637-335d-5abc-5a88d21b4db0@gmail.com>
Date:   Thu, 1 Apr 2021 16:52:33 -0700
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On 3/30/21 4:40 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.
> 
> Changelog:
> v8:
> * Add warn suppress parameter to arm_gen_branch_link()
> v7:
> * rebased
> v6:
> * rebased
> v5:
> * BUILD_BUG_ON() ensures fixed_plts[] always fits one PLT block
> * use "for" loop instead of "while"
> * scripts/recordmcount is filtering reloc types
> v4:
> * Fixed build without CONFIG_FUNCTION_TRACER
> * Reorganized pre-allocated PLTs handling in get_module_plt(),
>   now compiler eliminates the whole FTRACE-related handling code
>     if ARRAY_SIZE(fixed_plts) == 0
>     v3:
>     * Only extend struct dyn_arch_ftrace when ARM_MODULE_PLTS is enabled
>     v2:
>     * As suggested by Steven Rostedt, refrain from tree-wide API modification,
>       save module pointer in struct dyn_arch_ftrace instead (PowerPC way)

FWIW, ftracetest did not pick up new failures (nor were new tests fixed)
with this patch series.
-- 
Florian