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Date:   Wed, 22 Jul 2020 17:36:50 +0100
From:   Catalin Marinas <catalin.marinas@arm.com>
To:     Steven Rostedt <rostedt@goodmis.org>
Cc:     gregory.herrero@oracle.com, linux-kernel@vger.kernel.org,
        stable@vger.kernel.org, Will Deacon <will@kernel.org>,
        linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH] recordmcount: only record relocation of type
 R_AARCH64_CALL26 on arm64.
Message-ID: <20200722163650.GI27540@gaia>
References: <20200717143338.19302-1-gregory.herrero@oracle.com>
 <20200717133003.025f2096@oasis.local.home>
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On Fri, Jul 17, 2020 at 01:30:03PM -0400, Steven Rostedt wrote:
> On Fri, 17 Jul 2020 16:33:38 +0200
> gregory.herrero@oracle.com wrote:
> > From: Gregory Herrero <gregory.herrero@oracle.com>
> > Currently, if a section has a relocation to '_mcount' symbol, a new
> > __mcount_loc entry will be added whatever the relocation type is.
> > This is problematic when a relocation to '_mcount' is in the middle of a
> > section and is not a call for ftrace use.
> > 
> > Such relocation could be generated with below code for example:
> >     bool is_mcount(unsigned long addr)
> >     {
> >         return (target == (unsigned long) &_mcount);
> >     }
> > 
> > With this snippet of code, ftrace will try to patch the mcount location
> > generated by this code on module load and fail with:
> > 
> >     Call trace:
> >      ftrace_bug+0xa0/0x28c
> >      ftrace_process_locs+0x2f4/0x430
> >      ftrace_module_init+0x30/0x38
> >      load_module+0x14f0/0x1e78
> >      __do_sys_finit_module+0x100/0x11c
> >      __arm64_sys_finit_module+0x28/0x34
> >      el0_svc_common+0x88/0x194
> >      el0_svc_handler+0x38/0x8c
> >      el0_svc+0x8/0xc
> >     ---[ end trace d828d06b36ad9d59 ]---
> >     ftrace failed to modify
> >     [<ffffa2dbf3a3a41c>] 0xffffa2dbf3a3a41c
> >      actual:   66:a9:3c:90
> >     Initializing ftrace call sites
> >     ftrace record flags: 2000000
> >      (0)
> >     expected tramp: ffffa2dc6cf66724
> > 
> > So Limit the relocation type to R_AARCH64_CALL26 as in perl version of
> > recordmcount.
> 
> I'd rather have this go through the arm64 tree, as they can test it
> better than I can.
> 
> Acked-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

Thanks Steve.

> > Fixes: ed60453fa8f8 ("ARM: 6511/1: ftrace: add ARM support for C version of recordmcount")

This Fixes tag looks wrong. The above commit was for arm32.

-- 
Catalin