linux-kernel - Re: [PATCH] recordmcount: only record relocation of type R_AARCH64

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Message-ID: <20200722185047.GW17377@ltoracle>
Date:   Wed, 22 Jul 2020 20:50:47 +0200
From:   Gregory Herrero <gregory.herrero@...cle.com>
To:     Catalin Marinas <catalin.marinas@....com>
Cc:     Steven Rostedt <rostedt@...dmis.org>, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org, Will Deacon <will@...nel.org>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] recordmcount: only record relocation of type
 R_AARCH64_CALL26 on arm64.

On Wed, Jul 22, 2020 at 05:36:50PM +0100, Catalin Marinas wrote:
> On Fri, Jul 17, 2020 at 01:30:03PM -0400, Steven Rostedt wrote:
> > On Fri, 17 Jul 2020 16:33:38 +0200
> > gregory.herrero@...cle.com wrote:
> > > From: Gregory Herrero <gregory.herrero@...cle.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@...dmis.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.
> 
Thanks for catching this.
It should be as below instead:

  Fixes: af64d2aa872a ("ftrace: Add arm64 support to recordmcount")

Should I send a V2?

Thanks,
Greg