[PATCH] recordmcount: only record relocation of type R_AARCH64_CALL26 on arm64.

[PATCH] recordmcount: only record relocation of type R_AARCH64_CALL26 on arm64.

[gregory.herrero]

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.

Fixes: ed60453fa8f8 ("ARM: 6511/1: ftrace: add ARM support for C version of recordmcount")
Signed-off-by: Gregory Herrero <gregory.herrero@oracle.com>
---
 scripts/recordmcount.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/scripts/recordmcount.c b/scripts/recordmcount.c
index 7225107a9aaf..e59022b3f125 100644
--- a/scripts/recordmcount.c
+++ b/scripts/recordmcount.c
@@ -434,6 +434,11 @@ static int arm_is_fake_mcount(Elf32_Rel const *rp)
 	return 1;
 }
 
+static int arm64_is_fake_mcount(Elf64_Rel const *rp)
+{
+	return ELF64_R_TYPE(w(rp->r_info)) != R_AARCH64_CALL26;
+}
+
 /* 64-bit EM_MIPS has weird ELF64_Rela.r_info.
  * http://techpubs.sgi.com/library/manuals/4000/007-4658-001/pdf/007-4658-001.pdf
  * We interpret Table 29 Relocation Operation (Elf64_Rel, Elf64_Rela) [p.40]
@@ -547,6 +552,7 @@ static int do_file(char const *const fname)
 		make_nop = make_nop_arm64;
 		rel_type_nop = R_AARCH64_NONE;
 		ideal_nop = ideal_nop4_arm64;
+		is_fake_mcount64 = arm64_is_fake_mcount;
 		break;
 	case EM_IA_64:	reltype = R_IA64_IMM64; break;
 	case EM_MIPS:	/* reltype: e_class    */ break;
-- 
2.27.0

Re: [PATCH] recordmcount: only record relocation of type R_AARCH64_CALL26 on arm64.

[Greg KH]

On Fri, Jul 17, 2020 at 04:33:38PM +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.
> 
> Fixes: ed60453fa8f8 ("ARM: 6511/1: ftrace: add ARM support for C version of recordmcount")
> Signed-off-by: Gregory Herrero <gregory.herrero@oracle.com>
> ---
>  scripts/recordmcount.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/scripts/recordmcount.c b/scripts/recordmcount.c
> index 7225107a9aaf..e59022b3f125 100644
> --- a/scripts/recordmcount.c
> +++ b/scripts/recordmcount.c
> @@ -434,6 +434,11 @@ static int arm_is_fake_mcount(Elf32_Rel const *rp)
>  	return 1;
>  }
>  
> +static int arm64_is_fake_mcount(Elf64_Rel const *rp)
> +{
> +	return ELF64_R_TYPE(w(rp->r_info)) != R_AARCH64_CALL26;
> +}
> +
>  /* 64-bit EM_MIPS has weird ELF64_Rela.r_info.
>   * http://techpubs.sgi.com/library/manuals/4000/007-4658-001/pdf/007-4658-001.pdf
>   * We interpret Table 29 Relocation Operation (Elf64_Rel, Elf64_Rela) [p.40]
> @@ -547,6 +552,7 @@ static int do_file(char const *const fname)
>  		make_nop = make_nop_arm64;
>  		rel_type_nop = R_AARCH64_NONE;
>  		ideal_nop = ideal_nop4_arm64;
> +		is_fake_mcount64 = arm64_is_fake_mcount;
>  		break;
>  	case EM_IA_64:	reltype = R_IA64_IMM64; break;
>  	case EM_MIPS:	/* reltype: e_class    */ break;
> -- 
> 2.27.0
> 

<formletter>

This is not the correct way to submit patches for inclusion in the
stable kernel tree.  Please read:
    https://www.kernel.org/doc/html/latest/process/stable-kernel-rules.html
for how to do this properly.

</formletter>

Re: [PATCH] recordmcount: only record relocation of type R_AARCH64_CALL26 on arm64.

[Steven Rostedt]

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>

-- Steve


> Fixes: ed60453fa8f8 ("ARM: 6511/1: ftrace: add ARM support for C version of recordmcount")
> Signed-off-by: Gregory Herrero <gregory.herrero@oracle.com>
> ---
>  scripts/recordmcount.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/scripts/recordmcount.c b/scripts/recordmcount.c
> index 7225107a9aaf..e59022b3f125 100644
> --- a/scripts/recordmcount.c
> +++ b/scripts/recordmcount.c
> @@ -434,6 +434,11 @@ static int arm_is_fake_mcount(Elf32_Rel const *rp)
>  	return 1;
>  }
>  
> +static int arm64_is_fake_mcount(Elf64_Rel const *rp)
> +{
> +	return ELF64_R_TYPE(w(rp->r_info)) != R_AARCH64_CALL26;
> +}
> +
>  /* 64-bit EM_MIPS has weird ELF64_Rela.r_info.
>   * http://techpubs.sgi.com/library/manuals/4000/007-4658-001/pdf/007-4658-001.pdf
>   * We interpret Table 29 Relocation Operation (Elf64_Rel, Elf64_Rela) [p.40]
> @@ -547,6 +552,7 @@ static int do_file(char const *const fname)
>  		make_nop = make_nop_arm64;
>  		rel_type_nop = R_AARCH64_NONE;
>  		ideal_nop = ideal_nop4_arm64;
> +		is_fake_mcount64 = arm64_is_fake_mcount;
>  		break;
>  	case EM_IA_64:	reltype = R_IA64_IMM64; break;
>  	case EM_MIPS:	/* reltype: e_class    */ break;

Re: [PATCH] recordmcount: only record relocation of type R_AARCH64_CALL26 on arm64.

[Gregory Herrero]

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>
> 
> -- Steve
> 
Thanks Steve.
Should I send a V2 to add 'Cc: stable@vger.kernel.org' in the commit
description or can someone take care of it when adding the commit to
the tree?

Thanks,
Greg

> 
> > Fixes: ed60453fa8f8 ("ARM: 6511/1: ftrace: add ARM support for C version of recordmcount")
> > Signed-off-by: Gregory Herrero <gregory.herrero@oracle.com>
> > ---
> >  scripts/recordmcount.c | 6 ++++++
> >  1 file changed, 6 insertions(+)
> > 
> > diff --git a/scripts/recordmcount.c b/scripts/recordmcount.c
> > index 7225107a9aaf..e59022b3f125 100644
> > --- a/scripts/recordmcount.c
> > +++ b/scripts/recordmcount.c
> > @@ -434,6 +434,11 @@ static int arm_is_fake_mcount(Elf32_Rel const *rp)
> >  	return 1;
> >  }
> >  
> > +static int arm64_is_fake_mcount(Elf64_Rel const *rp)
> > +{
> > +	return ELF64_R_TYPE(w(rp->r_info)) != R_AARCH64_CALL26;
> > +}
> > +
> >  /* 64-bit EM_MIPS has weird ELF64_Rela.r_info.
> >   * http://techpubs.sgi.com/library/manuals/4000/007-4658-001/pdf/007-4658-001.pdf
> >   * We interpret Table 29 Relocation Operation (Elf64_Rel, Elf64_Rela) [p.40]
> > @@ -547,6 +552,7 @@ static int do_file(char const *const fname)
> >  		make_nop = make_nop_arm64;
> >  		rel_type_nop = R_AARCH64_NONE;
> >  		ideal_nop = ideal_nop4_arm64;
> > +		is_fake_mcount64 = arm64_is_fake_mcount;
> >  		break;
> >  	case EM_IA_64:	reltype = R_IA64_IMM64; break;
> >  	case EM_MIPS:	/* reltype: e_class    */ break;
> 

Re: [PATCH] recordmcount: only record relocation of type R_AARCH64_CALL26 on arm64.

[Steven Rostedt]

On Fri, 17 Jul 2020 22:01:19 +0200
Gregory Herrero <gregory.herrero@oracle.com> wrote:

> Thanks Steve.
> Should I send a V2 to add 'Cc: stable@vger.kernel.org' in the commit
> description or can someone take care of it when adding the commit to
> the tree?

If I was taking it, I would simply add the Cc: stable@vger.kernel.org
to the commit log, and no resend would be needed.

It's up to the ARM64 maintainers to decide in this case.

Cheers,

-- Steve

Re: [PATCH] recordmcount: only record relocation of type R_AARCH64_CALL26 on arm64.

[Catalin Marinas]

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

Re: [PATCH] recordmcount: only record relocation of type R_AARCH64_CALL26 on arm64.

[Gregory Herrero]

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@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.
> 
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

Re: [PATCH] recordmcount: only record relocation of type R_AARCH64_CALL26 on arm64.

[Mark Rutland]

Hi Gregory,

As a general thing, for patches affecting arm64 could you please Cc the
linx-arm-kernel mailing list (linux-arm-kernel@lists.infradead.org).
Some folk working on arm/arm64 aren't subscribed to LKML, and it means
patches like this may get missed.

On Fri, Jul 17, 2020 at 04:33:38PM +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

Which code specifically is this triggering for? Is this something in an
upstream kernel, or out-of-tree patches?

Can you say which toolchain you're using, too?

> So Limit the relocation type to R_AARCH64_CALL26 as in perl version of
> recordmcount.

Given our patching code expects each callsite to be:

	bl	_mcount

... this looks sane to me, and I *think* that's sound for modules too.

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

That's a 32-bit arm commit. I suspect that was meant to be:

Fixes: af64d2aa872a1747 ("ftrace: Add arm64 support to recordmcount")

> Signed-off-by: Gregory Herrero <gregory.herrero@oracle.com>
> ---
>  scripts/recordmcount.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/scripts/recordmcount.c b/scripts/recordmcount.c
> index 7225107a9aaf..e59022b3f125 100644
> --- a/scripts/recordmcount.c
> +++ b/scripts/recordmcount.c
> @@ -434,6 +434,11 @@ static int arm_is_fake_mcount(Elf32_Rel const *rp)
>  	return 1;
>  }
>  
> +static int arm64_is_fake_mcount(Elf64_Rel const *rp)
> +{
> +	return ELF64_R_TYPE(w(rp->r_info)) != R_AARCH64_CALL26;
> +}
> +
>  /* 64-bit EM_MIPS has weird ELF64_Rela.r_info.
>   * http://techpubs.sgi.com/library/manuals/4000/007-4658-001/pdf/007-4658-001.pdf
>   * We interpret Table 29 Relocation Operation (Elf64_Rel, Elf64_Rela) [p.40]
> @@ -547,6 +552,7 @@ static int do_file(char const *const fname)
>  		make_nop = make_nop_arm64;
>  		rel_type_nop = R_AARCH64_NONE;
>  		ideal_nop = ideal_nop4_arm64;
> +		is_fake_mcount64 = arm64_is_fake_mcount;
>  		break;

As above, I think this is sound, but if you could answer my questions
that'd be helpful.

Thanks,
Mark.

>  	case EM_IA_64:	reltype = R_IA64_IMM64; break;
>  	case EM_MIPS:	/* reltype: e_class    */ break;
> -- 
> 2.27.0
> 

Re: [PATCH] recordmcount: only record relocation of type R_AARCH64_CALL26 on arm64.

[Gregory Herrero]

Hi Mark,

On Thu, Jul 23, 2020 at 12:52:16PM +0100, Mark Rutland wrote:
> Hi Gregory,
> 
> As a general thing, for patches affecting arm64 could you please Cc the
> linx-arm-kernel mailing list (linux-arm-kernel@lists.infradead.org).
> Some folk working on arm/arm64 aren't subscribed to LKML, and it means
> patches like this may get missed.
> 
Got it, I will do that next time.

> On Fri, Jul 17, 2020 at 04:33:38PM +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
> 
> Which code specifically is this triggering for? Is this something in an
> upstream kernel, or out-of-tree patches?
> 
We faced this issue while porting Ksplice on ARM64 architecture.  So
that's an out-of-tree module. And we got this issue because we have
multiple references to '_mcount' like the one described in the commit
description of this patch.

> Can you say which toolchain you're using, too?
> 
We are using native gcc version: gcc (GCC) 7.3.0 20180125 (Red Hat 7.3.0-5)
And native binutils 2.31.1.

> > So Limit the relocation type to R_AARCH64_CALL26 as in perl version of
> > recordmcount.
> 
> Given our patching code expects each callsite to be:
> 
> 	bl	_mcount
> 
> ... this looks sane to me, and I *think* that's sound for modules too.
> 
Ok great.

> > Fixes: ed60453fa8f8 ("ARM: 6511/1: ftrace: add ARM support for C version of recordmcount")
> 
> That's a 32-bit arm commit. I suspect that was meant to be:
> 
> Fixes: af64d2aa872a1747 ("ftrace: Add arm64 support to recordmcount")
> 
Right.

> > Signed-off-by: Gregory Herrero <gregory.herrero@oracle.com>
> > ---
> >  scripts/recordmcount.c | 6 ++++++
> >  1 file changed, 6 insertions(+)
> > 
> > diff --git a/scripts/recordmcount.c b/scripts/recordmcount.c
> > index 7225107a9aaf..e59022b3f125 100644
> > --- a/scripts/recordmcount.c
> > +++ b/scripts/recordmcount.c
> > @@ -434,6 +434,11 @@ static int arm_is_fake_mcount(Elf32_Rel const *rp)
> >  	return 1;
> >  }
> >  
> > +static int arm64_is_fake_mcount(Elf64_Rel const *rp)
> > +{
> > +	return ELF64_R_TYPE(w(rp->r_info)) != R_AARCH64_CALL26;
> > +}
> > +
> >  /* 64-bit EM_MIPS has weird ELF64_Rela.r_info.
> >   * http://techpubs.sgi.com/library/manuals/4000/007-4658-001/pdf/007-4658-001.pdf
> >   * We interpret Table 29 Relocation Operation (Elf64_Rel, Elf64_Rela) [p.40]
> > @@ -547,6 +552,7 @@ static int do_file(char const *const fname)
> >  		make_nop = make_nop_arm64;
> >  		rel_type_nop = R_AARCH64_NONE;
> >  		ideal_nop = ideal_nop4_arm64;
> > +		is_fake_mcount64 = arm64_is_fake_mcount;
> >  		break;
> 
> As above, I think this is sound, but if you could answer my questions
> that'd be helpful.
> 
Thanks for the review,
Greg