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objtool warning: "duplicate frame pointer save"

Linus Torvalds-2
Josh,
 my current git version (with gcc 5.3.1) makes objtool warn about
"duplicate frame pointer save" in drivers/gpu/drm/vmwgfx/vmwgfx_msg.c
for both vmw_send_msg() and vmw_host_get_guestinfo().

The reason is that VMW_PORT_HB_OUT() uses a magic instruction sequence
(a "rep outsb") to communicate with the hypervisor (it's a virtual GPU
driver for vmware), and %rbp is part of the communication. So the
inline asm does a save-and-restore of the frame pointer around the
instruction sequence.

I actually find the objtool warning to be quite reasonable, so it's
not exactly a false positive, since in this case it actually does
point out that the frame pointer won't be reliable over that
instruction sequence.

But in this particular case it just ends up being the wrong thing -
the code is what it is, and %rbp just can't have the frame information
due to annoying magic calling conventions.

Is there some way to override objtool for this situation? I hate
seeing warnings that I need to ignore, it has just too often caused me
to mistakenly ignore warnings I *should* have reacted to.

I guess a STACK_FRAME_NON_STANDARD will shut objtool up (or just
disabling it entirely for the whole file), but I was wondering about
something more targeted that could be marked in the inline asm itself
(rather than have to mark the functions that use it)

               Linus
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Re: objtool warning: "duplicate frame pointer save"

Josh Poimboeuf
On Wed, May 25, 2016 at 10:14:24AM -0700, Linus Torvalds wrote:

> Josh,
>  my current git version (with gcc 5.3.1) makes objtool warn about
> "duplicate frame pointer save" in drivers/gpu/drm/vmwgfx/vmwgfx_msg.c
> for both vmw_send_msg() and vmw_host_get_guestinfo().
>
> The reason is that VMW_PORT_HB_OUT() uses a magic instruction sequence
> (a "rep outsb") to communicate with the hypervisor (it's a virtual GPU
> driver for vmware), and %rbp is part of the communication. So the
> inline asm does a save-and-restore of the frame pointer around the
> instruction sequence.
>
> I actually find the objtool warning to be quite reasonable, so it's
> not exactly a false positive, since in this case it actually does
> point out that the frame pointer won't be reliable over that
> instruction sequence.
>
> But in this particular case it just ends up being the wrong thing -
> the code is what it is, and %rbp just can't have the frame information
> due to annoying magic calling conventions.
>
> Is there some way to override objtool for this situation? I hate
> seeing warnings that I need to ignore, it has just too often caused me
> to mistakenly ignore warnings I *should* have reacted to.
>
> I guess a STACK_FRAME_NON_STANDARD will shut objtool up (or just
> disabling it entirely for the whole file), but I was wondering about
> something more targeted that could be marked in the inline asm itself
> (rather than have to mark the functions that use it)

I used to have a STACKTOOL_IGNORE_INSN macro that would tell the tool to
skip warnings for specific instructions in inline asm:

Here's an example of it how it was used:

  https://lkml.kernel.org/r/c0c1a92df921961cae5cce208247bb8d8a975d6d.1450442274.git.jpoimboe@...

And here's the implementation of the macro:

  https://lkml.kernel.org/r/cd7778181d2a5251c3dc21811fdbcaa2c16c98c3.1450442274.git.jpoimboe@...

As it turns out, we didn't need the macro, so I removed it.  But I can
easily add something like that again to get rid of the warnings.

There were objections to having "OBJTOOL" in the names of macros, so I
guess I'll call it STACK_INSN_NON_STANDARD, unless anybody has a better
idea.

--
Josh
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Re: objtool warning: "duplicate frame pointer save"

Linus Torvalds-2
On Wed, May 25, 2016 at 10:56 AM, Josh Poimboeuf <[hidden email]> wrote:
>
> I used to have a STACKTOOL_IGNORE_INSN macro that would tell the tool to
> skip warnings for specific instructions in inline asm:
>
> Here's an example of it how it was used:

Ok, looking at that, I'm starting to suspect that it is simpler to
just use STACK_FRAME_NON_STANDARD and mark the two functions that use
this particular inline asm with the odd %rbp problem.

It's a rather special case, after all.

                 Linus
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[PATCH] drm/vmwgfx: fix "duplicate frame pointer save" warning

Josh Poimboeuf
On Wed, May 25, 2016 at 04:51:21PM -0700, Linus Torvalds wrote:

> On Wed, May 25, 2016 at 10:56 AM, Josh Poimboeuf <[hidden email]> wrote:
> >
> > I used to have a STACKTOOL_IGNORE_INSN macro that would tell the tool to
> > skip warnings for specific instructions in inline asm:
> >
> > Here's an example of it how it was used:
>
> Ok, looking at that, I'm starting to suspect that it is simpler to
> just use STACK_FRAME_NON_STANDARD and mark the two functions that use
> this particular inline asm with the odd %rbp problem.
>
> It's a rather special case, after all.

That's fine with me, it is indeed a rare case.  We can always add the
per-instruction macro later if needed.  Here's a patch.

---

From: Josh Poimboeuf <[hidden email]>
Subject: [PATCH] drm/vmwgfx: fix "duplicate frame pointer save" warning

objtool reports the following warnings:

  drivers/gpu/drm/vmwgfx/vmwgfx_msg.o: warning: objtool: vmw_send_msg()+0x107: duplicate frame pointer save
  drivers/gpu/drm/vmwgfx/vmwgfx_msg.o: warning: objtool: vmw_host_get_guestinfo()+0x252: duplicate frame pointer save

To quote Linus:

 "The reason is that VMW_PORT_HB_OUT() uses a magic instruction sequence
  (a "rep outsb") to communicate with the hypervisor (it's a virtual GPU
  driver for vmware), and %rbp is part of the communication. So the
  inline asm does a save-and-restore of the frame pointer around the
  instruction sequence.

  I actually find the objtool warning to be quite reasonable, so it's
  not exactly a false positive, since in this case it actually does
  point out that the frame pointer won't be reliable over that
  instruction sequence.

  But in this particular case it just ends up being the wrong thing -
  the code is what it is, and %rbp just can't have the frame information
  due to annoying magic calling conventions."

Silence the warnings by telling objtool to ignore the two functions
which use the VMW_PORT_HB_{IN,OUT} macros.

Reported-by: Linus Torvalds <[hidden email]>
Signed-off-by: Josh Poimboeuf <[hidden email]>
---
 drivers/gpu/drm/vmwgfx/vmwgfx_msg.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/vmwgfx/vmwgfx_msg.c b/drivers/gpu/drm/vmwgfx/vmwgfx_msg.c
index 6de283c..f0374f9 100644
--- a/drivers/gpu/drm/vmwgfx/vmwgfx_msg.c
+++ b/drivers/gpu/drm/vmwgfx/vmwgfx_msg.c
@@ -28,6 +28,7 @@
 #include <linux/slab.h>
 #include <linux/module.h>
 #include <linux/kernel.h>
+#include <linux/frame.h>
 #include <asm/hypervisor.h>
 #include "drmP.h"
 #include "vmwgfx_msg.h"
@@ -194,7 +195,7 @@ static int vmw_send_msg(struct rpc_channel *channel, const char *msg)
 
  return -EINVAL;
 }
-
+STACK_FRAME_NON_STANDARD(vmw_send_msg);
 
 
 /**
@@ -304,6 +305,7 @@ static int vmw_recv_msg(struct rpc_channel *channel, void **msg,
 
  return 0;
 }
+STACK_FRAME_NON_STANDARD(vmw_recv_msg);
 
 
 /**
--
2.4.11

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Re: [PATCH] drm/vmwgfx: fix "duplicate frame pointer save" warning

Linus Torvalds-2
On Thu, May 26, 2016 at 11:43 AM, Josh Poimboeuf <[hidden email]> wrote:
>
> That's fine with me, it is indeed a rare case.  We can always add the
> per-instruction macro later if needed.  Here's a patch.

Ingo, I can take this directly, unless you have other things pending
that you want to send anyway and would  just add this to that existing
pile.

Just let me know.

                 Linus
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