[PATCH v3] Makefile: Tell compiler to generate bare-metal code

[PATCH v3] Makefile: Tell compiler to generate bare-metal code

[Andre Przywara]

Our GCC invocation does not provide many parameters, which lets the
toolchain fill in its own default setup.
In case of a native build or when using a full-featured cross-compiler,
this probably means Linux userland, which is not what we want for a
bare-metal application like boot-wrapper.

Tell the compiler to forget about those standard settings, and only use
what we explicitly ask for. In particular that means to not use toolchain
provided libraries, since they might pull in more code than we want, and
might not run well in the boot-wrapper environment.

Disable the stack protector, as this adds code that relies on userland:
"If a guard check fails, an error message is printed and the program
exits." (from the gcc manpage).

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
---
Hi,

compared to [PATCH v2 3/9] this drops the more contentious options (for
now, at least), and focuses on what's really needed. Including
-fno-stack-protector, as the need for this showed up in the cleanup series
already.

Cheers,
Andre

 Makefile.am | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/Makefile.am b/Makefile.am
index f941b07..581840e 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -125,6 +125,8 @@ CHOSEN_NODE	:= chosen {						\
 CPPFLAGS	+= $(INITRD_FLAGS)
 CFLAGS		+= -I$(top_srcdir)/include/ -I$(top_srcdir)/$(ARCH_SRC)/include/
 CFLAGS		+= -Wall -fomit-frame-pointer
+CFLAGS		+= -ffreestanding -nostdlib	# this is not for userland
+CFLAGS		+= -fno-stack-protector		# no terminal to print
 CFLAGS		+= -ffunction-sections -fdata-sections
 CFLAGS		+= -fno-pic -fno-pie
 LDFLAGS		+= --gc-sections
-- 
2.25.1


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Re: [PATCH v3] Makefile: Tell compiler to generate bare-metal code

[Mark Rutland]

Hi Andre,

On Mon, Jan 17, 2022 at 03:33:48PM +0000, Andre Przywara wrote:
> Our GCC invocation does not provide many parameters, which lets the
> toolchain fill in its own default setup.
> In case of a native build or when using a full-featured cross-compiler,
> this probably means Linux userland, which is not what we want for a
> bare-metal application like boot-wrapper.
> 
> Tell the compiler to forget about those standard settings, and only use
> what we explicitly ask for. In particular that means to not use toolchain
> provided libraries, since they might pull in more code than we want, and
> might not run well in the boot-wrapper environment.

Thanks for splitting this out; I'd like to apply this with two minor
fixups (which I'll apply locally if you agree).

> Disable the stack protector, as this adds code that relies on userland:
> "If a guard check fails, an error message is printed and the program
> exits." (from the gcc manpage).

I think we should say:

| Disable the stack protector, as this relies on support code, e.g.
| a __stack_chk_guard variable and __stack_chk_fail function, which the
| boot-wrapper does not implement.

> Signed-off-by: Andre Przywara <andre.przywara@arm.com>
> ---
> Hi,
> 
> compared to [PATCH v2 3/9] this drops the more contentious options (for
> now, at least), and focuses on what's really needed. Including
> -fno-stack-protector, as the need for this showed up in the cleanup series
> already.
> 
> Cheers,
> Andre
> 
>  Makefile.am | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/Makefile.am b/Makefile.am
> index f941b07..581840e 100644
> --- a/Makefile.am
> +++ b/Makefile.am
> @@ -125,6 +125,8 @@ CHOSEN_NODE	:= chosen {						\
>  CPPFLAGS	+= $(INITRD_FLAGS)
>  CFLAGS		+= -I$(top_srcdir)/include/ -I$(top_srcdir)/$(ARCH_SRC)/include/
>  CFLAGS		+= -Wall -fomit-frame-pointer
> +CFLAGS		+= -ffreestanding -nostdlib	# this is not for userland
> +CFLAGS		+= -fno-stack-protector		# no terminal to print

For now, I'd like to drop these comments, since I think the "no terminal
to print" comment is a bit confusing, and the "this is not for userland"
comment is arguably the most obvious.

Then as a future follow-up we can reoganise this for clarity:

| # Build code suitable for a bare-metal environment which does not
| # depend on any support libraries
| CFLAGS	+= ffreestanding -nostdlib
| 
| # Disable unnecessary features which require runtime support
| CFLAGS	+= -fno-stack-protector
| CFLAGS	+= ...
|
| # Disable unnecessary features which impact code-size
| CFLAGS	+= -fomit-frame-pointer
| CFLAGS	+= ...
|
| # Allow the linker to remove unused code and data to shrink the
| # resulting binary
| CFLAGS	+= -ffunction-sections -fdata-sections
| LDFLAGS	+= --gc-sections


>  CFLAGS		+= -ffunction-sections -fdata-sections
>  CFLAGS		+= -fno-pic -fno-pie
>  LDFLAGS		+= --gc-sections
> -- 
> 2.25.1
> 

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Re: [PATCH v3] Makefile: Tell compiler to generate bare-metal code

[Andre Przywara]

On Mon, 17 Jan 2022 16:47:20 +0000
Mark Rutland <mark.rutland@arm.com> wrote:

> Hi Andre,
> 
> On Mon, Jan 17, 2022 at 03:33:48PM +0000, Andre Przywara wrote:
> > Our GCC invocation does not provide many parameters, which lets the
> > toolchain fill in its own default setup.
> > In case of a native build or when using a full-featured cross-compiler,
> > this probably means Linux userland, which is not what we want for a
> > bare-metal application like boot-wrapper.
> > 
> > Tell the compiler to forget about those standard settings, and only use
> > what we explicitly ask for. In particular that means to not use toolchain
> > provided libraries, since they might pull in more code than we want, and
> > might not run well in the boot-wrapper environment.  
> 
> Thanks for splitting this out; I'd like to apply this with two minor
> fixups (which I'll apply locally if you agree).

Yes, I am fine with those changes, please go ahead!

> > Disable the stack protector, as this adds code that relies on userland:
> > "If a guard check fails, an error message is printed and the program
> > exits." (from the gcc manpage).  
> 
> I think we should say:
> 
> | Disable the stack protector, as this relies on support code, e.g.
> | a __stack_chk_guard variable and __stack_chk_fail function, which the
> | boot-wrapper does not implement.
> 
> > Signed-off-by: Andre Przywara <andre.przywara@arm.com>
> > ---
> > Hi,
> > 
> > compared to [PATCH v2 3/9] this drops the more contentious options (for
> > now, at least), and focuses on what's really needed. Including
> > -fno-stack-protector, as the need for this showed up in the cleanup series
> > already.
> > 
> > Cheers,
> > Andre
> > 
> >  Makefile.am | 2 ++
> >  1 file changed, 2 insertions(+)
> > 
> > diff --git a/Makefile.am b/Makefile.am
> > index f941b07..581840e 100644
> > --- a/Makefile.am
> > +++ b/Makefile.am
> > @@ -125,6 +125,8 @@ CHOSEN_NODE	:= chosen {						\
> >  CPPFLAGS	+= $(INITRD_FLAGS)
> >  CFLAGS		+= -I$(top_srcdir)/include/ -I$(top_srcdir)/$(ARCH_SRC)/include/
> >  CFLAGS		+= -Wall -fomit-frame-pointer
> > +CFLAGS		+= -ffreestanding -nostdlib	# this is not for userland
> > +CFLAGS		+= -fno-stack-protector		# no terminal to print  
> 
> For now, I'd like to drop these comments, since I think the "no terminal
> to print" comment is a bit confusing, and the "this is not for userland"
> comment is arguably the most obvious.
> 
> Then as a future follow-up we can reoganise this for clarity:
> 
> | # Build code suitable for a bare-metal environment which does not
> | # depend on any support libraries
> | CFLAGS	+= ffreestanding -nostdlib
> | 
> | # Disable unnecessary features which require runtime support
> | CFLAGS	+= -fno-stack-protector
> | CFLAGS	+= ...
> |
> | # Disable unnecessary features which impact code-size
> | CFLAGS	+= -fomit-frame-pointer
> | CFLAGS	+= ...
> |
> | # Allow the linker to remove unused code and data to shrink the
> | # resulting binary
> | CFLAGS	+= -ffunction-sections -fdata-sections
> | LDFLAGS	+= --gc-sections

Yes, that looks good to me as a plan. I agree that trying to fit
meaningful comment to the rest of the line was not a great idea.

Cheers,
Andre

> 
> 
> >  CFLAGS		+= -ffunction-sections -fdata-sections
> >  CFLAGS		+= -fno-pic -fno-pie
> >  LDFLAGS		+= --gc-sections
> > -- 
> > 2.25.1
> >   


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Re: [PATCH v3] Makefile: Tell compiler to generate bare-metal code

[Mark Rutland]

On Mon, Jan 17, 2022 at 04:53:10PM +0000, Andre Przywara wrote:
> On Mon, 17 Jan 2022 16:47:20 +0000
> Mark Rutland <mark.rutland@arm.com> wrote:
> 
> > Hi Andre,
> > 
> > On Mon, Jan 17, 2022 at 03:33:48PM +0000, Andre Przywara wrote:
> > > Our GCC invocation does not provide many parameters, which lets the
> > > toolchain fill in its own default setup.
> > > In case of a native build or when using a full-featured cross-compiler,
> > > this probably means Linux userland, which is not what we want for a
> > > bare-metal application like boot-wrapper.
> > > 
> > > Tell the compiler to forget about those standard settings, and only use
> > > what we explicitly ask for. In particular that means to not use toolchain
> > > provided libraries, since they might pull in more code than we want, and
> > > might not run well in the boot-wrapper environment.  
> > 
> > Thanks for splitting this out; I'd like to apply this with two minor
> > fixups (which I'll apply locally if you agree).
> 
> Yes, I am fine with those changes, please go ahead!

Thanks; I've applied those and pushed that out.

Mark.

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Re: [PATCH v3] Makefile: Tell compiler to generate bare-metal code

[Russell King (Oracle)]

On Mon, Jan 17, 2022 at 03:33:48PM +0000, Andre Przywara wrote:
> Our GCC invocation does not provide many parameters, which lets the
> toolchain fill in its own default setup.
> In case of a native build or when using a full-featured cross-compiler,
> this probably means Linux userland, which is not what we want for a
> bare-metal application like boot-wrapper.
> 
> Tell the compiler to forget about those standard settings, and only use
> what we explicitly ask for. In particular that means to not use toolchain
> provided libraries, since they might pull in more code than we want, and
> might not run well in the boot-wrapper environment.
> 
> Disable the stack protector, as this adds code that relies on userland:
> "If a guard check fails, an error message is printed and the program
> exits." (from the gcc manpage).
> 
> Signed-off-by: Andre Przywara <andre.przywara@arm.com>
> ---
> Hi,
> 
> compared to [PATCH v2 3/9] this drops the more contentious options (for
> now, at least), and focuses on what's really needed. Including
> -fno-stack-protector, as the need for this showed up in the cleanup series
> already.

There is history here. -ffreestanding has been tried before. See:

https://patchwork.kernel.org/project/linux-kbuild/patch/20200817220212.338670-5-ndesaulniers@google.com/

Have the issues in that thread been addressed?

If not, -ffreestanding does not belong in the top-level makefile.

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Re: [PATCH v3] Makefile: Tell compiler to generate bare-metal code

[Russell King (Oracle)]

On Mon, Jan 17, 2022 at 05:49:43PM +0000, Russell King (Oracle) wrote:
> On Mon, Jan 17, 2022 at 03:33:48PM +0000, Andre Przywara wrote:
> > Our GCC invocation does not provide many parameters, which lets the
> > toolchain fill in its own default setup.
> > In case of a native build or when using a full-featured cross-compiler,
> > this probably means Linux userland, which is not what we want for a
> > bare-metal application like boot-wrapper.
> > 
> > Tell the compiler to forget about those standard settings, and only use
> > what we explicitly ask for. In particular that means to not use toolchain
> > provided libraries, since they might pull in more code than we want, and
> > might not run well in the boot-wrapper environment.
> > 
> > Disable the stack protector, as this adds code that relies on userland:
> > "If a guard check fails, an error message is printed and the program
> > exits." (from the gcc manpage).
> > 
> > Signed-off-by: Andre Przywara <andre.przywara@arm.com>
> > ---
> > Hi,
> > 
> > compared to [PATCH v2 3/9] this drops the more contentious options (for
> > now, at least), and focuses on what's really needed. Including
> > -fno-stack-protector, as the need for this showed up in the cleanup series
> > already.
> 
> There is history here. -ffreestanding has been tried before. See:
> 
> https://patchwork.kernel.org/project/linux-kbuild/patch/20200817220212.338670-5-ndesaulniers@google.com/
> 
> Have the issues in that thread been addressed?
> 
> If not, -ffreestanding does not belong in the top-level makefile.

Oh, this is not the kernel. Please make it clear as you're posting to
the linux-arm-kernel mailing list...

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Re: [PATCH v3] Makefile: Tell compiler to generate bare-metal code

[Andre Przywara]

On Mon, 17 Jan 2022 17:49:43 +0000
"Russell King (Oracle)" <linux@armlinux.org.uk> wrote:

Hi Russell,

> On Mon, Jan 17, 2022 at 03:33:48PM +0000, Andre Przywara wrote:
> > Our GCC invocation does not provide many parameters, which lets the
> > toolchain fill in its own default setup.
> > In case of a native build or when using a full-featured cross-compiler,
> > this probably means Linux userland, which is not what we want for a
> > bare-metal application like boot-wrapper.
> > 
> > Tell the compiler to forget about those standard settings, and only use
> > what we explicitly ask for. In particular that means to not use toolchain
> > provided libraries, since they might pull in more code than we want, and
> > might not run well in the boot-wrapper environment.
> > 
> > Disable the stack protector, as this adds code that relies on userland:
> > "If a guard check fails, an error message is printed and the program
> > exits." (from the gcc manpage).
> > 
> > Signed-off-by: Andre Przywara <andre.przywara@arm.com>
> > ---
> > Hi,
> > 
> > compared to [PATCH v2 3/9] this drops the more contentious options (for
> > now, at least), and focuses on what's really needed. Including
> > -fno-stack-protector, as the need for this showed up in the cleanup series
> > already.  
> 
> There is history here. -ffreestanding has been tried before. See:
> 
> https://patchwork.kernel.org/project/linux-kbuild/patch/20200817220212.338670-5-ndesaulniers@google.com/

Thanks for the link, interesting insight.

> Have the issues in that thread been addressed?

My apologies, I just see that I accidentally dropped the "boot-wrapper"
part from the subject line. So this is not a Linux patch, we just use the
mailing list for boot-wrapper discussion as well.
As the boot-wrapper is a really minimal and small code base, I don't think
we bother too much about lost optimisation opportunities, instead want to
become more robust. At the moment we really don't use any options telling
the compiler to not use the standard library (which caused issues
already), so we'd like to play safe here.

Thanks,
Andre

> If not, -ffreestanding does not belong in the top-level makefile.

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