[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 at 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
>