Re: [PATCH] xen/build: Fix build failure from LDFLAGS mismatch

[Jan Beulich <jbeulich@xxxxxxxx>]

On 26.04.2022 01:06, Andrew Cooper wrote:
> In a GNU compatbile makefile, $(LDFLAGS) are passed to $(CC), not $(LD).

I have to admit that I have always been puzzled by this anomaly.

> In a default CentOS 7 build environment, $(LDFLAGS) is set to -Wl,-z,relro,
> which causes the Xen build to explode with:
> 
>   ld: unrecognized option '-Wl,-z,relro'
>   ld: use the --help option for usage information
> 
> It turns out that many downstreams identify this as a breakage in Xen's build
> system and bodge around it in various ways, mostly by unsetting all of
> $(CFLAGS), $(AFLAGS) and $(LDFLAGS).
> 
> However, that is a security issue because it means that tools/ is not built
> with the distro-wide hardening flags that are otherwise expected of
> packages (relro, _FORTIFY_SOURCE, stack-protector, etc).

This "security issue" is introduced by them, I would say. They simply
shouldn't build everything in one go, but rather build "tools" with
the flags left intact and "xen" with the flags suitably pruned. (We
do build "xen" separately, albeit for different reasons.) The way
./Config.mk works this would look to be advisable anyway. And
xen/Makefile should perhaps arrange for Config.mk to skip this
massaging when including it.

> tools/ specifically should honour the packaging environment's choice of flags,
> while xen/ must not pass $(LDFLAGS) to $(LD), and should not be influenced by
> the others either.

I'm not convinced of the last part of what you say. Why should it be
impossible to say "make CFLAGS=... xen" just like "make CC=... xen"
can be used?

> --- a/xen/Makefile
> +++ b/xen/Makefile
> @@ -254,6 +254,8 @@ export KBUILD_DEFCONFIG := $(ARCH)_defconfig
>  # reparsing Config.mk by e.g. arch/x86/boot/.
>  export XEN_TREEWIDE_CFLAGS := $(CFLAGS)
>  
> +LDFLAGS :=

Instead of this, why don't we do away with the few uses of $(LDFLAGS)?
If I haven't overlooked anything, there are exactly two lines (three
if also counting a comment) which would need changing.

Or why don't we transform -Wl,... into the form understood by $(LD)?
-z relro, for example, looks to be benign to the linking of Xen, the
more that this option can also be enabled by default and we haven't
found a need to disable it (afaics this option solely determines
which linker script to use when none was specified).

Jan