Re: [PATCH] configure: check whether linker works too

[Rich Felker <dalias@libc.org>]

On Mon, Oct 24, 2016 at 06:05:02PM -0500, Laine Gholson wrote:
> As a result of http://www.openwall.com/lists/musl/2016/10/22/3
> it has been shown that musl's current compiler sanity checks
> are inadequate. This patch checks if the compiler as a whole
> works properly, and moves the CPPFLAGS/CFLAGS/LDFLAGS sanity
> checks after the 'compiler works' check.
> 
> Patch and patch signature attached
> 
> Thanks,
>  Laine Gholson

> >From 99fd54d6f2b7c84a02c1ca0d5f1397d709c14313 Mon Sep 17 00:00:00 2001
> From: Laine Gholson <laine.gholson@gmail.com>
> Date: Mon, 24 Oct 2016 17:49:58 -0500
> Subject: [PATCH] configure: check whether linker works too
> 
> and move flag sanity checks to a seperate test
> ---
>  configure | 13 +++++++++++--
>  1 file changed, 11 insertions(+), 2 deletions(-)
> 
> diff --git a/configure b/configure
> index 707eb12..e7d565c 100755
> --- a/configure
> +++ b/configure
> @@ -236,8 +236,17 @@ printf "%s\n" "$CC"
>  test -n "$CC" || { echo "$0: cannot find a C compiler" ; exit 1 ; }
>  
>  printf "checking whether C compiler works... "
> -echo "typedef int x;" > "$tmpc"
> -if output=$($CC $CPPFLAGS $CFLAGS -c -o /dev/null "$tmpc" 2>&1) ; then
> +echo 'int main(void) { return 0; }' > "$tmpc"
> +if output=$($CC -o /dev/null "$tmpc" 2>&1) ; then
> +printf "yes\n"
> +else
> +printf "no; compiler output follows:\n%s\n" "$output"
> +exit 1
> +fi

As-is, this is not an acceptable change. When bootstrapping a new
toolchain, linking executables does not work because there is no libc
to link to. That's why we only test compiling and not linking.

The only need for a linker in the build process is building a shared
libc.so, and there are some linking tests that use -shared (which can
work without a libc.so, as long as -nostdlib is also being used). It
would be acceptable to add a linking test on which support for shared
library output depends; it should probably cause configure to fail if
--enable-shared was used (shared=yes) and otherwise should cause
shared=auto to become shared=no.

> +printf 'checking whether *FLAGS are sane... '
> +echo 'int main(void) { return 0; }' > "$tmpc"
> +if output=$($CC $CFLAGS $CPPFLAGS $LDFLAGS -o /dev/null "$tmpc" 2>&1) ; then
>  printf "yes\n"
>  else
>  printf "no; compiler output follows:\n%s\n" "$output"

I don't think this test is needed; you'll get a failure at make time
if you included bad flags. If there is a reason to prefer doing such a
test, there need to be separate ones for compiling and linking (since
linking might not work, e.g. if you have a compiler that can't produce
shared libraries).

Rich