Re: [Qemu-devel] [PATCH for-2.0] configure: add option to disable -fstack-protector flags

[Laurent Desnogues]

On Fri, Mar 28, 2014 at 5:19 PM, Paolo Bonzini <address@hidden> wrote:
> From: Steven Noonan <address@hidden>
>
> The -fstack-protector flag family is useful for ensuring safety and for
> debugging, but has a performance impact. Here are some boot time comparisons 
> of
> the various versions of -fstack-protector using qemu-system-arm on an x86_64
> host:
>
>     # -fstack-protector-all
>     Startup finished in 1.810s (kernel) + 12.331s (initrd) + 49.016s 
> (userspace) = 1min 3.159s
>     Startup finished in 1.801s (kernel) + 12.287s (initrd) + 47.925s 
> (userspace) = 1min 2.013s
>     Startup finished in 1.812s (kernel) + 12.302s (initrd) + 47.995s 
> (userspace) = 1min 2.111s
>
>     # -fstack-protector-strong
>     Startup finished in 1.744s (kernel) + 11.223s (initrd) + 44.688s 
> (userspace) = 57.657s
>     Startup finished in 1.721s (kernel) + 11.222s (initrd) + 44.194s 
> (userspace) = 57.138s
>     Startup finished in 1.693s (kernel) + 11.250s (initrd) + 44.426s 
> (userspace) = 57.370s
>
>     # -fstack-protector
>     Startup finished in 1.705s (kernel) + 11.409s (initrd) + 43.563s 
> (userspace) = 56.677s
>     Startup finished in 1.877s (kernel) + 11.137s (initrd) + 43.719s 
> (userspace) = 56.734s
>     Startup finished in 1.708s (kernel) + 11.141s (initrd) + 43.628s 
> (userspace) = 56.478s
>
>     # no stack protector
>     Startup finished in 1.743s (kernel) + 11.190s (initrd) + 43.709s 
> (userspace) = 56.643s
>     Startup finished in 1.763s (kernel) + 11.216s (initrd) + 43.767s 
> (userspace) = 56.747s
>     Startup finished in 1.711s (kernel) + 11.283s (initrd) + 43.878s 
> (userspace) = 56.873s
>
> This patch introduces a configure option to disable the stack protector
> entirely, and conditional stack protector flag selection (in order,
> based on availability): -fstack-protector-strong, -fstack-protector-all,
> no stack protector.
>
> Signed-off-by: Steven Noonan <address@hidden>
> Cc: Anthony Liguori <address@hidden>
> Reviewed-by: Stefan Weil <address@hidden>
> [Prefer -fstack-protector-all to -fstack-protector, suggested by
>  Laurent Desnogues. - Paolo]
> Signed-off-by: Paolo Bonzini <address@hidden>
> ---
>  configure | 18 +++++++++++++++---
>  1 file changed, 15 insertions(+), 3 deletions(-)
>
> diff --git a/configure b/configure
> index fb3bd05..eb0e7bb 100755
> --- a/configure
> +++ b/configure
> @@ -198,6 +198,7 @@ audio_win_int=""
>  cc_i386=i386-pc-linux-gnu-gcc
>  libs_qga=""
>  debug_info="yes"
> +stack_protector=""
>
>  # Don't accept a target_list environment variable.
>  unset target_list
> @@ -950,6 +951,10 @@ for opt do
>    ;;
>    --disable-werror) werror="no"
>    ;;
> +  --enable-stack-protector) stack_protector="yes"
> +  ;;
> +  --disable-stack-protector) stack_protector="no"
> +  ;;
>    --disable-curses) curses="no"
>    ;;
>    --enable-curses) curses="yes"
> @@ -1219,6 +1224,7 @@ Advanced options (experts only):
>    --disable-sparse         disable sparse checker (default)
>    --disable-strip          disable stripping binaries
>    --disable-werror         disable compilation abort on warning
> +  --disable-stack-protector disable compiler-provided stack protection
>    --disable-sdl            disable SDL
>    --enable-sdl             enable SDL
>    --with-sdlabi            select preferred SDL ABI 1.2 or 2.0
> @@ -1439,9 +1445,15 @@ for flag in $gcc_flags; do
>      fi
>  done
>
> -if compile_prog "-Werror -fstack-protector-all" "" ; then
> -    QEMU_CFLAGS="$QEMU_CFLAGS -fstack-protector-all"
> -    LIBTOOLFLAGS="$LIBTOOLFLAGS -Wc,-fstack-protector-all"
> +if test "$stack_protector" != "no" ; then
> +  gcc_flags="-fstack-protector-strong -fstack-protector-all"
> +  for flag in $gcc_flags; do
> +    if compile_prog "-Werror $flag" "" ; then
> +      QEMU_CFLAGS="$QEMU_CFLAGS $flag"
> +      LIBTOOLFLAGS="$LIBTOOLFLAGS -Wc,$flag"
> +      break
> +    fi
> +  done
>  fi

My understanding is that -fstack-protector, -fstack-protector-strong,
and -fstack-protector-all are strictly ordered in terms of the number
of functions that are checked, so you have changed the default
behavior to check less functions for compilers that support
-fstack-protector-strong.  Is that what you had in mind?

Also aren't there some versions of gcc that have -fstack-protector
but not the other two options?

Thanks,

Laurent

>  # Workaround for http://gcc.gnu.org/PR55489.  Happens with -fPIE/-fPIC and
> --
> 1.8.5.3
>