Re: [PATCH 1/2] meson: detect broken clang 17 with -fzero-call-used-regs

[Peter Maydell]

On Mon, 4 Mar 2024 at 14:46, Daniel P. Berrangé <berrange@redhat.com> wrote:
>
> Clang 17 will segv if given  -fzero-call-used-regs and optimization
> is enabled. Since upstream hasn't triaged the bug, distros are
> increasingly shipping with broken Clang.
>
> https://github.com/llvm/llvm-project/issues/75168
> https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=277474
> Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
> ---
>  meson.build | 17 ++++++++++++-----
>  1 file changed, 12 insertions(+), 5 deletions(-)
>
> diff --git a/meson.build b/meson.build
> index 0ef1654e86..762798f2ee 100644
> --- a/meson.build
> +++ b/meson.build
> @@ -555,17 +555,24 @@ endif
>  # Check further flags that make QEMU more robust against malicious parties
>
>  hardening_flags = [
> -    # Zero out registers used during a function call
> -    # upon its return. This makes it harder to assemble
> -    # ROP gadgets into something usable
> -    '-fzero-call-used-regs=used-gpr',
> -
>      # Initialize all stack variables to zero. This makes
>      # it harder to take advantage of uninitialized stack
>      # data to drive exploits
>      '-ftrivial-auto-var-init=zero',
>  ]
>
> +# Zero out registers used during a function call
> +# upon its return. This makes it harder to assemble
> +# ROP gadgets into something usable
> +#
> +# NB: CLang 17 is broken and SEGVs

"Clang"

> +# https://github.com/llvm/llvm-project/issues/75168
> +if cc.compiles('extern struct { void (*cb)(void); } s; void f(void) { 
> s.cb(); }',
> +               name: '-fzero-call-used-regs=used-gpr',
> +               args: ['-O2', '-fzero-call-used-regs=used-gpr'])
> +    hardening_flags += '-fzero-call-used-regs=used-gpr'
> +endif
> +
>  qemu_common_flags += cc.get_supported_arguments(hardening_flags)
>
>  add_global_arguments(qemu_common_flags, native: false, language: 
> all_languages)

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>

The stable releases don't have the use of -fzero-call-used-regs,
so we don't need to backport this.

thanks
-- PMM