Re: [PATCH] meson.build: Add -fzero-init-padding-bits=all

["Daniel P. Berrangé" <berrange@redhat.com>]

On Fri, May 08, 2026 at 11:47:23AM +0100, Peter Maydell wrote:
> The C standard doesn't always guarantee that struct and union padding
> bits are zero initialized, even if the code initializes a struct.
> For QEMU, this is potentially problematic, because we often have
> structs that match data structures in guest memory, where we
> initialize them and then bulk copy them into the guest.  If the
> compiler didn't zero init the whole of the memory containing the
> struct, we could potentially leak random data from the host into the
> guest via the padding bytes.
> 
> We already use -ftrivial-auto-var-init=zero, which will zero out
> padding in many of these cases, but -fzero-init-padding-bits=all
> closes some gaps, for example cases where we initialize a
> variable with a struct initializer, and cases involving unions.
> 
> Follow the Linux kernel in using both options. Compare kernel
> commit dce4aab8441 ("kbuild: Use -fzero-init-padding-bits=all").
> 
> This option exists in gcc-15 and above; it's not supported
> by clang, but clang documents that it guarantees zero init
> of these cases always:
> https://clang.llvm.org/docs/LanguageExtensions.html#union-and-aggregate-initialization-in-c
> Older gcc which don't have the option behave as if it were set.
> 
> (These options are passed through the cc.get_supported_arguments()
> filter, so we don't need to do anything extra to avoid passing it to
> a compiler that doesn't recognize it.)
> 
> Cc: qemu-stable@nongnu.org
> Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
> ---
> CC stable just as a precautionary thing; it's safe and we
> might as well make sure the hardening options are set there.
> ---
>  meson.build | 6 ++++++
>  1 file changed, 6 insertions(+)

Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>


With regards,
Daniel
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