meson.build: Add -fzero-init-padding-bits=all

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> Reviewed-by: Daniel P. Berrangé <berrange@redhat.com> Reviewed-by: Richard Henderson <richard.henderson@linaro.org> Reviewed-by: Pierrick Bouvier <pierrick.bouvier@oss.qualcomm.com> Message-id: 20260508104723.2144051-1-peter.maydell@linaro.org