Re: [PATCH v4 1/4] compiler_types: Introduce the Clang __preserve_most function attribute

[Kees Cook]

On Fri, Aug 11, 2023 at 05:18:38PM +0200, Marco Elver wrote:
> [1]: "On X86-64 and AArch64 targets, this attribute changes the calling
> convention of a function. The preserve_most calling convention attempts
> to make the code in the caller as unintrusive as possible. This
> convention behaves identically to the C calling convention on how
> arguments and return values are passed, but it uses a different set of
> caller/callee-saved registers. This alleviates the burden of saving and
> recovering a large register set before and after the call in the caller.
> If the arguments are passed in callee-saved registers, then they will be
> preserved by the callee across the call. This doesn't apply for values
> returned in callee-saved registers.
> 
>  * On X86-64 the callee preserves all general purpose registers, except
>    for R11. R11 can be used as a scratch register. Floating-point
>    registers (XMMs/YMMs) are not preserved and need to be saved by the
>    caller.
> 
>  * On AArch64 the callee preserve all general purpose registers, except
>    x0-X8 and X16-X18."
> 
> [1] https://clang.llvm.org/docs/AttributeReference.html#preserve-most
> 
> Introduce the attribute to compiler_types.h as __preserve_most.
> 
> Use of this attribute results in better code generation for calls to
> very rarely called functions, such as error-reporting functions, or
> rarely executed slow paths.
> 
> Beware that the attribute conflicts with instrumentation calls inserted
> on function entry which do not use __preserve_most themselves. Notably,
> function tracing which assumes the normal C calling convention for the
> given architecture.  Where the attribute is supported, __preserve_most
> will imply notrace. It is recommended to restrict use of the attribute
> to functions that should or already disable tracing.
> 
> Note: The additional preprocessor check against architecture should not
> be necessary if __has_attribute() only returns true where supported;
> also see https://github.com/ClangBuiltLinux/linux/issues/1908. But until
> __has_attribute() does the right thing, we also guard by known-supported
> architectures to avoid build warnings on other architectures.
> 
> The attribute may be supported by a future GCC version (see
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110899).
> 
> Signed-off-by: Marco Elver <elver@xxxxxxxxxx>
> Reviewed-by: Miguel Ojeda <ojeda@xxxxxxxxxx>
> Reviewed-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
> Acked-by: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>
> Acked-by: Mark Rutland <mark.rutland@xxxxxxx>

Should this go via -mm, the hardening tree, or something else? I'm happy
to carry it if no one else wants it?

-Kees

-- 
Kees Cook