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

[Marco Elver]

On Tue, 15 Aug 2023 at 01:21, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>
> 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?

v3 of this series is already in mm-unstable, and has had some -next
exposure (which was helpful in uncovering some additional issues).
Therefore, I think it's appropriate that it continues in mm and Andrew
picks up the latest v4 here.

Your official Ack would nevertheless be much appreciated!

Thanks,
-- Marco