Messages in this thread | | | Date | Mon, 14 Aug 2023 16:21:43 -0700 | From | Kees Cook <> | Subject | Re: [PATCH v4 1/4] compiler_types: Introduce the Clang __preserve_most function attribute |
| |
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@google.com> > Reviewed-by: Miguel Ojeda <ojeda@kernel.org> > Reviewed-by: Nick Desaulniers <ndesaulniers@google.com> > Acked-by: Steven Rostedt (Google) <rostedt@goodmis.org> > Acked-by: Mark Rutland <mark.rutland@arm.com>
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
| |