linux-kernel - Re: [PATCH v3] kbuild: rust: add PROCMACROLDFLAGS

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Message-ID: <CAABy=s1u75ywAECbkCmGfyt+Yp5khnF0UVcezA-_BEDWUVrHkw@mail.gmail.com>
Date: Thu, 5 Dec 2024 15:39:25 -0800
From: "Hong, Yifan" <elsk@...gle.com>
To: Miguel Ojeda <ojeda@...nel.org>
Cc: Masahiro Yamada <masahiroy@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>, 
	Jonathan Corbet <corbet@....net>, Nathan Chancellor <nathan@...nel.org>, Nicolas Schier <nicolas@...sle.eu>, 
	linux-kbuild@...r.kernel.org, Boqun Feng <boqun.feng@...il.com>, 
	Gary Guo <gary@...yguo.net>, Björn Roy Baron <bjorn3_gh@...tonmail.com>, 
	Benno Lossin <benno.lossin@...ton.me>, Andreas Hindborg <a.hindborg@...nel.org>, 
	Alice Ryhl <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>, rust-for-linux@...r.kernel.org, 
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org, 
	patches@...ts.linux.dev
Subject: Re: [PATCH v3] kbuild: rust: add PROCMACROLDFLAGS

On Tue, Nov 12, 2024 at 10:45 AM Miguel Ojeda <ojeda@...nel.org> wrote:
>
> From: HONG Yifan <elsk@...gle.com>
>
> These are flags to be passed when linking proc macros for the Rust
> toolchain. If unset, it defaults to $(KBUILD_HOSTLDFLAGS).
>
> This is needed because the list of flags to link hostprogs is not
> necessarily the same as the list of flags used to link libmacros.so.
> When we build proc macros, we need the latter, not the former (e.g. when
> using a Rust compiler binary linked to a different C library than host
> programs).
>
> To distinguish between the two, introduce this new variable to stand
> out from KBUILD_HOSTLDFLAGS used to link other host progs.
>
> Signed-off-by: HONG Yifan <elsk@...gle.com>
> Link: https://lore.kernel.org/r/20241017210430.2401398-2-elsk@google.com
> [ v3:
>
>   - `export`ed the variable. Otherwise it would not be visible in
>     `rust/Makefile`.
>
>   - Removed "additional" from the documentation and commit message,
>     since this actually replaces the other flags, unlike other cases.
>
>   - Added example of use case to documentation and commit message.
>     Thanks Alice for the details on what Google needs!
>
>   - Instead of `HOSTLDFLAGS`, used `KBUILD_HOSTLDFLAGS` as the fallback
>     to preserve the previous behavior as much as possible, as discussed
>     with Alice/Yifan. Thus moved the variable down too (currently we
>     do not modify `KBUILD_HOSTLDFLAGS` elsewhere) and avoided
>     mentioning `HOSTLDFLAGS` directly in the documentation.
>
>   - Fixed documentation header formatting.
>
>   - Reworded slightly.
>
>          - Miguel ]
> Signed-off-by: Miguel Ojeda <ojeda@...nel.org>
> ---
> Masahiro: if Kbuild wants to pick this up, that is great. Otherwise, I am happy
> picking this up early next cycle, if you give an `Acked-by` since this is
> changing the interface for Kbuild users given we are introducing a new
> environment variable. Thanks!
>
> Note that the `or` means if the string is empty, we will use the default rather
> than nothing. I didn't change that from Yifan's version, but maybe we want to do
> otherwise. Users can still provide e.g. an empty space to avoid any flag.

I am not sure if I understand the implications here.
https://www.gnu.org/software/make/manual/html_node/Conditional-Functions.html
says:

The or function provides a “short-circuiting” OR operation. Each
argument is expanded, in order. If an argument expands to a non-empty
string the processing stops and the result of the expansion is that
string. If, after all arguments are expanded, all of them are false
(empty), then the result of the expansion is the empty string.

I am assuming that this means:
- If PROCMACROLDFLAGS is not empty, KBUILD_PROCMACROLDFLAGS evaluates
to PROCMACROLDFLAGS
- Otherwise if KBUILD_HOSTLDFLAGS is not empty,
KBUILD_PROCMACROLDFLAGS evaluates to KBUILD_HOSTLDFLAGS
- Otherwise KBUILD_PROCMACROLDFLAGS is set to empty.

What do you mean by "use the default"?


>
> Yifan/Alice: please double-check the changes. Thanks!
>
> v3: see changes above.
> v2: https://lore.kernel.org/rust-for-linux/20241017210430.2401398-2-elsk@google.com/
> v1: https://lore.kernel.org/rust-for-linux/20241017200138.2390077-2-elsk@google.com/
>
>  Documentation/kbuild/kbuild.rst | 11 +++++++++++
>  Makefile                        |  3 ++-
>  rust/Makefile                   |  2 +-
>  3 files changed, 14 insertions(+), 2 deletions(-)
>
> diff --git a/Documentation/kbuild/kbuild.rst b/Documentation/kbuild/kbuild.rst
> index 1796b3eba37b..9cb876ccc363 100644
> --- a/Documentation/kbuild/kbuild.rst
> +++ b/Documentation/kbuild/kbuild.rst
> @@ -91,6 +91,17 @@ HOSTRUSTFLAGS
>  -------------
>  Additional flags to be passed to $(HOSTRUSTC) when building host programs.
>
> +PROCMACROLDFLAGS
> +----------------
> +Flags to be passed when linking Rust proc macros. Since proc macros are loaded
> +by rustc at build time, they must be linked in a way that is compatible with
> +the rustc toolchain being used.
> +
> +For instance, it can be useful when rustc uses a different C library than
> +the one the user wants to use for host programs.
> +
> +If unset, it defaults to the flags passed when linking host programs.
> +
>  HOSTLDFLAGS
>  -----------
>  Additional flags to be passed when linking host programs.
> diff --git a/Makefile b/Makefile
> index a9e723cb0596..3efb001bada5 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -471,6 +471,7 @@ KBUILD_HOSTRUSTFLAGS := $(rust_common_flags) -O -Cstrip=debuginfo \
>                         -Zallow-features= $(HOSTRUSTFLAGS)
>  KBUILD_HOSTLDFLAGS  := $(HOST_LFS_LDFLAGS) $(HOSTLDFLAGS)
>  KBUILD_HOSTLDLIBS   := $(HOST_LFS_LIBS) $(HOSTLDLIBS)
> +KBUILD_PROCMACROLDFLAGS := $(or $(PROCMACROLDFLAGS),$(KBUILD_HOSTLDFLAGS))
>
>  # Make variables (CC, etc...)
>  CPP            = $(CC) -E
> @@ -595,7 +596,7 @@ export HOSTRUSTC KBUILD_HOSTRUSTFLAGS
>  export CPP AR NM STRIP OBJCOPY OBJDUMP READELF PAHOLE RESOLVE_BTFIDS LEX YACC AWK INSTALLKERNEL
>  export PERL PYTHON3 CHECK CHECKFLAGS MAKE UTS_MACHINE HOSTCXX
>  export KGZIP KBZIP2 KLZOP LZMA LZ4 XZ ZSTD
> -export KBUILD_HOSTCXXFLAGS KBUILD_HOSTLDFLAGS KBUILD_HOSTLDLIBS LDFLAGS_MODULE
> +export KBUILD_HOSTCXXFLAGS KBUILD_HOSTLDFLAGS KBUILD_HOSTLDLIBS KBUILD_PROCMACROLDFLAGS LDFLAGS_MODULE
>  export KBUILD_USERCFLAGS KBUILD_USERLDFLAGS
>
>  export KBUILD_CPPFLAGS NOSTDINC_FLAGS LINUXINCLUDE OBJCOPYFLAGS KBUILD_LDFLAGS
> diff --git a/rust/Makefile b/rust/Makefile
> index f349e7b067ea..9f55c470aa2c 100644
> --- a/rust/Makefile
> +++ b/rust/Makefile
> @@ -344,7 +344,7 @@ quiet_cmd_rustc_procmacro = $(RUSTC_OR_CLIPPY_QUIET) P $@
>        cmd_rustc_procmacro = \
>         $(RUSTC_OR_CLIPPY) $(rust_common_flags) \
>                 -Clinker-flavor=gcc -Clinker=$(HOSTCC) \
> -               -Clink-args='$(call escsq,$(KBUILD_HOSTLDFLAGS))' \
> +               -Clink-args='$(call escsq,$(KBUILD_PROCMACROLDFLAGS))' \
>                 --emit=dep-info=$(depfile) --emit=link=$@ --extern proc_macro \
>                 --crate-type proc-macro \
>                 --crate-name $(patsubst lib%.so,%,$(notdir $@)) $<
>
> base-commit: d072acda4862f095ec9056979b654cc06a22cc68
> --
> 2.47.0