[PATCH v7] kbuild: host: use single executable for rustc -C linker

[PATCH v7] kbuild: host: use single executable for rustc -C linker

[Mohamad Alsadhan]

rustc's -C linker= option expects a single executable path. When
HOSTCC contains a wrapper (e.g. "ccache gcc"), passing
`-Clinker=$(HOSTCC)` results in the shell splitting the value into
multiple words, and rustc interprets the additional word as an
input filename:

  error: multiple input filenames provided ...

Generate a small wrapper script and pass it to -Clinker e.g.

  ```
  #!/bin/sh
  exec ccache gcc "$@"
  ```

This fix should be general enough to address most if not all cases
(incl. wrappers or subcommands) and avoids surprises of simpler fixes
like just defaulting to gcc.

This avoids passing the user command as an environment variable as
that would be more challenging to trace and debug shell expansions.

Link: https://github.com/Rust-for-Linux/linux/issues/1224
Suggested-by: Yoann Congal <yoann.congal@smile.fr>
Signed-off-by: Mohamad Alsadhan <mo@sdhn.cc>
---
v6 -> v7:
  - Always generate `scripts/rustc-wrapper` when `CONFIG_RUST=y`,
    including cases with no Rust host programs in `scripts/`.
  - Use `$(obj)/rustc-wrapper` for host Rust programs, fixing builds
    outside `scripts/`, e.g. `samples/rust/hostprogs/`.
  - remove leading exec (again) to allow leading env vars e.g.
    HOSTRUSTC_LD="VAR=VAL ccache gcc".
  - Track, clean and ignore generated wrapper as needed.

v5 -> v6:
  - Add fix to `rust/Makefile` as well (Yoann)
  - Include script to `.gitignore` and make clean (Nicolas)
  - Add back the outer `exec` to the command

v4 -> v5:
  - Fix word splitting issues
  - Remove unnecessary `exec sh -c` and simplify generated script

v3 -> v4:
  - Use filechk instead of if_changed macro to regenerate script
  - Remove trailing space at EOL

v2 -> v3:
  - Scrap previous hacky approaches (e.g. using lastword) and go with
    a proper fix (Gary) which turned out not that complex to
    implement.

v1 -> v2:
  - Rename HOSTRUSTC_LINKER to HOSTRUSTC_LD for consistency
  - Introduce explicit HOSTRUSTC_LD override
  - Warn when falling back due to multi-argument HOSTCC
  - Error out if a user-specified HOSTRUSTC_LD is not an executable

v1: https://lore.kernel.org/all/20260225102819.16553-1-mo@sdhn.cc/
v2: https://lore.kernel.org/all/20260227132713.23106-1-mo@sdhn.cc/
v3: https://lore.kernel.org/all/20260312002852.11292-1-mo@sdhn.cc/
v4: https://lore.kernel.org/all/20260317112021.14353-1-mo@sdhn.cc/
v5: https://lore.kernel.org/all/20260321150034.9915-1-mo@sdhn.cc/
v6: https://lore.kernel.org/all/20260331000802.380-1-mo@sdhn.cc/
---
 Makefile                          |  3 ++-
 rust/Makefile                     |  8 +++++---
 samples/rust/hostprogs/.gitignore |  1 +
 samples/rust/hostprogs/Makefile   |  2 ++
 scripts/.gitignore                |  1 +
 scripts/Makefile                  |  3 +++
 scripts/Makefile.host             | 26 +++++++++++++++++++++++---
 7 files changed, 37 insertions(+), 7 deletions(-)

diff --git a/Makefile b/Makefile
index 1a219bf1c..5ebeef67f 100644
--- a/Makefile
+++ b/Makefile
@@ -1651,7 +1651,8 @@ CLEAN_FILES += vmlinux.symvers modules-only.symvers \
 	       modules.builtin.ranges vmlinux.o.map vmlinux.unstripped \
 	       compile_commands.json rust/test \
 	       rust-project.json .vmlinux.objs .vmlinux.export.c \
-               .builtin-dtbs-list .builtin-dtb.S
+               .builtin-dtbs-list .builtin-dtb.S \
+	       scripts/rustc-wrapper
 
 # Directories & files removed with 'make mrproper'
 MRPROPER_FILES += include/config include/generated          \
diff --git a/rust/Makefile b/rust/Makefile
index 5eca6a817..6bc761a31 100644
--- a/rust/Makefile
+++ b/rust/Makefile
@@ -565,7 +565,7 @@ $(obj)/libsyn.rlib: $(src)/syn/lib.rs $(obj)/libquote.rlib FORCE
 quiet_cmd_rustc_procmacro = $(RUSTC_OR_CLIPPY_QUIET) P $@
       cmd_rustc_procmacro = \
 	$(RUSTC_OR_CLIPPY) $(rust_common_flags) $(rustc_target_flags) \
-		-Clinker-flavor=gcc -Clinker=$(HOSTCC) \
+		-Clinker-flavor=gcc -Clinker=scripts/rustc-wrapper \
 		-Clink-args='$(call escsq,$(KBUILD_PROCMACROLDFLAGS))' \
 		--emit=dep-info=$(depfile) --emit=link=$@ --extern proc_macro \
 		--crate-type proc-macro -L$(objtree)/$(obj) \
@@ -576,12 +576,14 @@ quiet_cmd_rustc_procmacro = $(RUSTC_OR_CLIPPY_QUIET) P $@
 $(obj)/$(libmacros_name): private rustc_target_flags = \
     --extern proc_macro2 --extern quote --extern syn
 $(obj)/$(libmacros_name): $(src)/macros/lib.rs $(obj)/libproc_macro2.rlib \
-    $(obj)/libquote.rlib $(obj)/libsyn.rlib FORCE
+    $(obj)/libquote.rlib $(obj)/libsyn.rlib \
+    scripts/rustc-wrapper FORCE
 	+$(call if_changed_dep,rustc_procmacro)
 
 $(obj)/$(libpin_init_internal_name): private rustc_target_flags = $(pin_init_internal-flags)
 $(obj)/$(libpin_init_internal_name): $(src)/pin-init/internal/src/lib.rs \
-    $(obj)/libproc_macro2.rlib $(obj)/libquote.rlib $(obj)/libsyn.rlib FORCE
+    $(obj)/libproc_macro2.rlib $(obj)/libquote.rlib $(obj)/libsyn.rlib \
+    scripts/rustc-wrapper FORCE
 	+$(call if_changed_dep,rustc_procmacro)
 
 # `rustc` requires `-Zunstable-options` to use custom target specifications
diff --git a/samples/rust/hostprogs/.gitignore b/samples/rust/hostprogs/.gitignore
index a6c173da5..d88a75790 100644
--- a/samples/rust/hostprogs/.gitignore
+++ b/samples/rust/hostprogs/.gitignore
@@ -1,3 +1,4 @@
 # SPDX-License-Identifier: GPL-2.0
 
 single
+rustc-wrapper
diff --git a/samples/rust/hostprogs/Makefile b/samples/rust/hostprogs/Makefile
index 8ddcbd741..9c9663a77 100644
--- a/samples/rust/hostprogs/Makefile
+++ b/samples/rust/hostprogs/Makefile
@@ -3,3 +3,5 @@
 hostprogs-always-y := single
 
 single-rust := y
+
+clean-files += rustc-wrapper
diff --git a/scripts/.gitignore b/scripts/.gitignore
index 4215c2208..b3948b148 100644
--- a/scripts/.gitignore
+++ b/scripts/.gitignore
@@ -6,6 +6,7 @@
 /kallsyms
 /module.lds
 /recordmcount
+/rustc-wrapper
 /rustdoc_test_builder
 /rustdoc_test_gen
 /sign-file
diff --git a/scripts/Makefile b/scripts/Makefile
index 0941e5ce7..287421322 100644
--- a/scripts/Makefile
+++ b/scripts/Makefile
@@ -29,6 +29,9 @@ generate_rust_target-rust := y
 rustdoc_test_builder-rust := y
 rustdoc_test_gen-rust := y
 
+always-$(CONFIG_RUST) += rustc-wrapper
+clean-files += rustc-wrapper
+
 HOSTCFLAGS_tracepoint-update.o = -I$(srctree)/tools/include
 HOSTCFLAGS_elf-parse.o = -I$(srctree)/tools/include
 HOSTCFLAGS_sorttable.o = -I$(srctree)/tools/include
diff --git a/scripts/Makefile.host b/scripts/Makefile.host
index c1dedf646..84fd87a53 100644
--- a/scripts/Makefile.host
+++ b/scripts/Makefile.host
@@ -87,11 +87,30 @@ hostcxx_flags  = -Wp,-MMD,$(depfile) \
                  $(KBUILD_HOSTCXXFLAGS) $(HOST_EXTRACXXFLAGS) \
                  $(HOSTCXXFLAGS_$(target-stem).o)
 
+# rustc's `-Clinker=` expects a single executable path, not a command line.
+# `HOSTCC` may be a multi-word command when wrapped (e.g. "ccache gcc"), which
+# would otherwise be split by the shell and mis-parsed by rustc.
+# To work around this, we generate a wrapper script that forwards arguments to
+# `HOSTRUSTC_LD` so that such commands can be used safely.
+#
+# Set `HOSTRUSTC_LD` for a different rustc linker command than `HOSTCC`
+HOSTRUSTC_LD ?= $(HOSTCC)
+
+define filechk_rustc-wrapper
+	printf "%s\n" \
+		'#!/bin/sh' \
+		'$(call escsq,$(HOSTRUSTC_LD)) "$$@"'
+endef
+
+$(obj)/rustc-wrapper: FORCE
+	$(call filechk,rustc-wrapper)
+	$(Q)chmod +x $@
+
 # `--out-dir` is required to avoid temporaries being created by `rustc` in the
 # current working directory, which may be not accessible in the out-of-tree
 # modules case.
 hostrust_flags = --out-dir $(dir $@) --emit=dep-info=$(depfile) \
-		 -Clinker-flavor=gcc -Clinker=$(HOSTCC) \
+		 -Clinker-flavor=gcc -Clinker=$(obj)/rustc-wrapper \
 		 -Clink-args='$(call escsq,$(KBUILD_HOSTLDFLAGS))' \
                  $(KBUILD_HOSTRUSTFLAGS) $(HOST_EXTRARUSTFLAGS) \
                  $(HOSTRUSTFLAGS_$(target-stem))
@@ -153,10 +172,11 @@ $(host-cxxobjs): $(obj)/%.o: $(obj)/%.cc FORCE
 quiet_cmd_host-rust	= HOSTRUSTC $@
       cmd_host-rust	= \
 	$(HOSTRUSTC) $(hostrust_flags) --emit=link=$@ $<
-$(host-rust): $(obj)/%: $(src)/%.rs FORCE
+$(host-rust): $(obj)/%: $(src)/%.rs $(obj)/rustc-wrapper FORCE
 	+$(call if_changed_dep,host-rust)
 
-targets += $(host-csingle) $(host-cmulti) $(host-cobjs) \
+targets += $(obj)/rustc-wrapper \
+	   $(host-csingle) $(host-cmulti) $(host-cobjs) \
 	   $(host-cxxmulti) $(host-cxxobjs) $(host-rust)
 
 # %.lex.o <- %.lex.c <- %.l
-- 
2.53.0