[PATCH v2] kbuild: host: use single executable for rustc -C linker

[Mohamad Alsadhan <mo@sdhn.cc>]

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 ...

Introduce HOSTRUSTC_LD to select the linker executable for host Rust
tools. When HOSTCC expands to multiple arguments and HOSTRUSTC_LD is
not set, fall back to the last word of HOSTCC and warn. Users needing
wrapper semantics can point HOSTRUSTC_LD at a wrapper script.

Closes: https://github.com/Rust-for-Linux/linux/issues/1224
Suggested-by: Yoann Congal <yoann.congal@smile.fr>

Acked-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Mohamad Alsadhan <mo@sdhn.cc>
---
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/
---
 scripts/Makefile.host | 21 ++++++++++++++++++++-
 1 file changed, 20 insertions(+), 1 deletion(-)

diff --git a/scripts/Makefile.host b/scripts/Makefile.host
index c1dedf646..f029f3f0c 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 `-C linker=` 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.
+#
+# Allow users to override the linker used by host Rust tools via HOSTRUSTC_LD.
+# If HOSTCC is multi-word and HOSTRUSTC_LD is not set, default to the last
+# word of HOSTCC (typically the underlying compiler) and emit a warning.
+ifndef HOSTRUSTC_LD
+HOSTRUSTC_LD := $(HOSTCC)
+endif
+
+ifneq ($(words $(HOSTRUSTC_LD)),1)
+ifneq ($(filter command\ line environment,$(origin HOSTRUSTC_LD)),)
+$(error HOSTRUSTC_LD must be a single executable for rustc -C linker=. Got: "$(HOSTRUSTC_LD)")
+endif
+HOSTRUSTC_LD := $(lastword $(HOSTCC))
+$(warning HOSTCC expands to multiple arguments. Trying HOSTRUSTC_LD="$(HOSTRUSTC_LD)" for rustc. Set HOSTRUSTC_LD to override.)
+endif
+
 # `--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=$(HOSTCC) \
+		 -Clinker-flavor=gcc -Clinker=$(HOSTRUSTC_LD) \
 		 -Clink-args='$(call escsq,$(KBUILD_HOSTLDFLAGS))' \
                  $(KBUILD_HOSTRUSTFLAGS) $(HOST_EXTRARUSTFLAGS) \
                  $(HOSTRUSTFLAGS_$(target-stem))
-- 
2.52.0