rust: build: remap path to avoid absolute path

When building with an out directory (O=), absolute paths can end up in the
file name in `#[track_caller]` or the panic message. This is not desirable
as this leaks the exact path being used to build the kernel and means that
the same location can appear in two forms (relative or absolute).

This is reported by Asahi [1] and is being workaround in [2] previously to
force everything to be absolute path. Using absolute path for everything
solves the inconsistency, however it does not address the reproducibility
issue. So, fix this by remap all absolute paths to srctree to relative path
instead.

This is previously attempted in commit dbdffaf ("kbuild, rust: use
-fremap-path-prefix to make paths relative") but that was reverted as
remapping debug info causes some tool (e.g. objdump) to be unable to find
sources. Therefore, use `--remap-path-scope` to only remap macros but leave
debuginfo untouched. `--remap-path-scope` is only stable in Rust 1.95, so
use `rustc-option` to detect its presence. This feature has been available
as  `-Zremap-path-scope` for all versions that we support; however due to
bugs in the Rust compiler, it does not work reliably until 1.94. I opted to
not enable it for 1.94 as it's just a single version that we missed.

This change can be validated by building a kernel with O=, strip debug info
on vmlinux, and then check if the absolute path exists in `strings
vmlinux`, e.g. `strings vmlinux |grep \/home`.

Reported-by: Janne Grunau <j@jannau.net>
Reported-by: Asahi Lina <lina+kernel@asahilina.net>
Closes: https://rust-for-linux.zulipchat.com/#narrow/channel/288089-General/topic/Per-call-site.20data.20and.20lock.20class.20keys/near/572466559 [1]
Link: AsahiLinux@54ab888 [2]
Signed-off-by: Gary Guo <gary@garyguo.net>
Acked-by: Nicolas Schier <nsc@kernel.org> # kbuild
Link: https://patch.msgid.link/20260226152112.3222886-1-gary@kernel.org
[ Reworded for few typos. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: nbdd0121

Makefile

@@ -1113,6 +1113,9 @@ KBUILD_CFLAGS += -fno-builtin-wcslen
 # change __FILE__ to the relative path to the source directory
 ifdef building_out_of_srctree
 KBUILD_CPPFLAGS += -fmacro-prefix-map=$(srcroot)/=
+ifeq ($(call rustc-option-yn, --remap-path-scope=macro),y)
+KBUILD_RUSTFLAGS += --remap-path-prefix=$(srcroot)/= --remap-path-scope=macro
+endif
 endif
 
 # include additional Makefiles when needed

rust/Makefile

@@ -148,7 +148,8 @@ doctests_modifiers_workaround := $(rustdoc_modifiers_workaround)$(if $(call rust
 quiet_cmd_rustdoc = RUSTDOC $(if $(rustdoc_host),H, ) $<
       cmd_rustdoc = \
 	OBJTREE=$(abspath $(objtree)) \
-	$(RUSTDOC) $(filter-out $(skip_flags) --remap-path-prefix=%,$(if $(rustdoc_host),$(rust_common_flags),$(rust_flags))) \
+	$(RUSTDOC) $(filter-out $(skip_flags) --remap-path-prefix=% --remap-path-scope=%, \
+			$(if $(rustdoc_host),$(rust_common_flags),$(rust_flags))) \
 		$(rustc_target_flags) -L$(objtree)/$(obj) \
 		-Zunstable-options --generate-link-to-definition \
 		--output $(rustdoc_output) \
@@ -334,7 +335,7 @@ quiet_cmd_rustdoc_test_kernel = RUSTDOC TK $<
 	rm -rf $(objtree)/$(obj)/test/doctests/kernel; \
 	mkdir -p $(objtree)/$(obj)/test/doctests/kernel; \
 	OBJTREE=$(abspath $(objtree)) \
-	$(RUSTDOC) --test $(filter-out --remap-path-prefix=%,$(rust_flags)) \
+	$(RUSTDOC) --test $(filter-out --remap-path-prefix=% --remap-path-scope=%,$(rust_flags)) \
 		-L$(objtree)/$(obj) --extern ffi --extern pin_init \
 		--extern kernel --extern build_error --extern macros \
 		--extern bindings --extern uapi \