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 dbdffaf50ff9 ("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: https://github.com/AsahiLinux/linux/commit/54ab88878869036c9d6620101bfe17a81e88c2f9
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>

diff --git a/Makefile b/Makefile
index 2446085983f7e8ebe14027afe5f032cb23d5ca37..73a39592f112b2169477a52ab616adfe86191670 100644 (file)
--- a/Makefile
+++ b/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

diff --git a/rust/Makefile b/rust/Makefile
index 1500993d7ecc6fde8a1bcb93ff43fc7232d5c026..9801af2e1e027a720918284361b598ad846834f5 100644 (file)
--- a/rust/Makefile
+++ b/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 \