[PATCH] rust: build: remap path to avoid absolute path

[Gary Guo <gary@kernel.org>]

From: Gary Guo <gary@garyguo.net>

When building with a 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 into the binary
which is not very helpful to achieve reproducibility. It also 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
sovles 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 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 [2]
Signed-off-by: Gary Guo <gary@garyguo.net>
---
 rust/Makefile          | 1 +
 scripts/Makefile.build | 1 +
 2 files changed, 2 insertions(+)

diff --git a/rust/Makefile b/rust/Makefile
index 629b3bdd2b20..598d2efede32 100644
--- a/rust/Makefile
+++ b/rust/Makefile
@@ -582,6 +582,7 @@ quiet_cmd_rustc_library = $(if $(skip_clippy),RUSTC,$(RUSTC_OR_CLIPPY_QUIET)) L
 		--crate-type rlib -L$(objtree)/$(obj) \
 		--crate-name $(patsubst %.o,%,$(notdir $@)) $< \
 		--sysroot=/dev/null \
+		--remap-path-prefix=$(abspath $(srctree))= \
 		-Zunstable-options \
 	$(if $(rustc_objcopy),;$(OBJCOPY) $(rustc_objcopy) $@) \
 	$(cmd_objtool)
diff --git a/scripts/Makefile.build b/scripts/Makefile.build
index 204e58dd1bb0..03dde30e953c 100644
--- a/scripts/Makefile.build
+++ b/scripts/Makefile.build
@@ -333,6 +333,7 @@ rust_common_cmd = \
 	--crate-type rlib -L $(objtree)/rust/ \
 	--crate-name $(basename $(notdir $@)) \
 	--sysroot=/dev/null \
+	--remap-path-prefix=$(abspath $(srctree))= \
 	--out-dir $(dir $@) --emit=dep-info=$(depfile)
 
 # `--emit=obj`, `--emit=asm` and `--emit=llvm-ir` imply a single codegen unit

base-commit: 36896083ef4dbc302e406f01975a227784160cf8
-- 
2.51.2