kleaf: use /proc/self/cwd as root directory in debug info

We use "-ffile-prefix-map" option to remove build directory from debug
info and to allow debugging on any machine with source code available.
But this rule was transforming an absolute path to a relative path,
which triggers a bug in DWARF and compilers, see
https://github.com/llvm/llvm-project/issues/83174.

"/proc/self/cwd" is an absolute path that resolves to a directory where
debugger runs, so debug information remains "relocatable" and the bug is
not triggered.

Original rules were relative to a "${{KERNEL_DIR}}", but in this change
"/proc/self/cwd" corresponds to the top repo directory, because:

* Users start Bazel in top directory, it would be convenient not to
  change directory when switching to debugger.

* Generated files live outside of "${{KERNEL_DIR}}", and in non-sandbox
  build (config=local or config=fast) they are accessible from the top
  directory.

* No need to have a separate rule to rewrite
  "${{ROOT_DIR}}/${{KERNEL_DIR}}" except for unsanboxed builds, where
  "${{KERNEL_DIR}}" is a symlink to a directory outside of
  "${{ROOT_DIR}}".

Test: TH, manual builds with config=local and config=fast
Bug: 315009365
Change-Id: I7727fa93ddf39301f9f755a748d2ef3ad91007bb

1 files changed, 14 insertions, 4 deletions

diff --git a/kleaf/impl/kernel_env.bzl b/kleaf/impl/kernel_env.bzl
index 56573e7..0577263 100644
--- a/kleaf/impl/kernel_env.bzl
+++ b/kleaf/impl/kernel_env.bzl
@@ -385,15 +385,25 @@ def _get_env_setup_cmds(ctx):
 
         ## Set up KCPPFLAGS
 
-        # use relative paths for file name references in the binaries
-        # (e.g. debug info)
-        export KCPPFLAGS="-ffile-prefix-map=${{ROOT_DIR}}/${{KERNEL_DIR}}/= -ffile-prefix-map=${{ROOT_DIR}}/="
+        # Replace ${{ROOT_DIR}} with "/proc/self/cwd" in the file name
+        # references in the binaries (e.g. debug info).
+        # "/proc/self/cwd" is an absolute path that resolves to a directory
+        # where debugger runs. And ${{ROOT_DIR}} layout should be the same as
+        # layout on the top of the repo, so if you start a debugger from the
+        # top directory, all paths should resolve correctly even on another
+        # machine.
+        export KCPPFLAGS="-ffile-prefix-map=${{ROOT_DIR}}=/proc/self/cwd"
 
         # For Kleaf local (non-sandbox) builds, $ROOT_DIR is under execroot but
         # $ROOT_DIR/$KERNEL_DIR is a symlink to the real source tree under
         # workspace root, making $abs_srctree not under $ROOT_DIR.
+        # Because compiler puts a real path to a binary, it should be a real
+        # path in -ffile-prefix-map. Also we would like to leave
+        # ${{KERNEL_DIR}} part in the path to be able to run debugger from the
+        # top directory, so we go one directory up from
+        # ${{ROOT_DIR}}/${{KERNEL_DIR}} before calling realpath.
         if [[ "$(realpath ${{ROOT_DIR}}/${{KERNEL_DIR}})" != "${{ROOT_DIR}}/${{KERNEL_DIR}}" ]]; then
-            export KCPPFLAGS="$KCPPFLAGS -ffile-prefix-map=$(realpath ${{ROOT_DIR}}/${{KERNEL_DIR}})/="
+            export KCPPFLAGS="$KCPPFLAGS -ffile-prefix-map=$(realpath ${{ROOT_DIR}}/${{KERNEL_DIR}}/..)=/proc/self/cwd"
         fi
     """.format(
         get_make_jobs_cmd = status.get_volatile_status_cmd(ctx, "MAKE_JOBS"),