[build] F: Pin newlib to _do_start-weak commit

Dockerfile

@@ -44,7 +44,10 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
 # Pinned commits for each component.
 ARG BINUTILS_COMMIT=686c3b0673c89fae3d03ee22a9d9ac9ce29bfa75
 ARG GCC_COMMIT=d6995f34118ad2a5ed2f0408a3a8af44568bf2ee
-ARG NEWLIB_COMMIT=e12d84a6789c07f938db4f6440ea0b427914c735
+# Pinned to the merge commit of nanvix/newlib#TBD ("[libc] F: Weaken
+# _do_start so libnvx_crt0.a can override it cleanly").  Update this
+# SHA to the upstream merge commit before merging the present PR.
+ARG NEWLIB_COMMIT=881840f880350e6fd929629cef9a5a3d1708bbf4
 ARG NEWLIB_BRANCH=dev
 
 ENV PREFIX=/opt/nanvix
@@ -61,9 +64,14 @@ RUN git clone https://github.com/nanvix/binutils /build/binutils && \
 RUN git clone https://github.com/nanvix/gcc /build/gcc && \
     cd /build/gcc && git checkout ${GCC_COMMIT}
 
-# Clone Newlib.
-RUN git clone --branch ${NEWLIB_BRANCH} --single-branch --depth=1 \
-    https://github.com/nanvix/newlib /build/newlib
+# Clone Newlib and pin to ${NEWLIB_COMMIT} (prior versions of this
+# Dockerfile cloned with --depth=1 only and silently used the tip of
+# ${NEWLIB_BRANCH} regardless of the declared pin, which made the
+# build non-reproducible).  Fetching with sufficient depth and then
+# checking out ${NEWLIB_COMMIT} restores the pin.
+RUN git clone --branch ${NEWLIB_BRANCH} \
+    https://github.com/nanvix/newlib /build/newlib && \
+    cd /build/newlib && git checkout ${NEWLIB_COMMIT}
 
 # Build Binutils.
 RUN cd /build/binutils && \

Dockerfile.localpatch-do_start

@@ -0,0 +1,56 @@
+# Local-only Dockerfile to validate the nanvix/newlib `_do_start` weakening
+# fix WITHOUT a full toolchain rebuild.  Starts from the official base image
+# and surgically rebuilds the `crt0.S` source from a patched newlib tree,
+# then overlays the resulting `crt0.o` onto the toolchain sysroot.
+#
+# After this patch:
+#   * `/opt/nanvix/i686-nanvix/lib/crt0.o` defines `_do_start` as a WEAK
+#     (`W`) symbol instead of STRONG (`T`).
+#   * `libnvx_crt0.a`'s strong `_do_start` override now wins at link time
+#     without `-Wl,--allow-multiple-definition`.
+#
+# Usage (run from `D:\src\toolchain-gcc-dev`):
+#   docker build --build-context newlib=D:\src\newlib-dev `
+#       -f Dockerfile.localpatch-do_start `
+#       -t local-nanvix/toolchain-gcc:do_start-weak .
+#
+# Verification:
+#   docker run --rm local-nanvix/toolchain-gcc:do_start-weak `
+#       i686-nanvix-nm /opt/nanvix/i686-nanvix/lib/crt0.o
+#   Expected: `00000000 W _do_start` (weak, not strong T).
+
+FROM ghcr.io/nanvix/toolchain-gcc:sha-34a3641
+
+# DO NOT set `ENV TARGET=` or `ENV PREFIX=` here.  The published
+# toolchain-gcc runtime image deliberately drops those build-time
+# variables; setting them again leaks into autotools-based downstream
+# builds.  In particular, libffi's `AX_ENABLE_BUILDDIR` macro
+# (m4/ax_enable_builddir.m4) computes its build subdir from `$TARGET`
+# if the variable is set, defeating config.sub's canonicalization of
+# `i686-nanvix` -> `i686-pc-nanvix`.  The result is that the build
+# lands in `./i686-nanvix/` while `Makefile.nanvix` and `z.py` look
+# in `./i686-pc-nanvix/`, breaking the link step.
+#
+# The `PATH` extension is the only inherited env override that is
+# safe and necessary.
+
+# Copy just the patched crt0.S source.
+COPY --from=newlib newlib/libc/sys/nanvix/crt0.S /build/patch/crt0.S
+
+# Normalize line endings — Windows checkouts may carry CRLF.
+RUN sed -i 's/\r$//' /build/patch/crt0.S
+
+# Reassemble crt0.S into crt0.o.
+RUN /opt/nanvix/bin/i686-nanvix-as \
+        /build/patch/crt0.S \
+        -o /build/patch/crt0.o
+
+# Replace the toolchain sysroot's crt0.o.
+RUN cp /build/patch/crt0.o /opt/nanvix/i686-nanvix/lib/crt0.o
+
+# Verify: _do_start must now be WEAK, not STRONG.
+RUN result=$(/opt/nanvix/bin/i686-nanvix-nm /opt/nanvix/i686-nanvix/lib/crt0.o); \
+    echo "$result"; \
+    echo "$result" | grep -qE '^[0-9a-f]+ W _do_start$' \
+        || (echo "VERIFY FAILED: _do_start is not WEAK in patched crt0.o" && exit 1) \
+    && echo "VERIFY OK: _do_start is now WEAK in crt0.o"