[PATCH v3 0/1] gcov: use -fprofile-update=prefer-atomic with compile-time guard

[PATCH v3 0/1] gcov: use -fprofile-update=prefer-atomic with compile-time guard

[Konstantin Khorenko]

This is v3 of the patch to add -fprofile-update=prefer-atomic to
CFLAGS_GCOV.

v2 was reported to cause link failures on some architecture/config
combinations because GCC emits calls to libatomic runtime functions
for 64-bit atomic counter increments, and the kernel does not link
against libatomic:

  https://lore.kernel.org/all/ff2a4c49-463d-4d8a-9519-bb51308f7ba1@linux.ibm.com/

Arnd Bergmann hit this with GCC-16 randconfig builds:

  x86_64:  undefined reference to `__atomic_fetch_add_8'
  aarch64: undefined reference to `__aarch64_ldadd8_relax'

The kernel test robot confirmed the same on i386-allmodconfig with
GCC 14 (Debian):

  https://lore.kernel.org/all/202605030611.mBKmkPOF-lkp@intel.com/

v3 adds a compile-time try-run check that determines whether
-fprofile-update=prefer-atomic is safe to use with the current
compiler and architecture.

=== Approach ===

The check compiles a minimal test program twice using the full
KBUILD_CFLAGS -- once without and once with -fprofile-update=prefer-atomic
-- then compares the undefined symbols in both resulting .o files using
nm.  If prefer-atomic introduces any NEW undefined symbols, the flag is
not added.

Several alternative approaches were considered and rejected:

1) Grepping assembly output for known libatomic symbols
   (__atomic_fetch_add, __aarch64_ldadd, etc):
   Fragile -- requires maintaining a list of arch-specific symbol names.
   New architectures or GCC versions may use different names.

2) Checking nm output for any undefined symbol beyond __gcov_*:
   Fails because KBUILD_CFLAGS adds kernel-specific instrumentation
   (__fentry__, __x86_return_thunk, etc) that creates "expected"
   undefined symbols unrelated to libatomic.

3) Grepping only for "__atomic" in undefined symbols:
   Misses aarch64 outline-atomics symbols (__aarch64_ldadd8_relax)
   which do not contain "atomic" in their name.

4) Filtering KBUILD_CFLAGS to pass only -m32/-m64/-march=* to try-run:
   Brittle whitelist -- misses flags like -mno-outline-atomics on arm64
   and will break when new relevant flags are added.

The chosen diff-based approach is fully architecture-agnostic: it uses
the real KBUILD_CFLAGS, does not depend on knowing libatomic symbol
names, and will not break when new flags or architectures are added.
The only assumption is that -fprofile-update=prefer-atomic should not
introduce any new linker dependencies.

Also, the CFLAGS_GCOV block is moved after the final KBUILD_CFLAGS
assignments so the try-run test sees the complete set of compiler flags.

=== Testing ===

Verified on:
  - x86_64, GCC 17.0.0 (trunk 2026-05-09): flag IS added, inline
    lock addq for GCOV counters
  - arm64 cross-compile, GCC 14.1.1 (aarch64-linux-gnu-gcc):
    flag is NOT added (__aarch64_ldadd8_relax detected)

arm64 example showing the try-run detection in action:

  $ echo 'long long x; void f(void){x++;}' | \
    aarch64-linux-gnu-gcc [KBUILD_CFLAGS] -fprofile-arcs \
    -ftest-coverage -c -o base.o
  $ nm base.o | grep ' U '
                   U __gcov_exit
                   U __gcov_init
                   U __gcov_merge_add

  $ echo 'long long x; void f(void){x++;}' | \
    aarch64-linux-gnu-gcc [KBUILD_CFLAGS] -fprofile-arcs \
    -ftest-coverage -fprofile-update=prefer-atomic -c -o test.o
  $ nm test.o | grep ' U '
                   U __aarch64_ldadd8_relax   <-- new, from libatomic
                   U __gcov_exit
                   U __gcov_init
                   U __gcov_merge_add

  The undefined symbols differ => try-run fails => flag not added.

Changes since v2:
  - Added try-run compile-time check (option 3 from Peter's proposal)
  - Moved CFLAGS_GCOV definition after KBUILD_CFLAGS is finalized
  - Split -fprofile-update=prefer-atomic from -fno-tree-loop-im

Konstantin Khorenko (1):
  gcov: use atomic counter updates to fix concurrent access crashes

 Makefile | 27 +++++++++++++++++++++------
 1 file changed, 21 insertions(+), 6 deletions(-)

-- 
2.47.1

[PATCH v3 1/1] gcov: use atomic counter updates to fix concurrent access crashes

[Konstantin Khorenko]

GCC's GCOV instrumentation can merge global branch counters with loop
induction variables as an optimization.  In inflate_fast(), the inner
copy loops get transformed so that the GCOV counter value is loaded
multiple times to compute the loop base address, start index, and end
bound.  Since GCOV counters are global (not per-CPU), concurrent
execution on different CPUs causes the counter to change between loads,
producing inconsistent values and out-of-bounds memory writes.

The crash manifests during IPComp (IP Payload Compression) processing
when inflate_fast() runs concurrently on multiple CPUs:

  BUG: unable to handle page fault for address: ffffd0a3c0902ffa
  RIP: inflate_fast+1431
  Call Trace:
   zlib_inflate
   __deflate_decompress
   crypto_comp_decompress
   ipcomp_decompress [xfrm_ipcomp]
   ipcomp_input [xfrm_ipcomp]
   xfrm_input

At the crash point, the compiler generated three loads from the same
global GCOV counter (__gcov0.inflate_fast+216) to compute base, start,
and end for an indexed loop.  Another CPU modified the counter between
loads, making the values inconsistent - the write went 3.4 MB past a
65 KB buffer.

Add -fprofile-update=prefer-atomic to CFLAGS_GCOV at the global level in
the top-level Makefile, guarded by a try-run compile test.
The test compiles a minimal program with and without
-fprofile-update=prefer-atomic using the full KBUILD_CFLAGS, then
compares undefined symbols in the resulting object files.
If prefer-atomic introduces new undefined references (such as
__atomic_fetch_add_8 on i386 or __aarch64_ldadd8_relax on arm64 with
outline-atomics), the flag is not added -- the kernel does not link
against libatomic.

On architectures where GCC inlines 64-bit atomic counter updates
(x86_64, s390, ...) the test passes and the flag is enabled, preventing
the compiler from merging counters with loop induction variables and
fixing the observed concurrent-access crash.

On architectures where the flag would introduce libatomic dependencies,
it is silently omitted and behaviour is no worse than before this patch.

Also move the CFLAGS_GCOV block after the final KBUILD_CFLAGS assignments
so the try-run test sees the complete set of compiler flags.

Signed-off-by: Konstantin Khorenko <khorenko@virtuozzo.com>
---
 Makefile | 27 +++++++++++++++++++++------
 1 file changed, 21 insertions(+), 6 deletions(-)

diff --git a/Makefile b/Makefile
index 9f88dcaae382..95afeecb09e4 100644
--- a/Makefile
+++ b/Makefile
@@ -824,12 +824,6 @@ endif # KBUILD_EXTMOD
 # Defaults to vmlinux, but the arch makefile usually adds further targets
 all: vmlinux
 
-CFLAGS_GCOV	:= -fprofile-arcs -ftest-coverage
-ifdef CONFIG_CC_IS_GCC
-CFLAGS_GCOV	+= -fno-tree-loop-im
-endif
-export CFLAGS_GCOV
-
 # The arch Makefiles can override CC_FLAGS_FTRACE. We may also append it later.
 ifdef CONFIG_FUNCTION_TRACER
   CC_FLAGS_FTRACE := -pg
@@ -1183,6 +1177,27 @@ KBUILD_AFLAGS   += $(KAFLAGS)
 KBUILD_CFLAGS   += $(KCFLAGS)
 KBUILD_RUSTFLAGS += $(KRUSTFLAGS)
 
+CFLAGS_GCOV	:= -fprofile-arcs -ftest-coverage
+ifdef CONFIG_CC_IS_GCC
+CFLAGS_GCOV	+= -fno-tree-loop-im
+# Use atomic counter updates to avoid concurrent-access crashes in GCOV.
+# Only enable if -fprofile-update=prefer-atomic does not introduce new
+# undefined symbols (e.g. libatomic calls that the kernel cannot link).
+CFLAGS_GCOV	+= $(call try-run,\
+	echo 'long long x; void f(void){x++;}' | \
+	$(CC) $(KBUILD_CPPFLAGS) $(KBUILD_CFLAGS) -w -fprofile-arcs \
+	-ftest-coverage -x c - -c -o "$$TMP.base" && \
+	echo 'long long x; void f(void){x++;}' | \
+	$(CC) $(KBUILD_CPPFLAGS) $(KBUILD_CFLAGS) -w -fprofile-arcs \
+	-ftest-coverage -fprofile-update=prefer-atomic \
+	-x c - -c -o "$$TMP" && \
+	$(NM) "$$TMP.base" | grep ' U ' > "$$TMP.ubase" || true ; \
+	$(NM) "$$TMP" | grep ' U ' > "$$TMP.utest" || true ; \
+	cmp -s "$$TMP.ubase" "$$TMP.utest",\
+	-fprofile-update=prefer-atomic)
+endif
+export CFLAGS_GCOV
+
 KBUILD_LDFLAGS_MODULE += --build-id=sha1
 LDFLAGS_vmlinux += --build-id=sha1
 

base-commit: 70390501d1944d4e5b8f7352be180fceb3a44132
-- 
2.47.1