CVE-2026-63825: gcov: use atomic counter updates to fix concurrent access crashes

In the Linux kernel, the following vulnerability has been resolved:

gcov: use atomic counter updates to fix concurrent access crashes

GCC's GCOV instrumentation can merge global branch counters with loop induction variables as an optimization. In inflatefast(), 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 inflatefast() runs concurrently on multiple CPUs:

BUG: unable to handle page fault for address: ffffd0a3c0902ffa RIP: inflatefast+1431 Call Trace: zlibinflate deflatedecompress cryptocompdecompress ipcompdecompress [xfrmipcomp] ipcompinput [xfrmipcomp] xfrminput

At the crash point, the compiler generated three loads from the same global GCOV counter (gcov0.inflatefast+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 CFLAGSGCOV 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 KBUILDCFLAGS, then compares undefined symbols in the resulting object files. If prefer-atomic introduces new undefined references (such as atomicfetchadd8 on i386 or aarch64ldadd8relax 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 (x8664, 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.

Move the CFLAGSGCOV block from its original position (before the arch Makefile include) to after the core KBUILDCFLAGS assignments but before the scripts/Makefile.gcc-plugins include. This placement ensures the try-run test sees arch-specific flags (-m32, -march=, -mno-outline-atomics) while avoiding GCC plugin flags (-fplugin=) that would break the test on clean builds when plugin shared objects do not yet exist.