[Devel] [PATCH vz10 4/4] 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. On architectures where the target supports
atomic profile updates (x86_64, arm64, ...) GCC emits atomic
instructions (e.g. lock addq) for GCOV counter updates instead of plain
load/store, which prevents the compiler from merging counters with loop
induction variables and fixes the observed concurrent-access crash.

On architectures that do not support atomic profile updates (m68k and
other small/UP targets) GCC silently falls back to the non-atomic
'single' mode, so behaviour there is no worse than before this patch.

Applying this globally rather than per-subsystem not only addresses the
observed crash in zlib but makes GCOV coverage data more consistent
overall, preventing similar issues in any kernel code path that may
execute concurrently.

Signed-off-by: Konstantin Khorenko <khorenko at virtuozzo.com>
Tested-by: Peter Oberparleiter <oberpar at linux.ibm.com>
Reviewed-by: Peter Oberparleiter <oberpar at linux.ibm.com>

Not committed to the ms yet.
https://lore.kernel.org/lkml/20260422125112.3583649-1-khorenko@virtuozzo.com/T/#t
https://virtuozzo.atlassian.net/browse/VSTOR-127788
https://virtuozzo.atlassian.net/browse/VSTOR-128012

Feature: fix ms/gcov
---
 Makefile | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Makefile b/Makefile
index e23ce694b8f9..11cf2b64e552 100644
--- a/Makefile
+++ b/Makefile
@@ -766,7 +766,7 @@ all: vmlinux
 
 CFLAGS_GCOV	:= -fprofile-arcs -ftest-coverage
 ifdef CONFIG_CC_IS_GCC
-CFLAGS_GCOV	+= -fno-tree-loop-im
+CFLAGS_GCOV	+= -fno-tree-loop-im -fprofile-update=prefer-atomic
 endif
 export CFLAGS_GCOV
 
-- 
2.47.1