Re: [PATCH v3 4/4] gcov: use atomic counter updates to fix concurrent access crashes

[Peter Oberparleiter]

On 01.04.2026 16:20, Konstantin Khorenko wrote:
> 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=atomic to CFLAGS_GCOV at the global level in the
> top-level Makefile.  This tells GCC that GCOV counters may be
> concurrently accessed, causing counter updates to use atomic
> instructions (lock addq) instead of plain load/store.  This prevents
> the compiler from merging counters with loop induction variables.
> 
> 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@xxxxxxxxxxxxx>

Thanks, this looks good to me!

Successfully tested this series on s390 (except for patch 3 which
depends on x86) using GCC 15.2.0, GCC 10.1.0, and current Clang from git
(20260401).

Tested-by: Peter Oberparleiter <oberpar@xxxxxxxxxxxxx>
Reviewed-by: Peter Oberparleiter <oberpar@xxxxxxxxxxxxx>

-- 
Peter Oberparleiter
Linux on IBM Z Development - IBM Germany R&D