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

[Nathan Chancellor]

On Thu, Apr 02, 2026 at 05:18:31PM +0300, 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>
> Tested-by: Peter Oberparleiter <oberpar@xxxxxxxxxxxxx>
> Reviewed-by: Peter Oberparleiter <oberpar@xxxxxxxxxxxxx>

While this is obviously a fix, what are the chances of regressions from
this change? As this should only impact GCOV, this could go via whatever
tree carries GCOV patches. If Kbuild is to take this change, my vote
would be to defer it to 7.2 at this point in the development cycle so
that it can have most of a cycle to sit in -next.

> ---
>  Makefile | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/Makefile b/Makefile
> index 6b1d9fb1a6b4..a55ad668d6ba 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -806,7 +806,7 @@ 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=atomic
>  endif
>  export CFLAGS_GCOV
>  
> -- 
> 2.43.5
>