[PATCH 0/1] gcov: add -fprofile-update=atomic to fix concurrent access crashes

[PATCH 0/1] gcov: add -fprofile-update=atomic to fix concurrent access crashes

[Konstantin Khorenko]

This patch adds -fprofile-update=atomic to global CFLAGS_GCOV in the
top-level Makefile to fix crashes caused by GCC merging GCOV counters
with loop induction variables in concurrent code paths.

History
-------

This was originally posted as a zlib-only fix:
  https://lore.kernel.org/lkml/20260330143256.306326-1-khorenko@virtuozzo.com/T/#t

During review, it was suggested to apply the flag globally instead of
per-subsystem, as it not only fixes the observed crash but makes GCOV
coverage data more consistent overall.  A combined series was posted:
  https://lore.kernel.org/lkml/20260401142020.1434243-1-khorenko@virtuozzo.com/T/#t

That combined series is now split per subsystem as requested by
reviewers.

The GCC bug report for the underlying compiler issue:
  https://gcc.gnu.org/bugzilla/show_bug.cgi?id=124749

Dependencies
------------

This patch requires the following preparatory fixes to be applied first,
otherwise CONFIG_GCOV_PROFILE_ALL=y builds will fail:

 - net: fix skb_ext BUILD_BUG_ON failures with GCOV (sent to netdev):
   __no_profile on skb_ext_total_length() and skb_extensions_init()
   https://lore.kernel.org/lkml/20260402140558.1437002-1-khorenko@virtuozzo.com/T/#t

 - iommu/generic_pt: disable GCOV for iommu_amdv1.o (sent to iommu):
   GCOV_PROFILE_iommu_amdv1.o := n
   https://lore.kernel.org/lkml/20260402141012.1437095-1-khorenko@virtuozzo.com/T/#t

Without those patches, -fprofile-update=atomic prevents GCC from
constant-folding expressions inside profiled inline functions, breaking
BUILD_BUG_ON / FIELD_PREP compile-time checks.

The crash
---------

Observed during LTP IPComp stress testing on a GCOV-enabled kernel:

  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

GCC merged a global GCOV counter with the loop induction variable.
Another CPU modified the counter between loads, causing a write 3.4 MB
past a 65 KB buffer.  -fprofile-update=atomic forces atomic counter
updates and prevents this merging.

Testing
-------

Build-tested with CONFIG_GCOV_PROFILE_ALL=y using GCC 11.4.1 and
GCC 16.0.1 20260327 (experimental). Both fail without the full set
of patches, both succeed with all three series applied.

Assembly-verified that -fprofile-update=atomic prevents counter-IV
merging in inflate_fast() on both compiler versions.

Also tested by Peter Oberparleiter:

Quote: "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)."

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

 Makefile | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

-- 
2.43.5

[PATCH] 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=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@virtuozzo.com>
Tested-by: Peter Oberparleiter <oberpar@linux.ibm.com>
Reviewed-by: Peter Oberparleiter <oberpar@linux.ibm.com>

---
 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

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@virtuozzo.com>
> Tested-by: Peter Oberparleiter <oberpar@linux.ibm.com>
> Reviewed-by: Peter Oberparleiter <oberpar@linux.ibm.com>

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
>