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 zlib Makefiles. 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. The flag is scoped to zlib only to avoid unnecessary overhead in the rest of the kernel. https://virtuozzo.atlassian.net/browse/VSTOR-127788 Feature: fix selftests Signed-off-by: Konstantin Khorenko <[email protected]> --- lib/zlib_deflate/Makefile | 6 ++++++ lib/zlib_dfltcc/Makefile | 6 ++++++ lib/zlib_inflate/Makefile | 7 +++++++ 3 files changed, 19 insertions(+) diff --git a/lib/zlib_deflate/Makefile b/lib/zlib_deflate/Makefile index 2622e03c0b942..dc0b3e5660e9e 100644 --- a/lib/zlib_deflate/Makefile +++ b/lib/zlib_deflate/Makefile @@ -7,6 +7,12 @@ # decompression code. # +# Force atomic GCOV counter updates to prevent GCC from merging global +# counters with loop induction variables (see lib/zlib_inflate/Makefile). +ifdef CONFIG_GCOV_KERNEL +ccflags-y += -fprofile-update=atomic +endif + obj-$(CONFIG_ZLIB_DEFLATE) += zlib_deflate.o zlib_deflate-objs := deflate.o deftree.o deflate_syms.o diff --git a/lib/zlib_dfltcc/Makefile b/lib/zlib_dfltcc/Makefile index 66e1c96387c40..fb08749d2ee7b 100644 --- a/lib/zlib_dfltcc/Makefile +++ b/lib/zlib_dfltcc/Makefile @@ -6,6 +6,12 @@ # This is the code for s390 zlib hardware support. # +# Force atomic GCOV counter updates to prevent GCC from merging global +# counters with loop induction variables (see lib/zlib_inflate/Makefile). +ifdef CONFIG_GCOV_KERNEL +ccflags-y += -fprofile-update=atomic +endif + obj-$(CONFIG_ZLIB_DFLTCC) += zlib_dfltcc.o zlib_dfltcc-objs := dfltcc.o dfltcc_deflate.o dfltcc_inflate.o diff --git a/lib/zlib_inflate/Makefile b/lib/zlib_inflate/Makefile index 27327d3e9f541..8707c649adda5 100644 --- a/lib/zlib_inflate/Makefile +++ b/lib/zlib_inflate/Makefile @@ -14,6 +14,13 @@ # uncompression can be done without blocking on allocation). # +# Force atomic GCOV counter updates to prevent GCC from merging global +# counters with loop induction variables — concurrent inflate_fast() +# execution on multiple CPUs causes out-of-bounds writes otherwise. +ifdef CONFIG_GCOV_KERNEL +ccflags-y += -fprofile-update=atomic +endif + obj-$(CONFIG_ZLIB_INFLATE) += zlib_inflate.o zlib_inflate-objs := inffast.o inflate.o infutil.o \ -- 2.43.0 _______________________________________________ Devel mailing list [email protected] https://lists.openvz.org/mailman/listinfo/devel
