Fix strict aliasing UB in MurMur hash implementation. #459
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This was spotted when trying to upgrade the libseccomp fedora package to version 2.6.0 in fedora rawhide. It comes with gcc 15 and LTO enabled by default. When running the test 61-sim-transactions we get plenty of such errors in valgrind: ==265507== Use of uninitialised value of size 8 ==265507== at 0x4096AD: _hsh_add (gen_bpf.c:599) ==265507== by 0x40A557: UnknownInlinedFun (gen_bpf.c:2016) ==265507== by 0x40A557: gen_bpf_generate (gen_bpf.c:2341) ==265507== by 0x400CDE: UnknownInlinedFun (db.c:2685) ==265507== by 0x400CDE: UnknownInlinedFun (db.c:2682) ==265507== by 0x400CDE: UnknownInlinedFun (api.c:756) ==265507== by 0x400CDE: UnknownInlinedFun (util.c:162) ==265507== by 0x400CDE: UnknownInlinedFun (util.c:153) ==265507== by 0x400CDE: main (61-sim-transactions.c:128) ==265507== Uninitialised value was created by a stack allocation ==265507== at 0x409590: _hsh_add (gen_bpf.c:573) Investigating this a bit, it seems that because of LTO the MurMur hash implementation is being inlined in _hsh_add. The way we call getblock32 with the explicit cast to const uint32_t* is a strict aliasing violation. This is reproducible on a "fedora:rawhide" container (gcc 15) and using: export CFLAGS='-O2 -flto=auto -ffat-lto-objects -g' Signed-off-by: Romain Geissler <[email protected]>
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This was spotted when trying to upgrade the libseccomp fedora package to version 2.6.0 in fedora rawhide. It comes with gcc 15 and LTO enabled by default. When running the test 61-sim-transactions we get plenty of such errors in valgrind:
Investigating this a bit, it seems that because of LTO the MurMur hash implementation is being inlined in _hsh_add. The way we call
getblock32with the explicit cast toconst uint32_t*is a strict aliasing violation.This is reproducible on a "fedora:rawhide" container (gcc 15) and using:
export CFLAGS='-O2 -flto=auto -ffat-lto-objects -g'