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envelope-from=alxndr@bu.edu; helo=NAM10-BN7-obe.outbound.protection.outlook.com X-Spam_score_int: -8 X-Spam_score: -0.9 X-Spam_bar: / X-Spam_report: (-0.9 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, HK_RANDOM_ENVFROM=0.001, HK_RANDOM_FROM=0.999, MSGID_FROM_MTA_HEADER=0.001, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H2=-0.001, SPF_HELO_PASS=-0.001, SPF_PASS=-0.001 autolearn=no autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Laurent Vivier , Thomas Huth , Alexander Bulekov , f4bug@amsat.org, darren.kenny@oracle.com, Bandan Das , Stefan Hajnoczi , Paolo Bonzini Errors-To: qemu-devel-bounces+incoming=patchwork.ozlabs.org@nongnu.org Sender: "Qemu-devel" I noticed that with a sufficiently small timeout, the fuzzer fork-server sometimes locks up. On closer inspection, the issue appeared to be caused by entering our SIGALRM handler, while libfuzzer is in it's crash handlers. Because libfuzzer relies on pipe communication with an external child process to print out stack-traces, we shouldn't exit early, and leave an orphan child. Check for children in the SIGALRM handler to avoid this issue. Signed-off-by: Alexander Bulekov Acked-by: Thomas Huth Reviewed-by: Darren Kenny --- tests/qtest/fuzz/generic_fuzz.c | 15 +++++++++++++++ 1 file changed, 15 insertions(+) diff --git a/tests/qtest/fuzz/generic_fuzz.c b/tests/qtest/fuzz/generic_fuzz.c index ee8c17a04c..387ae2020a 100644 --- a/tests/qtest/fuzz/generic_fuzz.c +++ b/tests/qtest/fuzz/generic_fuzz.c @@ -583,6 +583,21 @@ static void handle_timeout(int sig) fprintf(stderr, "[Timeout]\n"); fflush(stderr); } + + /* + * If there is a crash, libfuzzer/ASAN forks a child to run an + * "llvm-symbolizer" process for printing out a pretty stacktrace. It + * communicates with this child using a pipe. If we timeout+Exit, while + * libfuzzer is still communicating with the llvm-symbolizer child, we will + * be left with an orphan llvm-symbolizer process. Sometimes, this appears + * to lead to a deadlock in the forkserver. Use waitpid to check if there + * are any waitable children. If so, exit out of the signal-handler, and + * let libfuzzer finish communicating with the child, and exit, on its own. + */ + if (waitpid(-1, NULL, WNOHANG) == 0) { + return; + } + _Exit(0); }