On 14.05.2020 23:02, Joerg Sonnenberger wrote: > On Thu, May 14, 2020 at 07:21:35PM +0000, Kamil Rytarowski wrote: >> Module Name: src >> Committed By: kamil >> Date: Thu May 14 19:21:35 UTC 2020 >> >> Modified Files: >> src/tests/lib/libc/sys: t_ptrace_fork_wait.h >> >> Log Message: >> Ignore interception of the SIGCHLD signals. >> >> SIGCHLD once blocked is discarded by the kernel as it has the >> SA_IGNORE property. During the fork(2) operation all signals can be >> shortly blocked and missed (unless there is a registered signal >> handler in the traced child). This leads to a race in this test if >> there would be an intention to catch SIGCHLD. > > This doesn't sound right. sigprocmask and pthread_sigmask shouldn't > affect whether the signal handler is called, only when. Temporarily > masking a signal is different from setting it to SIG_IGN. > > Joerg >
I was looking into it and as there is no signal handler for SIGCHLD, the signal is discarded. Another approach to fix the race would be to: 1. Add SIGCHLD signal handler in the traced child. 2. Explicitly masking SIGCHLD in the traced child. If a signal would not reach the child process (as it is ignored or masked+SA_IGNOREd) and it is not a crash signal, it is dropped. As I checked, it's the design in UNIX to overlook SIGCHLD signals in UNIX. Race free signals could be maybe possible, but with some design rethinking.
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