runtime: when disabling SIGPROF handler, ignore SIGPROF

If the runtime disables the SIGPROF handler, because this is Go code
that is linked into a non-Go program, then don't go back to the
default handling of SIGPROF; just start ignoring SIGPROF.
Otherwise the program can get killed by a stray SIGPROF that is
delivered, presumably to a different thread, after profiling is disabled.

Fixes #19320

Change-Id: Ifebae477d726699c8c82c867604b73110c1cf262
Reviewed-on: https://go-review.googlesource.com/c/go/+/200740
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

diff --git a/src/runtime/signal_unix.go b/src/runtime/signal_unix.go
index a9a65d51640b1da6f2239cb07d903158d9669f53..3db6133af0afad82666efd8f75d2f77cb0a86b87 100644 (file)
--- a/src/runtime/signal_unix.go
+++ b/src/runtime/signal_unix.go
@@ -242,10 +242,26 @@ func setProcessCPUProfiler(hz int32) {
                }
        } else {
                // If the Go signal handler should be disabled by default,
-               // disable it if it is enabled.
+               // switch back to the signal handler that was installed
+               // when we enabled profiling. We don't try to handle the case
+               // of a program that changes the SIGPROF handler while Go
+               // profiling is enabled.
+               //
+               // If no signal handler was installed before, then start
+               // ignoring SIGPROF signals. We do this, rather than change
+               // to SIG_DFL, because there may be a pending SIGPROF
+               // signal that has not yet been delivered to some other thread.
+               // If we change to SIG_DFL here, the program will crash
+               // when that SIGPROF is delivered. We assume that programs
+               // that use profiling don't want to crash on a stray SIGPROF.
+               // See issue 19320.
                if !sigInstallGoHandler(_SIGPROF) {
                        if atomic.Cas(&handlingSig[_SIGPROF], 1, 0) {
-                               setsig(_SIGPROF, atomic.Loaduintptr(&fwdSig[_SIGPROF]))
+                               h := atomic.Loaduintptr(&fwdSig[_SIGPROF])
+                               if h == _SIG_DFL {
+                                       h = _SIG_IGN
+                               }
+                               setsig(_SIGPROF, h)
                        }
                }
        }