On Tue, Nov 28, 2023 at 06:37:50PM -0800, Andres Freund wrote: > For a moment I was, wrongly, worried this would break signal handlers we > intentionally inherit from postmaster. It's fine though, because we block > signals in fork_process() until somewhere in InitPostmasterChild(), after > we've called InitProcessGlobals(). But perhaps that should be commented upon > somewhere?
Good call. I expanded on the MyProcPid assertion in wrapper_handler() a bit. -- Nathan Bossart Amazon Web Services: https://aws.amazon.com
>From 8c36c9470bee558bcab3fa70d456ca976dc02ac2 Mon Sep 17 00:00:00 2001 From: Nathan Bossart <nat...@postgresql.org> Date: Tue, 28 Nov 2023 14:58:20 -0600 Subject: [PATCH v4 1/3] Check that MyProcPid == getpid() in all signal handlers. In commit 97550c0711, we added a similar check to the SIGTERM handler for the startup process. This commit adds this check to all signal handlers installed with pqsignal(). This is done by using a wrapper function that performs the check before calling the actual handler. The hope is that this will offer more general protection against child processes of Postgres backends inadvertently modifying shared memory due to inherited signal handlers. Another potential follow-up improvement is to use this wrapper handler function to restore errno instead of relying on each individual handler function to do so. This commit makes the changes in commit 97550c0711 obsolete but leaves reverting it for a follow-up commit. Reviewed-by: Andres Freund Discussion: https://postgr.es/m/20231121212008.GA3742740%40nathanxps13 --- src/port/pqsignal.c | 86 +++++++++++++++++++++++++++++++++++++++++++-- 1 file changed, 84 insertions(+), 2 deletions(-) diff --git a/src/port/pqsignal.c b/src/port/pqsignal.c index 23e7f685c1..f04a7153de 100644 --- a/src/port/pqsignal.c +++ b/src/port/pqsignal.c @@ -46,23 +46,99 @@ #include "c.h" #include <signal.h> +#ifndef FRONTEND +#include <unistd.h> +#endif #ifndef FRONTEND #include "libpq/pqsignal.h" +#include "miscadmin.h" +#endif + +#ifdef PG_SIGNAL_COUNT /* Windows */ +#define PG_NSIG (PG_SIGNAL_COUNT) +#elif defined(NSIG) +#define PG_NSIG (NSIG) +#else +#define PG_NSIG (64) /* XXX: wild guess */ +#endif + +/* Check a couple of common signals to make sure PG_NSIG is accurate. */ +StaticAssertDecl(SIGUSR2 < PG_NSIG, "SIGUSR2 >= PG_NSIG"); +StaticAssertDecl(SIGHUP < PG_NSIG, "SIGHUP >= PG_NSIG"); +StaticAssertDecl(SIGTERM < PG_NSIG, "SIGTERM >= PG_NSIG"); +StaticAssertDecl(SIGALRM < PG_NSIG, "SIGALRM >= PG_NSIG"); + +static volatile pqsigfunc pqsignal_handlers[PG_NSIG]; + +/* + * Except when called with SIG_IGN or SIG_DFL, pqsignal() sets up this function + * as the handler for all signals. This wrapper handler function checks that + * it is called within a process that the server knows about (i.e., any process + * that has called InitProcessGlobals(), such as a client backend), and not a + * child process forked by system(3), etc. This check ensures that such child + * processes do not modify shared memory, which is often detrimental. If the + * check succeeds, the function originally provided to pqsignal() is called. + * Otherwise, the default signal handler is installed and then called. + */ +static void +wrapper_handler(SIGNAL_ARGS) +{ +#ifndef FRONTEND + + /* + * We expect processes to set MyProcPid before calling pqsignal() or + * before accepting signals. + */ + Assert(MyProcPid); + Assert(MyProcPid != PostmasterPid || !IsUnderPostmaster); + + if (unlikely(MyProcPid != (int) getpid())) + { + pqsignal(postgres_signal_arg, SIG_DFL); + raise(postgres_signal_arg); + return; + } #endif + (*pqsignal_handlers[postgres_signal_arg]) (postgres_signal_arg); +} + /* * Set up a signal handler, with SA_RESTART, for signal "signo" * * Returns the previous handler. + * + * NB: If called within a signal handler, race conditions may lead to bogus + * return values. You should either avoid calling this within signal handlers + * or ignore the return value. + * + * XXX: Since no in-tree callers use the return value, and there is little + * reason to do so, it would be nice if we could convert this to a void + * function instead of providing potentially-bogus return values. + * Unfortunately, that requires modifying the pqsignal() in legacy-pqsignal.c, + * which in turn requires an SONAME bump, which is probably not worth it. */ pqsigfunc pqsignal(int signo, pqsigfunc func) { + pqsigfunc orig_func = pqsignal_handlers[signo]; /* assumed atomic */ #if !(defined(WIN32) && defined(FRONTEND)) struct sigaction act, oact; +#else + pqsigfunc ret; +#endif + Assert(signo < PG_NSIG); + + if (func != SIG_IGN && func != SIG_DFL) + { + pqsignal_handlers[signo] = func; /* assumed atomic */ + func = wrapper_handler; + } + +#if !(defined(WIN32) && defined(FRONTEND)) act.sa_handler = func; sigemptyset(&act.sa_mask); act.sa_flags = SA_RESTART; @@ -72,9 +148,15 @@ pqsignal(int signo, pqsigfunc func) #endif if (sigaction(signo, &act, &oact) < 0) return SIG_ERR; - return oact.sa_handler; + else if (oact.sa_handler == wrapper_handler) + return orig_func; + else + return oact.sa_handler; #else /* Forward to Windows native signal system. */ - return signal(signo, func); + if ((ret = signal(signo, func)) == wrapper_handler) + return orig_func; + else + return ret; #endif } -- 2.25.1
>From b9cf1ead0805a65657103be31fcff66dd5b2146b Mon Sep 17 00:00:00 2001 From: Nathan Bossart <nat...@postgresql.org> Date: Tue, 28 Nov 2023 15:18:13 -0600 Subject: [PATCH v4 2/3] Centralize logic for restoring errno in signal handlers. Presently, we rely on each individual signal handler to save the initial value of errno and then restore it before returning if needed. This is easily forgotten and, if missed, often goes undetected for a long time. In commit XXXXXXXXXX, we introduced a wrapper signal handler function that checks whether MyProcPid matches getpid(). This commit moves the aforementioned errno restoration code from the individual signal handlers to the new wrapper handler so that we no longer need to worry about missing it. Reviewed-by: Andres Freund Discussion: https://postgr.es/m/20231121212008.GA3742740%40nathanxps13 --- src/backend/postmaster/autovacuum.c | 4 ---- src/backend/postmaster/checkpointer.c | 4 ---- src/backend/postmaster/interrupt.c | 8 -------- src/backend/postmaster/pgarch.c | 4 ---- src/backend/postmaster/postmaster.c | 16 ---------------- src/backend/postmaster/startup.c | 12 ------------ src/backend/postmaster/syslogger.c | 4 ---- src/backend/replication/walsender.c | 4 ---- src/backend/storage/ipc/latch.c | 4 ---- src/backend/storage/ipc/procsignal.c | 4 ---- src/backend/tcop/postgres.c | 8 -------- src/backend/utils/misc/timeout.c | 4 ---- src/fe_utils/cancel.c | 3 --- src/port/pqsignal.c | 6 ++++++ 14 files changed, 6 insertions(+), 79 deletions(-) diff --git a/src/backend/postmaster/autovacuum.c b/src/backend/postmaster/autovacuum.c index 86a3b3d8be..ced7f72f63 100644 --- a/src/backend/postmaster/autovacuum.c +++ b/src/backend/postmaster/autovacuum.c @@ -1423,12 +1423,8 @@ AutoVacWorkerFailed(void) static void avl_sigusr2_handler(SIGNAL_ARGS) { - int save_errno = errno; - got_SIGUSR2 = true; SetLatch(MyLatch); - - errno = save_errno; } diff --git a/src/backend/postmaster/checkpointer.c b/src/backend/postmaster/checkpointer.c index 42c807d352..6d8c1b7ce4 100644 --- a/src/backend/postmaster/checkpointer.c +++ b/src/backend/postmaster/checkpointer.c @@ -834,15 +834,11 @@ IsCheckpointOnSchedule(double progress) static void ReqCheckpointHandler(SIGNAL_ARGS) { - int save_errno = errno; - /* * The signaling process should have set ckpt_flags nonzero, so all we * need do is ensure that our main loop gets kicked out of any wait. */ SetLatch(MyLatch); - - errno = save_errno; } diff --git a/src/backend/postmaster/interrupt.c b/src/backend/postmaster/interrupt.c index 6d4bd76bf8..fa7833f673 100644 --- a/src/backend/postmaster/interrupt.c +++ b/src/backend/postmaster/interrupt.c @@ -60,12 +60,8 @@ HandleMainLoopInterrupts(void) void SignalHandlerForConfigReload(SIGNAL_ARGS) { - int save_errno = errno; - ConfigReloadPending = true; SetLatch(MyLatch); - - errno = save_errno; } /* @@ -108,10 +104,6 @@ SignalHandlerForCrashExit(SIGNAL_ARGS) void SignalHandlerForShutdownRequest(SIGNAL_ARGS) { - int save_errno = errno; - ShutdownRequestPending = true; SetLatch(MyLatch); - - errno = save_errno; } diff --git a/src/backend/postmaster/pgarch.c b/src/backend/postmaster/pgarch.c index 46af349564..ff0958fa52 100644 --- a/src/backend/postmaster/pgarch.c +++ b/src/backend/postmaster/pgarch.c @@ -283,13 +283,9 @@ PgArchWakeup(void) static void pgarch_waken_stop(SIGNAL_ARGS) { - int save_errno = errno; - /* set flag to do a final cycle and shut down afterwards */ ready_to_stop = true; SetLatch(MyLatch); - - errno = save_errno; } /* diff --git a/src/backend/postmaster/postmaster.c b/src/backend/postmaster/postmaster.c index 7a5cd06c5c..1cc8fa2c43 100644 --- a/src/backend/postmaster/postmaster.c +++ b/src/backend/postmaster/postmaster.c @@ -2608,12 +2608,8 @@ InitProcessGlobals(void) static void handle_pm_pmsignal_signal(SIGNAL_ARGS) { - int save_errno = errno; - pending_pm_pmsignal = true; SetLatch(MyLatch); - - errno = save_errno; } /* @@ -2622,12 +2618,8 @@ handle_pm_pmsignal_signal(SIGNAL_ARGS) static void handle_pm_reload_request_signal(SIGNAL_ARGS) { - int save_errno = errno; - pending_pm_reload_request = true; SetLatch(MyLatch); - - errno = save_errno; } /* @@ -2705,8 +2697,6 @@ process_pm_reload_request(void) static void handle_pm_shutdown_request_signal(SIGNAL_ARGS) { - int save_errno = errno; - switch (postgres_signal_arg) { case SIGTERM: @@ -2723,8 +2713,6 @@ handle_pm_shutdown_request_signal(SIGNAL_ARGS) break; } SetLatch(MyLatch); - - errno = save_errno; } /* @@ -2884,12 +2872,8 @@ process_pm_shutdown_request(void) static void handle_pm_child_exit_signal(SIGNAL_ARGS) { - int save_errno = errno; - pending_pm_child_exit = true; SetLatch(MyLatch); - - errno = save_errno; } /* diff --git a/src/backend/postmaster/startup.c b/src/backend/postmaster/startup.c index 0e7de26bc2..83dbed86b9 100644 --- a/src/backend/postmaster/startup.c +++ b/src/backend/postmaster/startup.c @@ -96,32 +96,22 @@ static void StartupProcExit(int code, Datum arg); static void StartupProcTriggerHandler(SIGNAL_ARGS) { - int save_errno = errno; - promote_signaled = true; WakeupRecovery(); - - errno = save_errno; } /* SIGHUP: set flag to re-read config file at next convenient time */ static void StartupProcSigHupHandler(SIGNAL_ARGS) { - int save_errno = errno; - got_SIGHUP = true; WakeupRecovery(); - - errno = save_errno; } /* SIGTERM: set flag to abort redo and exit */ static void StartupProcShutdownHandler(SIGNAL_ARGS) { - int save_errno = errno; - if (in_restore_command) { /* @@ -140,8 +130,6 @@ StartupProcShutdownHandler(SIGNAL_ARGS) else shutdown_requested = true; WakeupRecovery(); - - errno = save_errno; } /* diff --git a/src/backend/postmaster/syslogger.c b/src/backend/postmaster/syslogger.c index 96dd03d9e0..a47838d505 100644 --- a/src/backend/postmaster/syslogger.c +++ b/src/backend/postmaster/syslogger.c @@ -1642,10 +1642,6 @@ RemoveLogrotateSignalFiles(void) static void sigUsr1Handler(SIGNAL_ARGS) { - int save_errno = errno; - rotation_requested = true; SetLatch(MyLatch); - - errno = save_errno; } diff --git a/src/backend/replication/walsender.c b/src/backend/replication/walsender.c index 3bc9c82389..a809c2202b 100644 --- a/src/backend/replication/walsender.c +++ b/src/backend/replication/walsender.c @@ -3245,12 +3245,8 @@ HandleWalSndInitStopping(void) static void WalSndLastCycleHandler(SIGNAL_ARGS) { - int save_errno = errno; - got_SIGUSR2 = true; SetLatch(MyLatch); - - errno = save_errno; } /* Set up signal handlers */ diff --git a/src/backend/storage/ipc/latch.c b/src/backend/storage/ipc/latch.c index d8a69990b3..359f648703 100644 --- a/src/backend/storage/ipc/latch.c +++ b/src/backend/storage/ipc/latch.c @@ -2243,12 +2243,8 @@ GetNumRegisteredWaitEvents(WaitEventSet *set) static void latch_sigurg_handler(SIGNAL_ARGS) { - int save_errno = errno; - if (waiting) sendSelfPipeByte(); - - errno = save_errno; } /* Send one byte to the self-pipe, to wake up WaitLatch */ diff --git a/src/backend/storage/ipc/procsignal.c b/src/backend/storage/ipc/procsignal.c index b7427906de..8e65009a89 100644 --- a/src/backend/storage/ipc/procsignal.c +++ b/src/backend/storage/ipc/procsignal.c @@ -638,8 +638,6 @@ CheckProcSignal(ProcSignalReason reason) void procsignal_sigusr1_handler(SIGNAL_ARGS) { - int save_errno = errno; - if (CheckProcSignal(PROCSIG_CATCHUP_INTERRUPT)) HandleCatchupInterrupt(); @@ -683,6 +681,4 @@ procsignal_sigusr1_handler(SIGNAL_ARGS) HandleRecoveryConflictInterrupt(PROCSIG_RECOVERY_CONFLICT_BUFFERPIN); SetLatch(MyLatch); - - errno = save_errno; } diff --git a/src/backend/tcop/postgres.c b/src/backend/tcop/postgres.c index e415cf1f34..4bfa3fc009 100644 --- a/src/backend/tcop/postgres.c +++ b/src/backend/tcop/postgres.c @@ -2970,8 +2970,6 @@ quickdie(SIGNAL_ARGS) void die(SIGNAL_ARGS) { - int save_errno = errno; - /* Don't joggle the elbow of proc_exit */ if (!proc_exit_inprogress) { @@ -2993,8 +2991,6 @@ die(SIGNAL_ARGS) */ if (DoingCommandRead && whereToSendOutput != DestRemote) ProcessInterrupts(); - - errno = save_errno; } /* @@ -3004,8 +3000,6 @@ die(SIGNAL_ARGS) void StatementCancelHandler(SIGNAL_ARGS) { - int save_errno = errno; - /* * Don't joggle the elbow of proc_exit */ @@ -3017,8 +3011,6 @@ StatementCancelHandler(SIGNAL_ARGS) /* If we're still here, waken anything waiting on the process latch */ SetLatch(MyLatch); - - errno = save_errno; } /* signal handler for floating point exception */ diff --git a/src/backend/utils/misc/timeout.c b/src/backend/utils/misc/timeout.c index 8ab755d363..c9790f8cdb 100644 --- a/src/backend/utils/misc/timeout.c +++ b/src/backend/utils/misc/timeout.c @@ -363,8 +363,6 @@ schedule_alarm(TimestampTz now) static void handle_sig_alarm(SIGNAL_ARGS) { - int save_errno = errno; - /* * Bump the holdoff counter, to make sure nothing we call will process * interrupts directly. No timeout handler should do that, but these @@ -452,8 +450,6 @@ handle_sig_alarm(SIGNAL_ARGS) } RESUME_INTERRUPTS(); - - errno = save_errno; } diff --git a/src/fe_utils/cancel.c b/src/fe_utils/cancel.c index 10c0cd4554..bffef24d63 100644 --- a/src/fe_utils/cancel.c +++ b/src/fe_utils/cancel.c @@ -152,7 +152,6 @@ ResetCancelConn(void) static void handle_sigint(SIGNAL_ARGS) { - int save_errno = errno; char errbuf[256]; CancelRequested = true; @@ -173,8 +172,6 @@ handle_sigint(SIGNAL_ARGS) write_stderr(errbuf); } } - - errno = save_errno; /* just in case the write changed it */ } /* diff --git a/src/port/pqsignal.c b/src/port/pqsignal.c index f04a7153de..775f2a40af 100644 --- a/src/port/pqsignal.c +++ b/src/port/pqsignal.c @@ -80,10 +80,14 @@ static volatile pqsigfunc pqsignal_handlers[PG_NSIG]; * processes do not modify shared memory, which is often detrimental. If the * check succeeds, the function originally provided to pqsignal() is called. * Otherwise, the default signal handler is installed and then called. + * + * This wrapper also handles restoring the value of errno. */ static void wrapper_handler(SIGNAL_ARGS) { + int save_errno = errno; + #ifndef FRONTEND /* @@ -102,6 +106,8 @@ wrapper_handler(SIGNAL_ARGS) #endif (*pqsignal_handlers[postgres_signal_arg]) (postgres_signal_arg); + + errno = save_errno; } /* -- 2.25.1
>From c47d4c30835412c39f4a06703cd8318629970d5a Mon Sep 17 00:00:00 2001 From: Nathan Bossart <nat...@postgresql.org> Date: Tue, 28 Nov 2023 15:25:03 -0600 Subject: [PATCH v4 3/3] Revert "Avoid calling proc_exit() in processes forked by system()." Thanks to commit XXXXXXXXXX, this check in the SIGTERM handler for the startup process is now obsolete and can be removed. Instead of leaving around the dead function write_stderr_signal_safe(), I've opted to just remove it for now. Thanks to modern version control software, it will be trivial to dig it up if it is ever needed in the future. This partially reverts commit 97550c0711. Reviewed-by: Andres Freund Discussion: https://postgr.es/m/20231121212008.GA3742740%40nathanxps13 --- src/backend/postmaster/startup.c | 17 +---------------- src/backend/utils/error/elog.c | 28 ---------------------------- src/include/utils/elog.h | 6 ------ 3 files changed, 1 insertion(+), 50 deletions(-) diff --git a/src/backend/postmaster/startup.c b/src/backend/postmaster/startup.c index 83dbed86b9..f40acd20ff 100644 --- a/src/backend/postmaster/startup.c +++ b/src/backend/postmaster/startup.c @@ -19,8 +19,6 @@ */ #include "postgres.h" -#include <unistd.h> - #include "access/xlog.h" #include "access/xlogrecovery.h" #include "access/xlogutils.h" @@ -113,20 +111,7 @@ static void StartupProcShutdownHandler(SIGNAL_ARGS) { if (in_restore_command) - { - /* - * If we are in a child process (e.g., forked by system() in - * RestoreArchivedFile()), we don't want to call any exit callbacks. - * The parent will take care of that. - */ - if (MyProcPid == (int) getpid()) - proc_exit(1); - else - { - write_stderr_signal_safe("StartupProcShutdownHandler() called in child process\n"); - _exit(1); - } - } + proc_exit(1); else shutdown_requested = true; WakeupRecovery(); diff --git a/src/backend/utils/error/elog.c b/src/backend/utils/error/elog.c index 6aeb855e49..ba9179da85 100644 --- a/src/backend/utils/error/elog.c +++ b/src/backend/utils/error/elog.c @@ -3733,34 +3733,6 @@ write_stderr(const char *fmt,...) } -/* - * Write a message to STDERR using only async-signal-safe functions. This can - * be used to safely emit a message from a signal handler. - * - * TODO: It is likely possible to safely do a limited amount of string - * interpolation (e.g., %s and %d), but that is not presently supported. - */ -void -write_stderr_signal_safe(const char *str) -{ - int nwritten = 0; - int ntotal = strlen(str); - - while (nwritten < ntotal) - { - int rc; - - rc = write(STDERR_FILENO, str + nwritten, ntotal - nwritten); - - /* Just give up on error. There isn't much else we can do. */ - if (rc == -1) - return; - - nwritten += rc; - } -} - - /* * Adjust the level of a recovery-related message per trace_recovery_messages. * diff --git a/src/include/utils/elog.h b/src/include/utils/elog.h index 0971d5ce33..b25ea65dff 100644 --- a/src/include/utils/elog.h +++ b/src/include/utils/elog.h @@ -536,10 +536,4 @@ extern void write_jsonlog(ErrorData *edata); */ extern void write_stderr(const char *fmt,...) pg_attribute_printf(1, 2); -/* - * Write a message to STDERR using only async-signal-safe functions. This can - * be used to safely emit a message from a signal handler. - */ -extern void write_stderr_signal_safe(const char *fmt); - #endif /* ELOG_H */ -- 2.25.1
