From 54fc9dca5b10e82cb85c7cd44ebe1eb0f28f795b Mon Sep 17 00:00:00 2001 From: Nathan Bossart Date: Tue, 17 Oct 2023 10:42:12 -0500 Subject: [PATCH] Avoid calling proc_exit() in processes forked by system(). The SIGTERM handler for the startup process immediately calls proc_exit() for the duration of the restore_command, i.e., a call to system(). This system() call forks a new process to execute the shell command, and this child process inherits the parent's signal handlers. If both the parent and child processes receive SIGTERM, both will attempt to call proc_exit(). This can end badly. For example, both processes will try to remove themselves from the PGPROC shared array. To fix this problem, this commit adds a check in StartupProcShutdownHandler() to see whether MyProcPid == getpid(). If they match, this is the parent process, and we can proc_exit() like before. If they do not match, this is a child process, and we just emit a message to STDERR (in a signal safe manner) and _exit(), thereby skipping any problematic exit callbacks. This commit also adds checks in proc_exit(), ProcKill(), and AuxiliaryProcKill() that verify they are not being called within such child processes. Suggested-by: Andres Freund Reviewed-by: Thomas Munro, Andres Freund Discussion: https://postgr.es/m/Y9nGDSgIm83FHcad%40paquier.xyz Discussion: https://postgr.es/m/20230223231503.GA743455%40nathanxps13 Backpatch-through: 11 --- src/backend/postmaster/startup.c | 17 ++++++++++++++++- src/backend/storage/ipc/ipc.c | 4 ++++ src/backend/storage/lmgr/proc.c | 8 ++++++++ src/backend/utils/error/elog.c | 28 ++++++++++++++++++++++++++++ src/include/utils/elog.h | 6 ++++++ 5 files changed, 62 insertions(+), 1 deletion(-) diff --git a/src/backend/postmaster/startup.c b/src/backend/postmaster/startup.c index 69077bd2075..c3f5e18b9c9 100644 --- a/src/backend/postmaster/startup.c +++ b/src/backend/postmaster/startup.c @@ -19,6 +19,8 @@ */ #include "postgres.h" +#include + #include "access/xlog.h" #include "libpq/pqsignal.h" #include "miscadmin.h" @@ -102,7 +104,20 @@ StartupProcShutdownHandler(SIGNAL_ARGS) int save_errno = errno; if (in_restore_command) - proc_exit(1); + { + /* + * 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); + } + } else shutdown_requested = true; WakeupRecovery(); diff --git a/src/backend/storage/ipc/ipc.c b/src/backend/storage/ipc/ipc.c index 4045d7d68a0..df89a28d2ff 100644 --- a/src/backend/storage/ipc/ipc.c +++ b/src/backend/storage/ipc/ipc.c @@ -103,6 +103,10 @@ static int on_proc_exit_index, void proc_exit(int code) { + /* not safe if forked by system(), etc. */ + if (MyProcPid != (int) getpid()) + elog(PANIC, "proc_exit() called in child process"); + /* Clean up everything that must be cleaned up */ proc_exit_prepare(code); diff --git a/src/backend/storage/lmgr/proc.c b/src/backend/storage/lmgr/proc.c index f323228602f..1e06d6580ce 100644 --- a/src/backend/storage/lmgr/proc.c +++ b/src/backend/storage/lmgr/proc.c @@ -826,6 +826,10 @@ ProcKill(int code, Datum arg) Assert(MyProc != NULL); + /* not safe if forked by system(), etc. */ + if (MyProc->pid != (int) getpid()) + elog(PANIC, "ProcKill() called in child process"); + /* Make sure we're out of the sync rep lists */ SyncRepCleanupAtProcExit(); @@ -955,6 +959,10 @@ AuxiliaryProcKill(int code, Datum arg) Assert(proctype >= 0 && proctype < NUM_AUXILIARY_PROCS); + /* not safe if forked by system(), etc. */ + if (MyProc->pid != (int) getpid()) + elog(PANIC, "AuxiliaryProcKill() called in child process"); + auxproc = &AuxiliaryProcs[proctype]; Assert(MyProc == auxproc); diff --git a/src/backend/utils/error/elog.c b/src/backend/utils/error/elog.c index a3e1c59a829..001b87ea999 100644 --- a/src/backend/utils/error/elog.c +++ b/src/backend/utils/error/elog.c @@ -3590,6 +3590,34 @@ 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 f53607e12eb..f98a1e8b629 100644 --- a/src/include/utils/elog.h +++ b/src/include/utils/elog.h @@ -453,4 +453,10 @@ extern void set_syslog_parameters(const char *ident, int facility); */ 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 */