[hackers] [tabbed][PATCH] fix: faulty zombie reaping

From: NRK <nrk_AT_disroot.org>
Date: Sat, 11 Mar 2023 06:20:07 +0600

issues with the current signal handler:

1. die() isn't async-signal-safe
2. there's a race-window during reinstating the signal handler allowing
   zombies to be produced (should be rare in practice).
3. if waitpid fails, it will clobber the errno which can lead to
   undesired codepath being taken on the resuming code (this one can
   actually occur in practice).

to reproduce the 3rd issue:

        ~> ./tabbed &
        [1] 20644
        0x1800003
        ~> while :; do kill -s SIGCHLD 20644; done >&2 2>/dev/null
        XIO: fatal IO error 10 (No child processes) on X server ":0"
              after 47 requests (47 known processed) with 0 events remaining.
        [1] + exit 1 ./tabbed

set the signal handler to SIG_IGN to reap zombies automatically
(according to POSIX.1-2001).

NOTE: this patch follows dwm's commit 712d663. according to the manpage,
none of the sa_flags being set are meaningful since we're using SIG_IGN.
they used to be meaningful on earlier version of the patch where it was
using SIG_DFL, i've kept them here just in case.

---
 Makefile |  2 +-
 tabbed.c | 21 +++++++++------------
 2 files changed, 10 insertions(+), 13 deletions(-)
diff --git a/Makefile b/Makefile
index f8f5ba4..54ba350 100644
--- a/Makefile
+++ b/Makefile
_AT_@ -11,7 +11,7 @@ DOCPREFIX = ${PREFIX}/share/doc/${NAME}
 # use system flags.
 TABBED_CFLAGS = -I/usr/X11R6/include -I/usr/include/freetype2 ${CFLAGS}
 TABBED_LDFLAGS = -L/usr/X11R6/lib -lX11 -lfontconfig -lXft ${LDFLAGS}
-TABBED_CPPFLAGS = -DVERSION=\"${VERSION}\" -D_DEFAULT_SOURCE
+TABBED_CPPFLAGS = -DVERSION=\"${VERSION}\" -D_DEFAULT_SOURCE -D_XOPEN_SOURCE=700L
 
 # OpenBSD (uncomment)
 #TABBED_CFLAGS = -I/usr/X11R6/include -I/usr/X11R6/include/freetype2 ${CFLAGS}
diff --git a/tabbed.c b/tabbed.c
index eafe28a..477a041 100644
--- a/tabbed.c
+++ b/tabbed.c
_AT_@ -125,7 +125,6 @@ static void run(void);
 static void sendxembed(int c, long msg, long detail, long d1, long d2);
 static void setcmd(int argc, char *argv[], int);
 static void setup(void);
-static void sigchld(int unused);
 static void spawn(const Arg *arg);
 static int textnw(const char *text, unsigned int len);
 static void toggle(const Arg *arg);
_AT_@ -976,9 +975,16 @@ setup(void)
 	XWMHints *wmh;
 	XClassHint class_hint;
 	XSizeHints *size_hint;
+	struct sigaction sa;
 
-	/* clean up any zombies immediately */
-	sigchld(0);
+	/* do not transform children into zombies when they terminate */
+	sigemptyset(&sa.sa_mask);
+	sa.sa_flags = SA_NOCLDSTOP | SA_NOCLDWAIT | SA_RESTART;
+	sa.sa_handler = SIG_IGN;
+	sigaction(SIGCHLD, &sa, NULL);
+
+	/* clean up any zombies that might have been inherited */
+	while (waitpid(-1, NULL, WNOHANG) > 0);
 
 	/* init screen */
 	screen = DefaultScreen(dpy);
_AT_@ -1078,15 +1084,6 @@ setup(void)
 	focus(-1);
 }
 
-void
-sigchld(int unused)
-{
-	if (signal(SIGCHLD, sigchld) == SIG_ERR)
-		die("%s: cannot install SIGCHLD handler", argv0);
-
-	while (0 < waitpid(-1, NULL, WNOHANG));
-}
-
 void
 spawn(const Arg *arg)
 {
-- 
2.39.1

Received on Sat Mar 11 2023 - 01:20:07 CET