Close stdin/out after child is forked
Testcase:
{ sleep 1; yes | head -n99999; } | strace -o/dev/null sh -c 'exec <&-; sleep 9'
The writer (head) will be able to perform writes to write end of the pipe.
With strace call removed, head will immediately get SIGPIPE.
This change fixes this: now writer immediately gets SIGPIPE with strace too.
* strace.c (startup_child): Close stdin/out and reopen them to /dev/null.
Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>
diff --git a/strace.c b/strace.c
index 614b85f..35a0423 100644
--- a/strace.c
+++ b/strace.c
@@ -1353,6 +1353,27 @@
* to create a genuine separate stack and execute on it.
*/
}
+ /*
+ * A case where straced process is part of a pipe:
+ * { sleep 1; yes | head -n99999; } | strace -o/dev/null sh -c 'exec <&-; sleep 9'
+ * If strace won't close its fd#0, closing it in tracee is not enough:
+ * the pipe is still open, it has a reader. Thus, "head" will not get its
+ * SIGPIPE at once, on the first write.
+ *
+ * Preventing it by closing strace's stdin/out.
+ * (Don't leave fds 0 and 1 closed, this is bad practice: future opens
+ * will reuse them, unexpectedly making a newly opened object "stdin").
+ */
+ close(0);
+ if (open("/dev/null", O_RDWR) != 0) /* /dev not populated? */
+ if (open("/", O_RDONLY) != 0) /* shouldn't happen... */
+ perror_msg_and_die("Can't open '/'");
+ dup2(0, 1);
+#if 0
+ /* A good idea too, but we sometimes need to print error messages */
+ if (shared_log != stderr)
+ dup2(0, 2);
+#endif
}
#if USE_SEIZE
