[PATCH v2] strace: expand -D option
Eugene Syromiatnikov
esyr at redhat.com
Sat Oct 5 22:55:13 UTC 2019
From: Fanda Uchytil <strace.t8xuewpmde at h4x.cz>
As of now, despit being stated that -D option runs strace as a "detached"
grandchild (and the option name being named after "daemon"), strace
still runs in the same process group and session, thus not being
"detached" in a common sense and being subjected to process group kill
and session termination kill. Quoting[1]:
I stumble upon unexpected behavior: if strace is used with option '-D'
(tracer as a detached grandchild) and process (leader) kills whole
process group, it will kill strace too.
It can be easily reproduced by `timeout` from "coreutils":
# timeout -s KILL 2 strace -D -o ./strace-inside.log /bin/sleep 10 &
Here we can see, that `strace` didn't finished its output (because it
was killed):
# tail -n 1 ./strace-inside.log
nanosleep({tv_sec=10, tv_nsec=0},
If `timeout` is not run in '--foreground' mode, it changes process group
and after "timeout" it sends two kills:
setpgid(0, 0) = 0
kill(37337, SIGKILL) = 0
kill(0, SIGKILL) = ?
The first kill is for the `sleep` and the second one is for the process
group (which is `strace` part of). PIDs and their relations are:
timeout pid=30595 [ppid=476 bash ] pgrp=30595
sleep pid=37337 [ppid=30595 timeout] pgrp=30595
strace pid=30603 [ppid=1 systemd] pgrp=30595
Here is "strace log" of `strace` inside `timeout`:
strace: Process 30603 attached
wait4(-1, <unfinished ...>) = ?
+++ killed by SIGKILL +++
I think that detached `strace` should not be killed like that -- it
should not be part of former grandparents' "job pipeline".
While this behaviour is not exactly intuitive, it is that way for quite
some time, so it might be relied upon by some of the strace users.
In order to address this issue, two new levels of "daemonisation"
are added, that put strace in a separate process group and session,
respectively.
[1] https://lists.strace.io/pipermail/strace-devel/2019-October/009160.html
* strace.1.in (.SH SYNOPSIS): Update.
(.SS Tracing): Document -DD and -DDD.
* strace.c (DAEMONIZE_NONE, DAEMONIZE_GRANDCHILD, DAEMONIZE_NEW_PGROUP,
* DAEMONIZE_NEW_SESSION, DAEMONIZE_OPTS_GUARD__, MAX_DAEMONIZE_OPTS):
* New enumeration entities.
(daemonized_tracer): Change type to unsigned int.
(usage): Document -DD and -DDD.
(startup_attach): Call setsid if daemonized_tracer is no less than
DAEMONIZE_NEW_SESSION; call setpgid if daemonized_tracer is equal
to DAEMONIZE_NEW_PGROUP.
(init) <case 'D'>: Increase daemonized_tracer instead of setting to 1.
(init): Bail out if to many -D's are given.
* tests/options-syntax.test: Add checks for -D option usage.
Co-authored-by: Eugene Syromyatnikov <evgsyr at gmail.com>
---
strace.1.in | 25 ++++++++++++++++++++++---
strace.c | 43 ++++++++++++++++++++++++++++++++++++++-----
tests/options-syntax.test | 4 ++++
3 files changed, 64 insertions(+), 8 deletions(-)
diff --git a/strace.1.in b/strace.1.in
index 86058a8..eb872df 100644
--- a/strace.1.in
+++ b/strace.1.in
@@ -53,7 +53,7 @@ strace \- trace system calls and signals
.BR "" {
.OR \-p pid
.BR "" |
-.OP \-D
+.OP \-DDD
.OM \-E var\fR[=\fIval\fR]
.OP \-u username
.IR command " [" args ]
@@ -73,7 +73,7 @@ strace \- trace system calls and signals
.BR "" {
.OR \-p pid
.BR "" |
-.OP \-D
+.OP \-DDD
.OM \-E var\fR[=\fIval\fR]
.OP -u username
.IR command " [" args ]
@@ -338,11 +338,30 @@ multi-threaded process and therefore require
but don't want to trace its (potentially very complex) children.
.TP
.B \-D
-Run tracer process as a detached grandchild, not as parent of the
+Run tracer process as a grandchild, not as the parent of the
tracee. This reduces the visible effect of
.B strace
by keeping the tracee a direct child of the calling process.
.TP
+.B \-DD
+Run tracer process as tracee's grandchild in a separate process group.
+In addition to reduction of the visible effect of
+.BR strace ,
+it also avoids killing of
+.B strace
+with
+.BR kill (2)
+issued to the whole process group.
+.TP
+.B \-DDD
+Run tracer process as tracee's grandchild in a separate session
+("true daemonisation").
+In addition to reduction of the visible effect of
+.BR strace ,
+it also avoids killing of
+.B strace
+upon session termination.
+.TP
.B \-f
Trace child processes as they are created by currently traced
processes as a result of the
diff --git a/strace.c b/strace.c
index b52a3db..b7388d1 100644
--- a/strace.c
+++ b/strace.c
@@ -93,6 +93,15 @@ static int opt_intr;
/* We play with signal mask only if this mode is active: */
#define interactive (opt_intr == INTR_WHILE_WAIT)
+enum {
+ DAEMONIZE_NONE = 0,
+ DAEMONIZE_GRANDCHILD = 1,
+ DAEMONIZE_NEW_PGROUP = 2,
+ DAEMONIZE_NEW_SESSION = 3,
+
+ DAEMONIZE_OPTS_GUARD__,
+ MAX_DAEMONIZE_OPTS = DAEMONIZE_OPTS_GUARD__ - 1
+};
/*
* daemonized_tracer supports -D option.
* With this option, strace forks twice.
@@ -105,7 +114,7 @@ static int opt_intr;
* wait() etc. Without -D, strace process gets lodged in between,
* disrupting parent<->child link.
*/
-static bool daemonized_tracer;
+static unsigned int daemonized_tracer;
static int post_attach_sigstop = TCB_IGNORE_ONE_SIGSTOP;
#define use_seize (post_attach_sigstop == 0)
@@ -241,10 +250,10 @@ usage(void)
usage: strace [-ACdffhi" K_OPT "qqrtttTvVwxxyyzZ] [-I n] [-b execve] [-e expr]...\n\
[-a column] [-o file] [-s strsize] [-X format] [-P path]...\n\
[-p pid]... [--seccomp-bpf]\n\
- { -p pid | [-D] [-E var=val]... [-u username] PROG [ARGS] }\n\
+ { -p pid | [-DDD] [-E var=val]... [-u username] PROG [ARGS] }\n\
or: strace -c[dfwzZ] [-I n] [-b execve] [-e expr]... [-O overhead]\n\
[-S sortby] [-P path]... [-p pid]... [--seccomp-bpf]\n\
- { -p pid | [-D] [-E var=val]... [-u username] PROG [ARGS] }\n\
+ { -p pid | [-DDD] [-E var=val]... [-u username] PROG [ARGS] }\n\
\n\
Output format:\n\
-A open the file provided in the -o option in append mode\n\
@@ -292,7 +301,9 @@ Filtering:\n\
\n\
Tracing:\n\
-b execve detach on execve syscall\n\
- -D run tracer process as a detached grandchild, not as parent\n\
+ -D run tracer process as a grandchild, not as parent\n\
+ -DD run tracer process in a separate process group\n\
+ -DDD run tracer process in a separate session\n\
-f follow forks\n\
-ff follow forks with output into separate files\n\
-I interruptible\n\
@@ -1131,6 +1142,23 @@ startup_attach(void)
_exit(0); /* paranoia */
}
/* grandchild */
+ if (daemonized_tracer >= DAEMONIZE_NEW_SESSION) {
+ /*
+ * If -D is passed thrice, create a new session,
+ * so we won't be killed upon session termination.
+ */
+ if (setsid() < 0)
+ perror_msg_and_die("Can't create "
+ "a new session");
+ } else if (daemonized_tracer == DAEMONIZE_NEW_PGROUP) {
+ /*
+ * If -D is passed twice, create a new process group,
+ * so we won't be killed by kill(0, ...).
+ */
+ if (setpgid(0, 0) < 0)
+ perror_msg_and_die("Can't create "
+ "a new process group");
+ }
/* We will be the tracer process. Remember our new pid: */
strace_tracer_pid = getpid();
}
@@ -1662,7 +1690,7 @@ init(int argc, char *argv[])
debug_flag = 1;
break;
case 'D':
- daemonized_tracer = 1;
+ daemonized_tracer++;
break;
case 'e':
qualify(optarg);
@@ -1786,6 +1814,11 @@ init(int argc, char *argv[])
error_msg_and_help("PROG [ARGS] must be specified with -D");
}
+ if (daemonized_tracer > (unsigned int) MAX_DAEMONIZE_OPTS)
+ error_msg_and_help("Too many -D's (%u), maximum supported -D "
+ "count is %d",
+ daemonized_tracer, MAX_DAEMONIZE_OPTS);
+
if (seccomp_filtering) {
if (nprocs && (!argc || debug_flag))
error_msg("--seccomp-bpf is not enabled for processes"
diff --git a/tests/options-syntax.test b/tests/options-syntax.test
index 141689a..6a916f1 100755
--- a/tests/options-syntax.test
+++ b/tests/options-syntax.test
@@ -24,6 +24,10 @@ check_e_using_grep "$ff_name: File *name too long" -ff -o "$ff_name" true
check_h 'must have PROG [ARGS] or -p PID'
check_h 'PROG [ARGS] must be specified with -D' -D -p $$
+check_h 'PROG [ARGS] must be specified with -D' -DD -p $$
+check_h 'PROG [ARGS] must be specified with -D' -DDD -p $$
+check_h 'PROG [ARGS] must be specified with -D' -DDDD -p $$
+check_h 'Too many -D'\''s (4), maximum supported -D count is 3' -DDDD /bin/true
check_h '-c and -C are mutually exclusive' -c -C true
check_h '-c and -C are mutually exclusive' -C -c true
check_h '(-c or -C) and -ff are mutually exclusive' -c -ff true
--
2.1.4
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