[PATCH 2/3] Stage output for -z and -Z options

Paul Chaignon paul.chaignon at gmail.com
Mon Apr 1 21:14:20 UTC 2019


-z and -Z options print only successful and failing syscalls respectively.
However, failure of syscall is only known after syscall return.  Thus, we
end up with something like this on, e.g., ENOENT:

  open("does_not_exist", O_RDONLY <unfinished ...>

whereas the intended result is that the open(...) line is not shown at
all.

This patch fixes this issue using open_memstream.  When either the -z or
the -Z option is used, the output is staged in memory (using
open_memstream) until we know whether the syscall was successful.  If the
open_memstream function is not available, strace retains the old behavior.

Signed-off-by: Paul Chaignon <paul.chaignon at gmail.com>
Co-Authored-by: Burkhard Kohl <burkhard.kohl at intel.com>
---
 Makefile.am  |  1 +
 configure.ac |  1 +
 defs.h       | 15 +++++++++++++
 strace.c     | 62 +++++++++++++++++++++++++++++++++++++++++++++-------
 syscall.c    | 19 ++++++++--------
 5 files changed, 81 insertions(+), 17 deletions(-)

diff --git a/Makefile.am b/Makefile.am
index 56987ae0..dd44b04c 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -298,6 +298,7 @@ strace_SOURCES =	\
 	sock.c		\
 	sockaddr.c	\
 	socketutils.c	\
+	stage_output.c	\
 	sparc.c		\
 	sram_alloc.c	\
 	stat.c		\
diff --git a/configure.ac b/configure.ac
index c95fd229..91f3152d 100644
--- a/configure.ac
+++ b/configure.ac
@@ -258,6 +258,7 @@ AC_CHECK_FUNCS(m4_normalize([
 	iconv_open
 	if_indextoname
 	open64
+	open_memstream
 	prctl
 	preadv
 	process_vm_readv
diff --git a/defs.h b/defs.h
index b5ebb343..8600dec2 100644
--- a/defs.h
+++ b/defs.h
@@ -204,6 +204,12 @@ struct tcb {
 	int sys_func_rval;	/* Syscall entry parser's return value */
 	int curcol;		/* Output column for this process */
 	FILE *outf;		/* Output file for this process */
+#if HAVE_OPEN_MEMSTREAM
+	FILE *memf;		/* Staged output file for this process */
+	char *memfptr;
+	size_t memfloc;
+#endif
+
 	const char *auxstr;	/* Auxiliary info from syscall (see RVAL_STR) */
 	void *_priv_data;	/* Private data for syscall decoding functions */
 	void (*_free_priv_data)(void *); /* Callback for freeing priv_data */
@@ -418,6 +424,8 @@ extern bool count_wallclock;
 extern unsigned int qflag;
 extern bool not_failing_only;
 extern bool failing_only;
+/* allows dropping of failed/successful syscalls output */
+extern bool stage_output;
 extern unsigned int show_fd_path;
 /* are we filtering traces based on paths? */
 extern struct path_set {
@@ -1165,6 +1173,13 @@ extern void tprints(const char *str);
 extern void tprintf_comment(const char *fmt, ...) ATTRIBUTE_FORMAT((printf, 1, 2));
 extern void tprints_comment(const char *str);
 
+/*
+ * Staging output for -z/-Z (not_failing_only/failing_only).
+ */
+extern FILE *strace_openmemstream(struct tcb *tcp);
+extern void drop_staged_out(struct tcb *tcp);
+extern void publish_staged_out(struct tcb *tcp);
+
 static inline void
 printaddr_comment(const kernel_ulong_t addr)
 {
diff --git a/strace.c b/strace.c
index bd2dd992..8c5eeb30 100644
--- a/strace.c
+++ b/strace.c
@@ -112,6 +112,7 @@ static int post_attach_sigstop = TCB_IGNORE_ONE_SIGSTOP;
 /* Sometimes we want to print succeeding/failing syscalls only. */
 bool not_failing_only;
 bool failing_only;
+bool stage_output;
 
 /* Show path associated with fd arguments */
 unsigned int show_fd_path;
@@ -275,7 +276,13 @@ Filtering:\n\
   -e expr        a qualifying expression: option=[!]all or option=[!]val1[,val2]...\n\
      options:    trace, abbrev, verbose, raw, signal, read, write, fault, inject, kvm\n\
   -P path        trace accesses to path\n\
-\n\
+"
+#if HAVE_OPEN_MEMSTREAM
+"  -z             print only succeeding syscalls\n\
+  -Z             print only failing syscalls\n\
+"
+#endif
+"\n\
 Tracing:\n\
   -b execve      detach on execve syscall\n\
   -D             run tracer process as a detached grandchild, not as parent\n\
@@ -303,10 +310,6 @@ Miscellaneous:\n\
 /* ancient, no one should use it
 -F -- attempt to follow vforks (deprecated, use -f)\n\
  */
-/* this is broken, so don't document it
--z -- print only succeeding syscalls\n\
--Z -- print only failing syscalls\n\
- */
 , DEFAULT_ACOLUMN, DEFAULT_STRLEN, DEFAULT_SORTBY);
 	exit(0);
 }
@@ -512,8 +515,17 @@ ATTRIBUTE_FORMAT((printf, 1, 0))
 static void
 tvprintf(const char *const fmt, va_list args)
 {
+	int n = 0;
 	if (current_tcp) {
-		int n = vfprintf(current_tcp->outf, fmt, args);
+		/* If open_memstream is not supported, stage_output will always
+		 * be false. */
+		if (stage_output) {
+			if (!current_tcp->memf)
+				error_msg_and_die("No file opened for current_tcp->memf\n");
+			n = vfprintf(current_tcp->memf, fmt, args);
+		} else
+			n = vfprintf(current_tcp->outf, fmt, args);
+
 		if (n < 0) {
 			/* very unlikely due to vfprintf buffering */
 			outf_perror(current_tcp);
@@ -539,7 +551,18 @@ void
 tprints(const char *str)
 {
 	if (current_tcp) {
-		int n = fputs_unlocked(str, current_tcp->outf);
+		int n;
+
+		/* If open_memstream is not supported, stage_output will always
+		 * be false. */
+		if (stage_output) {
+			if (!current_tcp->memf)
+				error_msg_and_die("No file opened for current_tcp->memf\n");
+			n = fputs_unlocked(str, current_tcp->memf);
+		} else {
+			n = fputs_unlocked(str, current_tcp->outf);
+		}
+
 		if (n >= 0) {
 			current_tcp->curcol += strlen(str);
 			return;
@@ -700,6 +723,11 @@ after_successful_attach(struct tcb *tcp, const unsigned int flags)
 		tcp->outf = strace_fopen(name);
 	}
 
+#if HAVE_OPEN_MEMSTREAM
+	if (not_failing_only || failing_only)
+		strace_openmemstream(tcp);
+#endif
+
 #ifdef ENABLE_STACKTRACE
 	if (stack_trace_enabled)
 		unwind_tcb_init(tcp);
@@ -1563,6 +1591,9 @@ init(int argc, char *argv[])
 	int c, i;
 	int optF = 0;
 
+	/* during init don't print to internal memf stream */
+	stage_output = false;
+
 	if (!program_invocation_name || !*program_invocation_name) {
 		static char name[] = "strace";
 		program_invocation_name =
@@ -1782,6 +1813,10 @@ init(int argc, char *argv[])
 		unwind_init();
 #endif
 
+#ifdef HAVE_OPEN_MEMSTREAM
+	stage_output = not_failing_only || failing_only;
+#endif
+
 	/* See if they want to run as another user. */
 	if (username != NULL) {
 		struct passwd *pent;
@@ -2108,6 +2143,10 @@ print_signalled(struct tcb *tcp, const int pid, int status)
 static void
 print_exited(struct tcb *tcp, const int pid, int status)
 {
+	bool saved_stage_output_flag = stage_output;
+
+	stage_output = false;
+
 	if (pid == strace_child) {
 		exit_code = WEXITSTATUS(status);
 		strace_child = 0;
@@ -2119,11 +2158,16 @@ print_exited(struct tcb *tcp, const int pid, int status)
 		tprintf("+++ exited with %d +++\n", WEXITSTATUS(status));
 		line_ended();
 	}
+
+	stage_output = saved_stage_output_flag;
 }
 
 static void
 print_stopped(struct tcb *tcp, const siginfo_t *si, const unsigned int sig)
 {
+	bool saved_stage_output_flag = stage_output;
+	stage_output = false;
+
 	if (cflag != CFLAG_ONLY_STATS
 	    && !hide_log(tcp)
 	    && is_number_in_set(sig, signal_set)) {
@@ -2141,6 +2185,8 @@ print_stopped(struct tcb *tcp, const siginfo_t *si, const unsigned int sig)
 			unwind_tcb_print(tcp);
 #endif
 	}
+
+	stage_output = saved_stage_output_flag;
 }
 
 static void
@@ -2793,7 +2839,7 @@ static void ATTRIBUTE_NORETURN
 terminate(void)
 {
 	int sig = interrupted;
-
+	stage_output = false;
 	cleanup(sig);
 	if (cflag)
 		call_summary(shared_log);
diff --git a/syscall.c b/syscall.c
index 7ea45292..3b4d3b44 100644
--- a/syscall.c
+++ b/syscall.c
@@ -753,18 +753,17 @@ syscall_exiting_trace(struct tcb *tcp, struct timespec *ts, int res)
 	if (raw(tcp)) {
 		/* sys_res = printargs(tcp); - but it's nop on sysexit */
 	} else {
-	/* FIXME: not_failing_only (IOW, option -z) is broken:
-	 * failure of syscall is known only after syscall return.
-	 * Thus we end up with something like this on, say, ENOENT:
-	 *     open("does_not_exist", O_RDONLY <unfinished ...>
-	 *     {next syscall decode}
-	 * whereas the intended result is that open(...) line
-	 * is not shown at all.
-	 */
 		if ((not_failing_only && syserror(tcp)) ||
-		    (failing_only && !syserror(tcp)))
+		    (failing_only && !syserror(tcp))) {
+			/* If open_memstream is not supported, stage_output
+			 * will always be false. */
+			if (stage_output) {
+				drop_staged_out(tcp);
+				line_ended();
+			}
 			return 0;	/* ignore failed/successful
 					 * syscalls */
+		}
 		if (tcp->sys_func_rval & RVAL_DECODED)
 			sys_res = tcp->sys_func_rval;
 		else
@@ -900,6 +899,8 @@ syscall_exiting_trace(struct tcb *tcp, struct timespec *ts, int res)
 	}
 	tprints("\n");
 	dumpio(tcp);
+	if (stage_output)
+		publish_staged_out(tcp);
 	line_ended();
 
 #ifdef ENABLE_STACKTRACE
-- 
2.17.1



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