[PATCH v4 2/4] Stage output for -z and -Z options

Paul Chaignon paul.chaignon at gmail.com
Sat Jun 15 17:31:24 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 the syscall return status.
If the open_memstream function is not available, these new options error out.

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         | 10 ++++++++
 stage_output.c | 66 ++++++++++++++++++++++++++++++++++++++++++++++++++
 strace.1.in    |  6 +++++
 strace.c       | 31 ++++++++++++++++++++----
 syscall.c      | 18 +++++++-------
 7 files changed, 119 insertions(+), 14 deletions(-)
 create mode 100644 stage_output.c

diff --git a/Makefile.am b/Makefile.am
index a8ace321..b53d795c 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -307,6 +307,7 @@ 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 621312ac..a5286378 100644
--- a/configure.ac
+++ b/configure.ac
@@ -262,6 +262,7 @@ 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 9745078b..213be9e6 100644
--- a/defs.h
+++ b/defs.h
@@ -204,6 +204,10 @@ 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 */
+	FILE *real_outf;	/* Backup for real outf while staging */
+	char *memfptr;
+	size_t memfloc;
+
 	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 */
@@ -1196,6 +1200,12 @@ 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 status qualifier.
+ */
+extern FILE *strace_open_memstream(struct tcb *tcp);
+extern void strace_close_memstream(struct tcb *tcp, bool publish);
+
 static inline void
 printaddr_comment(const kernel_ulong_t addr)
 {
diff --git a/stage_output.c b/stage_output.c
new file mode 100644
index 00000000..4000aa0c
--- /dev/null
+++ b/stage_output.c
@@ -0,0 +1,66 @@
+/*
+ * Copyright (c) 2017 Intel Corporation
+ * Copyright (c) 2019 Paul Chaignon <paul.chaignon at gmail.com>
+ * All rights reserved.
+ *
+ * SPDX-License-Identifier: LGPL-2.1-or-later
+ */
+
+/*
+ *  Output staging is based on the "open_memstream()" function, see:
+ *  http://man7.org/linux/man-pages/man3/open_memstream.3.html
+ *  Requires glibc version 2.7 mininal
+ *
+ *  open_memstream returns a FILE stream that allows writing to a
+ *  dynamically growing buffer, that can be either copied to
+ *  tcp->outf (syscall successful) or dropped (syscall failed)
+ */
+
+#include "defs.h"
+
+FILE *
+strace_open_memstream(struct tcb *tcp)
+{
+	FILE *fp = NULL;
+
+#if HAVE_OPEN_MEMSTREAM
+	tcp->memfptr = NULL;
+	fp = open_memstream(&tcp->memfptr, &tcp->memfloc);
+	if (!fp)
+		perror_msg_and_die("open_memstream");
+	/* Call to fflush required to update tcp->memfptr, see open_memstream
+	 * man page. */
+	fflush(fp);
+
+	/* Store the FILE pointer for later restauration. */
+	tcp->real_outf = tcp->outf;
+	tcp->outf = fp;
+#endif
+
+	return fp;
+}
+
+void
+strace_close_memstream(struct tcb *tcp, bool publish)
+{
+#if HAVE_OPEN_MEMSTREAM
+	if (!tcp->real_outf) {
+		debug_msg("memstream already closed");
+		return;
+	}
+
+	if (fclose(tcp->outf))
+		perror_msg("fclose(tcp->outf)");
+
+	tcp->outf = tcp->real_outf;
+	tcp->real_outf = NULL;
+	if (tcp->memfptr) {
+		if (publish)
+			fputs_unlocked(tcp->memfptr, tcp->outf);
+		else
+			debug_msg("syscall output dropped: %s", tcp->memfptr);
+		free(tcp->memfptr);
+		tcp->memfptr = NULL;
+	}
+#endif
+}
diff --git a/strace.1.in b/strace.1.in
index 76a74119..e1090a0f 100644
--- a/strace.1.in
+++ b/strace.1.in
@@ -771,6 +771,12 @@ Print unabbreviated versions of environment, stat, termios, etc.
 calls.  These structures are very common in calls and so the default
 behavior displays a reasonable subset of structure members.  Use
 this option to get all of the gory details.
+.TP
+.B \-z
+Print only syscalls that returned without an error code.
+.TP
+.B \-Z
+Print only syscalls that returned with an error code.
 .SS Tracing
 .TP 12
 .BI "\-b " syscall
diff --git a/strace.c b/strace.c
index d9cff343..6c3d68c0 100644
--- a/strace.c
+++ b/strace.c
@@ -275,6 +275,8 @@ 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\
+  -z             print only syscalls that returned without an error code\n\
+  -Z             print only syscalls that returned with an error code\n\
 \n\
 Tracing:\n\
   -b execve      detach on execve syscall\n\
@@ -303,10 +305,6 @@ 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);
 }
@@ -1771,6 +1769,11 @@ init(int argc, char *argv[])
 			error_msg("-%c has no effect with -c", 'y');
 	}
 
+#ifndef HAVE_OPEN_MEMSTREAM
+	if (not_failing_only || failing_only)
+		error_msg_and_help("open_memstream is required to use -z or -Z");
+#endif
+
 	acolumn_spaces = xmalloc(acolumn + 1);
 	memset(acolumn_spaces, ' ', acolumn);
 	acolumn_spaces[acolumn] = '\0';
@@ -2044,6 +2047,8 @@ static struct tcb *
 maybe_switch_tcbs(struct tcb *tcp, const int pid)
 {
 	FILE *fp;
+	char *memfptr;
+	size_t memfloc;
 	struct tcb *execve_thread;
 	long old_pid = tcb_wait_tab[tcp->wait_data_idx].msg;
 
@@ -2065,10 +2070,22 @@ maybe_switch_tcbs(struct tcb *tcp, const int pid)
 		fprintf(execve_thread->outf, " <pid changed to %d ...>\n", pid);
 		/*execve_thread->curcol = 0; - no need, see code below */
 	}
-	/* Swap output FILEs (needed for -ff) */
+	/* Swap output FILEs and memstream (needed for -ff) */
 	fp = execve_thread->outf;
 	execve_thread->outf = tcp->outf;
 	tcp->outf = fp;
+	if (execve_thread->real_outf || tcp->real_outf) {
+		fp = execve_thread->real_outf;
+		execve_thread->real_outf = tcp->real_outf;
+		tcp->real_outf = fp;
+		memfptr = execve_thread->memfptr;
+		execve_thread->memfptr = tcp->memfptr;
+		tcp->memfptr = memfptr;
+		memfloc = execve_thread->memfloc;
+		execve_thread->memfloc = tcp->memfloc;
+		tcp->memfloc = memfloc;
+	}
+
 	/* And their column positions */
 	execve_thread->curcol = tcp->curcol;
 	tcp->curcol = 0;
@@ -2081,6 +2098,10 @@ maybe_switch_tcbs(struct tcb *tcp, const int pid)
 		printleader(tcp);
 		tprintf("+++ superseded by execve in pid %lu +++\n", old_pid);
 		line_ended();
+		/* Need to reopen memstream for thread
+		 * as we closed it in droptcb. */
+		if (failing_only || not_failing_only)
+			strace_open_memstream(tcp);
 		tcp->flags |= TCB_REPRINT;
 	}
 
diff --git a/syscall.c b/syscall.c
index 29dfb7a7..2a471711 100644
--- a/syscall.c
+++ b/syscall.c
@@ -656,6 +656,9 @@ syscall_entering_trace(struct tcb *tcp, unsigned int *sig)
 	}
 #endif
 
+	if (not_failing_only || failing_only)
+		strace_open_memstream(tcp);
+
 	printleader(tcp);
 	tprintf("%s(", tcp_sysent(tcp)->sys_name);
 	int res = raw(tcp) ? printargs(tcp) : tcp_sysent(tcp)->sys_func(tcp);
@@ -757,18 +760,13 @@ 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))) {
+			strace_close_memstream(tcp, false);
+			line_ended();
 			return 0;	/* ignore failed/successful
 					 * syscalls */
+		}
 		if (tcp->sys_func_rval & RVAL_DECODED)
 			sys_res = tcp->sys_func_rval;
 		else
@@ -904,6 +902,8 @@ syscall_exiting_trace(struct tcb *tcp, struct timespec *ts, int res)
 	}
 	tprints("\n");
 	dumpio(tcp);
+	if (not_failing_only || failing_only)
+		strace_close_memstream(tcp, true);
 	line_ended();
 
 #ifdef ENABLE_STACKTRACE
-- 
2.17.1



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