[PATCH v3 2/3] The -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:
Paul Chaignon
paul.chaignon at gmail.com
Sun May 12 18:20:43 UTC 2019
On Thu, Apr 25, 2019 at 04:22PM, burkhard.kohl at intel.com wrote:
> From: Burkhard Kohl <burkhard.kohl at intel.com>
>
> 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.
>
> Combining both -z and -Z options may be useful in some cases, e.g.
> when tracing signals.
>
> Note: Syscalls returning void (e.g. exit_group) will be missed by
> both options.
>
> Signed-off-by: Burkhard Kohl <burkhard.kohl at intel.com>
> Co-Authored-by: Paul Chaignon <paul.chaignon at gmail.com>
> ---
> Makefile.am | 1 +
> configure.ac | 1 +
> defs.h | 10 ++++++++
> stage_output.c | 69 ++++++++++++++++++++++++++++++++++++++++++++++++++
> strace.1.in | 7 +++++
> strace.c | 12 ++++++++-
> syscall.c | 21 +++++++++++++--
> 7 files changed, 118 insertions(+), 3 deletions(-)
> create mode 100644 stage_output.c
>
> diff --git a/Makefile.am b/Makefile.am
> index b5946914..4e7ef4f4 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 86d3626b..523a6496 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 b5ebb343..ee5da8b3 100644
> --- a/defs.h
> +++ b/defs.h
> @@ -204,6 +204,11 @@ 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 *real_outf; /* backup for real outf while staging */
> + 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 */
> @@ -1165,6 +1170,11 @@ 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);
>
> +/* for staging output */
> +#ifdef HAVE_OPEN_MEMSTREAM
> +extern FILE *strace_open_memstream(struct tcb *tcp);
> +extern void strace_close_memstream(struct tcb *tcp, bool);
> +#endif
> 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..c353e265
> --- /dev/null
> +++ b/stage_output.c
> @@ -0,0 +1,69 @@
> +/*
> + * Copyright (c) 2017/2019 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
> + * Works with glibc version 2.7 or higher
> + *
> + * open_memstream returns a FILE stream that allows writing to a
> + * dynamically growing buffer, which then will be either copied
> + * to tcp->outf (syscall successful) or dropped (syscall failed)
> + * or vice versa.
> + */
> +
> +#include "defs.h"
> +#if HAVE_OPEN_MEMSTREAM
> +
> +FILE *
> +strace_open_memstream(struct tcb *tcp)
> +{
> + FILE *fp = NULL;
> +
> + 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 physical (open) FILE pointer for later restauration */
> + tcp->real_outf = tcp->outf;
> + tcp->outf = fp;
> + return fp;
> +}
> +
> +void
> +strace_close_memstream(struct tcb *tcp, bool drop)
> +{
> + if (!tcp->real_outf) {
If you want to detect such errors, it would be preferable to set real_outf
to NULL at the end of this function.
> + if (debug_flag)
> + error_msg("Strange, strace_closememstream called although FILE pointers not swapped!");
These two lines can be replaced with a call to debug_msg().
The error message could also be shortened. Maybe just "real_outf is NULL".
> + return;
> + }
> +
> + if (fclose(tcp->outf))
> + perror_msg("fclose(tcp->outf)");
> +
> + /* Restore the pyhisical FILE pointer */
Typo on "physical".
> + tcp->outf = tcp->real_outf;
> + if (tcp->memfptr) {
> + if (drop) {
> + if (debug_flag)
> + error_msg("staged syscall output dropped: %s", tcp->memfptr);
Ditto for debug_msg().
> + } else {
> + if (debug_flag)
> + error_msg("staged syscall output printed: %s", tcp->memfptr);
Ditto.
> + fprintf(tcp->outf, "%s", tcp->memfptr);
This can be replaced with a call to fputs_unlocked().
> + }
> + free(tcp->memfptr);
> + tcp->memfptr = NULL;
> + }
> +}
> +#endif
> diff --git a/strace.1.in b/strace.1.in
> index 76a74119..6ab95836 100644
> --- a/strace.1.in
> +++ b/strace.1.in
> @@ -771,6 +771,13 @@ 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 successful syscalls only.
It might be better to be precise here regarding what we mean by
"successful" exactly. Maybe "Print only syscalls that did not return an
error code"?
> +.TP
> +.BI "\-Z "
> +Print failed syscalls only. (-z/-Z options may be combined).
> +Can be useful e.g. to catch only signals.
I'm not sure this is a good idea. There's already -e trace=none to catch
only signals. Unless we have a strong use case for this, we should
probably avoid exposing new features; once users depend on it, we can't go
back and prohibit it. I think we should explicitly prohibit it for now
and allow it in the future if there's ever a need.
> .SS Tracing
> .TP 12
> .BI "\-b " syscall
> diff --git a/strace.c b/strace.c
> index 42cd1419..2b408bcb 100644
> --- a/strace.c
> +++ b/strace.c
> @@ -275,7 +275,13 @@ 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\
> @@ -1587,7 +1593,11 @@ init(int argc, char *argv[])
> #ifdef ENABLE_STACKTRACE
> "k"
> #endif
> +#ifdef HAVE_OPEN_MEMSTREAM
> "a:Ab:cCdDe:E:fFhiI:o:O:p:P:qrs:S:tTu:vVwxX:yzZ")) != EOF) {
> +#else
> + "a:Ab:cCdDe:E:fFhiI:o:O:p:P:qrs:S:tTu:vVwxX:yz")) != EOF) {
> +#endif
> switch (c) {
> case 'a':
> acolumn = string_to_uint(optarg);
> diff --git a/syscall.c b/syscall.c
> index 949a848b..61057796 100644
> --- a/syscall.c
> +++ b/syscall.c
> @@ -652,6 +652,10 @@ syscall_entering_trace(struct tcb *tcp, unsigned int *sig)
> }
> #endif
>
> +#if HAVE_OPEN_MEMSTREAM
> + if (not_failing_only || failing_only)
> + strace_open_memstream(tcp);
> +#endif
> printleader(tcp);
> tprintf("%s(", tcp_sysent(tcp)->sys_name);
> int res = raw(tcp) ? printargs(tcp) : tcp_sysent(tcp)->sys_func(tcp);
> @@ -753,6 +757,15 @@ 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 {
> +#if HAVE_OPEN_MEMSTREAM
> + if ((not_failing_only && syserror(tcp)) ||
> + (failing_only && !syserror(tcp))) {
> + line_ended();
> + strace_close_memstream(tcp, true);
> + return 0; /* ignore failed/successful
> + * syscalls */
> + }
> +#else
The below comment can now be removed.
> /* 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:
> @@ -763,8 +776,8 @@ syscall_exiting_trace(struct tcb *tcp, struct timespec *ts, int res)
> */
> if ((not_failing_only && syserror(tcp)) ||
> (failing_only && !syserror(tcp)))
> - return 0; /* ignore failed/successful
> - * syscalls */
> + return 0; /* ignore failed syscalls*/
Not sure I understand the change of comment here.
> +#endif
> if (tcp->sys_func_rval & RVAL_DECODED)
> sys_res = tcp->sys_func_rval;
> else
> @@ -900,6 +913,10 @@ syscall_exiting_trace(struct tcb *tcp, struct timespec *ts, int res)
> }
> tprints("\n");
> dumpio(tcp);
> +#ifdef HAVE_OPEN_MEMSTREAM
> + if (not_failing_only || failing_only)
> + strace_close_memstream(tcp, false);
> +#endif
> line_ended();
>
> #ifdef ENABLE_STACKTRACE
> --
> 2.17.1
>
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