[PATCH v3 0/3] Integrate stack tracer status in 'stat'

[PATCH v3 0/3] Integrate stack tracer status in 'stat'

[Vladislav Valtchev (VMware)]

This short patch series makes trace-cmd stat aware of the stack tracer: now,
when the stack tracker is ON, the command will report that.

Vladislav Valtchev (VMware) (3):
  trace-cmd: Make read_proc() to return int status via OUT arg
  trace-cmd: Remove the die() call from read_proc()
  trace-cmd: Making stat to report when the stack tracer is ON

 trace-cmd.h   |  2 ++
 trace-stack.c | 85 ++++++++++++++++++++++++++++++++++++++++++++++++-----------
 trace-stat.c  |  8 ++++++
 3 files changed, 79 insertions(+), 16 deletions(-)

-- 
2.14.1

[PATCH v3 1/3] trace-cmd: Make read_proc() to return int status via OUT arg

[Vladislav Valtchev (VMware)]

This patch changes both the implementation and the interface of read_proc()
in trace-stack.c. First, it makes the function to read a string from the proc
file and then parse it as an integer using strtol(). Then, it makes the function
to return the integer read from the proc file using the int *status OUT
parameter, in order to make possible its return value to be used by the caller
to check if the operation succeeded.

This new implementation relaxes the external contraints the function relies on,
making it possible to be used by trace stat. The point is that 'stat' should not
fail in case there is something wrong with the stack tracer. Only the call to
die() in case the file is empty has been left in this patch: it will be removed
as well in a separate commit.

Signed-off-by: Vladislav Valtchev (VMware) <vladislav.valtchev@gmail.com>
---
 trace-stack.c | 79 +++++++++++++++++++++++++++++++++++++++++++++++------------
 1 file changed, 63 insertions(+), 16 deletions(-)

diff --git a/trace-stack.c b/trace-stack.c
index aa79ae3..c1058ca 100644
--- a/trace-stack.c
+++ b/trace-stack.c
@@ -20,6 +20,7 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#include <limits.h>
 #include <getopt.h>
 #include <sys/types.h>
 #include <sys/stat.h>
@@ -49,37 +50,79 @@ static void test_available(void)
 		die("stack tracer not configured on running kernel");
 }
 
-static char read_proc(void)
+/*
+ * Returns:
+ *   -1 - Something went wrong
+ *    0 - File does not exist (stack tracer not enabled)
+ *    1 - Success
+ */
+static int read_proc(int *status)
 {
-	char buf[1];
+	struct stat stat_buf;
+	char buf[64];
+	long num;
 	int fd;
 	int n;
 
+	if (stat(PROC_FILE, &stat_buf) < 0) {
+		/* stack tracer not configured on running kernel */
+		*status = 0; /* not configured means disabled */
+		return 0;
+	}
+
 	fd = open(PROC_FILE, O_RDONLY);
-	if (fd < 0)
-		die("reading %s", PROC_FILE);
-	n = read(fd, buf, 1);
-	close(fd);
-	if (n != 1)
+
+	if (fd < 0) {
+		/* we cannot open the file: likely a permission problem. */
+		return -1;
+	}
+
+	n = read(fd, buf, sizeof(buf));
+
+	/* We assume that the file is never empty we got no errors. */
+	if (n <= 0)
 		die("error reading %s", PROC_FILE);
 
-	return buf[0];
+	/* Does this file have more than 63 characters?? */
+	if (n >= sizeof(buf))
+		return -1;
+
+	/* n is guaranteed to be in the range [1, sizeof(buf)-1]. */
+	buf[n] = 0;
+	close(fd);
+
+	errno = 0;
+
+	/* Read an integer from buf ignoring any non-digit trailing characters. */
+	num = strtol(buf, NULL, 10);
+
+	/* strtol() returned 0: we have to check for errors */
+	if (!num && (errno == EINVAL || errno == ERANGE))
+		return -1;
+
+	if (num > INT_MAX || num < INT_MIN)
+		return -1; /* the number is good but does not fit in 'int' */
+
+	*status = num;
+	return 1; /* full success */
 }
 
-static void start_stop_trace(char val)
+/* NOTE: this implementation only accepts new_status in the range [0..9]. */
+static void change_stack_tracer_status(int new_status)
 {
 	char buf[1];
+	int status;
 	int fd;
 	int n;
 
-	buf[0] = read_proc();
-	if (buf[0] == val)
-		return;
+	if (read_proc(&status) > 0 && status == new_status)
+		return; /* nothing to do */
 
 	fd = open(PROC_FILE, O_WRONLY);
+
 	if (fd < 0)
 		die("writing %s", PROC_FILE);
-	buf[0] = val;
+	buf[0] = new_status + '0';
 	n = write(fd, buf, 1);
 	if (n < 0)
 		die("writing into %s", PROC_FILE);
@@ -88,12 +131,12 @@ static void start_stop_trace(char val)
 
 static void start_trace(void)
 {
-	start_stop_trace('1');
+	change_stack_tracer_status(1);
 }
 
 static void stop_trace(void)
 {
-	start_stop_trace('0');
+	change_stack_tracer_status(0);
 }
 
 static void reset_trace(void)
@@ -123,8 +166,12 @@ static void read_trace(void)
 	char *buf = NULL;
 	size_t n;
 	int r;
+	int status;
 
-	if (read_proc() == '1')
+	if (read_proc(&status) <= 0)
+		die("Invalid stack tracer state");
+
+	if (status > 0)
 		printf("(stack tracer running)\n");
 	else
 		printf("(stack tracer not running)\n");
-- 
2.14.1

[PATCH v3 2/3] trace-cmd: Remove the die() call from read_proc()

[Vladislav Valtchev (VMware)]

As trace-stack.c's read_proc() function is going to be used by trace-cmd stat,
we don't want it to make the program die in case something went wrong.
Therefore, this simple patch makes read_proc() to just return -1 in case the
proc file was empty or read() failed with an error, instead of using die().

Signed-off-by: Vladislav Valtchev (VMware) <vladislav.valtchev@gmail.com>
---
 trace-stack.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/trace-stack.c b/trace-stack.c
index c1058ca..d55d994 100644
--- a/trace-stack.c
+++ b/trace-stack.c
@@ -79,9 +79,9 @@ static int read_proc(int *status)
 
 	n = read(fd, buf, sizeof(buf));
 
-	/* We assume that the file is never empty we got no errors. */
+	/* The file was empty or read() failed with an error. */
 	if (n <= 0)
-		die("error reading %s", PROC_FILE);
+		return -1;
 
 	/* Does this file have more than 63 characters?? */
 	if (n >= sizeof(buf))
-- 
2.14.1

[PATCH v3 3/3] trace-cmd: Making stat to report when the stack tracer is ON

[Vladislav Valtchev (VMware)]

trace-cmd stat is a handy way for users to see what tracing is currently going
on, but currently it does not say anything about the stack tracing. This patch
makes the command to show a message when the stack tracer is ON.

Signed-off-by: Vladislav Valtchev (VMware) <vladislav.valtchev@gmail.com>
---
 trace-cmd.h   | 2 ++
 trace-stack.c | 6 ++++++
 trace-stat.c  | 8 ++++++++
 3 files changed, 16 insertions(+)

diff --git a/trace-cmd.h b/trace-cmd.h
index 6fd34d7..9704b2e 100644
--- a/trace-cmd.h
+++ b/trace-cmd.h
@@ -358,6 +358,8 @@ void tracecmd_free_hooks(struct hook_list *hooks);
 /* --- Hack! --- */
 int tracecmd_blk_hack(struct tracecmd_input *handle);
 
+/* --- Stack tracer functions --- */
+int tracecmd_stack_tracer_status(int *status);
 
 /* --- Debugging --- */
 struct kbuffer *tracecmd_record_kbuf(struct tracecmd_input *handle,
diff --git a/trace-stack.c b/trace-stack.c
index d55d994..0028ecc 100644
--- a/trace-stack.c
+++ b/trace-stack.c
@@ -107,6 +107,12 @@ static int read_proc(int *status)
 	return 1; /* full success */
 }
 
+/* Public wrapper of read_proc() */
+int tracecmd_stack_tracer_status(int *status)
+{
+	return read_proc(status);
+}
+
 /* NOTE: this implementation only accepts new_status in the range [0..9]. */
 static void change_stack_tracer_status(int new_status)
 {
diff --git a/trace-stat.c b/trace-stat.c
index fd16354..23d7dd4 100644
--- a/trace-stat.c
+++ b/trace-stat.c
@@ -893,6 +893,7 @@ void trace_stat (int argc, char **argv)
 {
 	struct buffer_instance *instance = &top_instance;
 	int topt = 0;
+	int status;
 	int c;
 
 	for (;;) {
@@ -928,5 +929,12 @@ void trace_stat (int argc, char **argv)
 		stat_instance(instance);
 	}
 
+	if (tracecmd_stack_tracer_status(&status) >= 0) {
+		if (status > 0)
+			printf("Stack tracing is enabled\n\n");
+	} else {
+		printf("Error reading stack tracer status\n\n");
+	}
+
 	exit(0);
 }
-- 
2.14.1

Re: [PATCH v3 1/3] trace-cmd: Make read_proc() to return int status via OUT arg

[Steven Rostedt]

On Tue, 16 Jan 2018 09:47:42 +0200
"Vladislav Valtchev (VMware)" <vladislav.valtchev@gmail.com> wrote:

> +	errno = 0;
> +
> +	/* Read an integer from buf ignoring any non-digit trailing characters. */
> +	num = strtol(buf, NULL, 10);
> +
> +	/* strtol() returned 0: we have to check for errors */
> +	if (!num && (errno == EINVAL || errno == ERANGE))
> +		return -1;

Repeating again here. According to the man page of strtol():

RETURN VALUE
       The  strtol() function returns the result of the conversion, unless the
       value would underflow or overflow.  If an  underflow  occurs,  strtol()
       returns  LONG_MIN.   If  an overflow occurs, strtol() returns LONG_MAX.
       In both cases, errno is set to ERANGE.  Precisely the  same  holds  for
       strtoll()  (with  LLONG_MIN  and  LLONG_MAX  instead  of  LONG_MIN  and
       LONG_MAX).

and this:

       The implementation may also set errno to EINVAL in case  no  conversion
       was performed (no digits seen, and 0 returned).

Thus, !num is not enough. The example in the man page has:

           errno = 0;    /* To distinguish success/failure after call */
           val = strtol(str, &endptr, base);

           /* Check for various possible errors */

           if ((errno == ERANGE && (val == LONG_MAX || val == LONG_MIN))
                   || (errno != 0 && val == 0)) {
               perror("strtol");
               exit(EXIT_FAILURE);
           }

Let's follow this.

-- Steve

Re: [PATCH v3 1/3] trace-cmd: Make read_proc() to return int status via OUT arg

[Steven Rostedt]

On Tue, 16 Jan 2018 09:47:42 +0200
"Vladislav Valtchev (VMware)" <vladislav.valtchev@gmail.com> wrote:

> diff --git a/trace-stack.c b/trace-stack.c
> index aa79ae3..c1058ca 100644
> --- a/trace-stack.c
> +++ b/trace-stack.c
> @@ -20,6 +20,7 @@
>  #include <stdio.h>
>  #include <stdlib.h>
>  #include <string.h>
> +#include <limits.h>
>  #include <getopt.h>
>  #include <sys/types.h>
>  #include <sys/stat.h>
> @@ -49,37 +50,79 @@ static void test_available(void)
>  		die("stack tracer not configured on running kernel");
>  }
>  
> -static char read_proc(void)
> +/*
> + * Returns:
> + *   -1 - Something went wrong
> + *    0 - File does not exist (stack tracer not enabled)
> + *    1 - Success
> + */
> +static int read_proc(int *status)
>  {
> -	char buf[1];
> +	struct stat stat_buf;
> +	char buf[64];
> +	long num;
>  	int fd;
>  	int n;
>  
> +	if (stat(PROC_FILE, &stat_buf) < 0) {
> +		/* stack tracer not configured on running kernel */
> +		*status = 0; /* not configured means disabled */
> +		return 0;
> +	}
> +
>  	fd = open(PROC_FILE, O_RDONLY);
> -	if (fd < 0)
> -		die("reading %s", PROC_FILE);
> -	n = read(fd, buf, 1);
> -	close(fd);
> -	if (n != 1)
> +
> +	if (fd < 0) {
> +		/* we cannot open the file: likely a permission problem. */
> +		return -1;
> +	}

Let's follow Linux coding style. The comment is obvious and can be
removed, and we can remove the braces.

	if (fd < 0)
		return -1;

> +
> +	n = read(fd, buf, sizeof(buf));
> +
> +	/* We assume that the file is never empty we got no errors. */
> +	if (n <= 0)
>  		die("error reading %s", PROC_FILE);
>  
> -	return buf[0];
> +	/* Does this file have more than 63 characters?? */

The comment is too verbose, and not needed.

> +	if (n >= sizeof(buf))
> +		return -1;
> +
> +	/* n is guaranteed to be in the range [1, sizeof(buf)-1]. */

The comment is not needed. It would be a bug if it were not true, and
the code directly above the comment shows this guarantee.

> +	buf[n] = 0;
> +	close(fd);
> +
> +	errno = 0;
> +
> +	/* Read an integer from buf ignoring any non-digit trailing characters. */

Don't need to describe what strtol() does. Remove the comment please.

> +	num = strtol(buf, NULL, 10);
> +
> +	/* strtol() returned 0: we have to check for errors */

Already commented about this code in my last email.

> +	if (!num && (errno == EINVAL || errno == ERANGE))
> +		return -1;
> +
> +	if (num > INT_MAX || num < INT_MIN)
> +		return -1; /* the number is good but does not fit in 'int' */

Comment is redundant and unnecessary, please remove it.

Comments are good, but not if they are at the level of describing the
code for a beginner programmer. That is, if the code isn't obvious what
it is doing, it should be commented. But comments that describe the
definition of the code end up causing more noise than being helpful.


> +
> +	*status = num;
> +	return 1; /* full success */
>  }
>  
> -static void start_stop_trace(char val)
> +/* NOTE: this implementation only accepts new_status in the range [0..9]. */

For example, the above comment is good. It explicitly states that this
function requires the parameter new_status be in the range of 0-9, and
a reviewer or new developer doesn't have to go read the function to
figure that out.


> +static void change_stack_tracer_status(int new_status)
>  {
>  	char buf[1];
> +	int status;
>  	int fd;
>  	int n;
>  

Should enforce that new_status is between 0 and 9, by one of the
methods I discussed in the other email.

-- Steve

> -	buf[0] = read_proc();
> -	if (buf[0] == val)
> -		return;
> +	if (read_proc(&status) > 0 && status == new_status)
> +		return; /* nothing to do */
>  
>  	fd = open(PROC_FILE, O_WRONLY);
> +
>  	if (fd < 0)
>  		die("writing %s", PROC_FILE);
> -	buf[0] = val;
> +	buf[0] = new_status + '0';
>  	n = write(fd, buf, 1);
>  	if (n < 0)
>  		die("writing into %s", PROC_FILE);
> @@ -88,12 +131,12 @@ static void start_stop_trace(char val)
>  
>  static void start_trace(void)
>  {
> -	start_stop_trace('1');
> +	change_stack_tracer_status(1);
>  }
>  
>  static void stop_trace(void)
>  {
> -	start_stop_trace('0');
> +	change_stack_tracer_status(0);
>  }
>  
>  static void reset_trace(void)
> @@ -123,8 +166,12 @@ static void read_trace(void)
>  	char *buf = NULL;
>  	size_t n;
>  	int r;
> +	int status;
>  
> -	if (read_proc() == '1')
> +	if (read_proc(&status) <= 0)
> +		die("Invalid stack tracer state");
> +
> +	if (status > 0)
>  		printf("(stack tracer running)\n");
>  	else
>  		printf("(stack tracer not running)\n");

Re: [PATCH v3 1/3] trace-cmd: Make read_proc() to return int status via OUT arg

[Vladislav Valtchev]

On Tue, 2018-01-16 at 12:19 -0500, Steven Rostedt wrote:
> On Tue, 16 Jan 2018 09:47:42 +0200
> "Vladislav Valtchev (VMware)" <vladislav.valtchev@gmail.com> wrote:
> 
> > +	errno = 0;
> > +
> > +	/* Read an integer from buf ignoring any non-digit trailing characters. */
> > +	num = strtol(buf, NULL, 10);
> > +
> > +	/* strtol() returned 0: we have to check for errors */
> > +	if (!num && (errno == EINVAL || errno == ERANGE))
> > +		return -1;
> 
> Repeating again here. According to the man page of strtol():

v3 addresses only the comments for patch 3/3.
I'm sorry for that. All the other comments will be addressed in v4.

> 
> RETURN VALUE
>        The  strtol() function returns the result of the conversion, unless the
>        value would underflow or overflow.  If an  underflow  occurs,  strtol()
>        returns  LONG_MIN.   If  an overflow occurs, strtol() returns LONG_MAX.
>        In both cases, errno is set to ERANGE.  Precisely the  same  holds  for
>        strtoll()  (with  LLONG_MIN  and  LLONG_MAX  instead  of  LONG_MIN  and
>        LONG_MAX).
> 
> and this:
> 
>        The implementation may also set errno to EINVAL in case  no  conversion
>        was performed (no digits seen, and 0 returned).
> 
> Thus, !num is not enough. The example in the man page has:
> 
>            errno = 0;    /* To distinguish success/failure after call */
>            val = strtol(str, &endptr, base);
> 
>            /* Check for various possible errors */
> 
>            if ((errno == ERANGE && (val == LONG_MAX || val == LONG_MIN))
>                    || (errno != 0 && val == 0)) {
>                perror("strtol");
>                exit(EXIT_FAILURE);
>            }
> 
> Let's follow this.
> 
> -- Steve

Sure, I thought that:

	errno = 0;
	num = strtol(buf, NULL, 10);

	/* strtol() returned 0: we have to check for errors */
	if (!num && (errno == EINVAL || errno == ERANGE))
		return -1;

	if (num > INT_MAX || num < INT_MIN)
		return -1;

covered all the cases because the case:
(val == LONG_MAX || val == LONG_MIN)

is covered by: if (num > INT_MAX || num < INT_MIN)
[no matter the errno]

but that's not true for 32 bit systems where sizeof(long) == sizeof(int).
It had to be: if (num >= INT_MAX || num <= INT_MIN), but in that
case it would exclude two valid int32 values.

Therefore, let's go with:
	if ((errno == ERANGE && (val == LONG_MAX || val == LONG_MIN))
            || (errno != 0 && val == 0))


Just let me keep also the following check:

	if (num > INT_MAX || num < INT_MIN)
		return -1;

since [INT_MIN, INT_MAX] is a subset of [LONG_MIN, LONG_MAX].

Vlad


-- 
Vladislav Valtchev
VMware Open Source Technology Center

Re: [PATCH v3 1/3] trace-cmd: Make read_proc() to return int status via OUT arg

[Steven Rostedt]

On Tue, 16 Jan 2018 21:10:36 +0200
Vladislav Valtchev <vladislav.valtchev@gmail.com> wrote:

> On Tue, 2018-01-16 at 12:19 -0500, Steven Rostedt wrote:
> > On Tue, 16 Jan 2018 09:47:42 +0200
> > "Vladislav Valtchev (VMware)" <vladislav.valtchev@gmail.com> wrote:
> >   
> > > +	errno = 0;
> > > +
> > > +	/* Read an integer from buf ignoring any non-digit trailing characters. */
> > > +	num = strtol(buf, NULL, 10);
> > > +
> > > +	/* strtol() returned 0: we have to check for errors */
> > > +	if (!num && (errno == EINVAL || errno == ERANGE))
> > > +		return -1;  
> > 
> > Repeating again here. According to the man page of strtol():  
> 
> v3 addresses only the comments for patch 3/3.
> I'm sorry for that. All the other comments will be addressed in v4.
> 
> > 
> > RETURN VALUE
> >        The  strtol() function returns the result of the conversion, unless the
> >        value would underflow or overflow.  If an  underflow  occurs,  strtol()
> >        returns  LONG_MIN.   If  an overflow occurs, strtol() returns LONG_MAX.
> >        In both cases, errno is set to ERANGE.  Precisely the  same  holds  for
> >        strtoll()  (with  LLONG_MIN  and  LLONG_MAX  instead  of  LONG_MIN  and
> >        LONG_MAX).
> > 
> > and this:
> > 
> >        The implementation may also set errno to EINVAL in case  no  conversion
> >        was performed (no digits seen, and 0 returned).
> > 
> > Thus, !num is not enough. The example in the man page has:
> > 
> >            errno = 0;    /* To distinguish success/failure after call */
> >            val = strtol(str, &endptr, base);
> > 
> >            /* Check for various possible errors */
> > 
> >            if ((errno == ERANGE && (val == LONG_MAX || val == LONG_MIN))
> >                    || (errno != 0 && val == 0)) {
> >                perror("strtol");
> >                exit(EXIT_FAILURE);
> >            }
> > 
> > Let's follow this.
> > 
> > -- Steve  
> 
> Sure, I thought that:
> 
> 	errno = 0;
> 	num = strtol(buf, NULL, 10);
> 
> 	/* strtol() returned 0: we have to check for errors */
> 	if (!num && (errno == EINVAL || errno == ERANGE))
> 		return -1;
> 
> 	if (num > INT_MAX || num < INT_MIN)
> 		return -1;
> 
> covered all the cases because the case:
> (val == LONG_MAX || val == LONG_MIN)
> 
> is covered by: if (num > INT_MAX || num < INT_MIN)
> [no matter the errno]
> 
> but that's not true for 32 bit systems where sizeof(long) == sizeof(int).
> It had to be: if (num >= INT_MAX || num <= INT_MIN), but in that
> case it would exclude two valid int32 values.
> 
> Therefore, let's go with:
> 	if ((errno == ERANGE && (val == LONG_MAX || val == LONG_MIN))
>             || (errno != 0 && val == 0))
> 
> 
> Just let me keep also the following check:
> 
> 	if (num > INT_MAX || num < INT_MIN)
> 		return -1;
> 
> since [INT_MIN, INT_MAX] is a subset of [LONG_MIN, LONG_MAX].
> 

True. What about just doing:


	if (num > INT_MAX || num < INT_MIN || (!num && errno))

That should cover it all, and match what the man pages have.

-- Steve