Re: [PATCH v4 1/4] trace-cmd: Add new tracefs APIs for checking if a ftrace file / directory exists

[Steven Rostedt <rostedt@goodmis.org>]

On Tue, 25 Feb 2020 20:07:30 +0200
"Tzvetomir Stoyanov (VMware)" <tz.stoyanov@gmail.com> wrote:

> Some ftrace files and directories are optional, depending on specific kernel configuration or version.
> It is a good practice to check if the file / directory exist, before trying to access it. There are a lot of places in trace-cmd implementation with such checks.
> The new libtracefs APIs can be used for this, they are ftarce instance aware:
>  bool tracefs_file_exist(struct tracefs_instance *instance, char *name);
>  bool tracefs_dir_exist(struct tracefs_instance *instance, char *name);
> 
> Signed-off-by: Tzvetomir Stoyanov (VMware) <tz.stoyanov@gmail.com>
> ---
>  include/tracefs/tracefs.h      |  3 +++
>  lib/tracefs/tracefs-instance.c | 49 +++++++++++++++++++++++++++++++++-
>  2 files changed, 51 insertions(+), 1 deletion(-)
> 
> diff --git a/include/tracefs/tracefs.h b/include/tracefs/tracefs.h
> index 85690b66..bc8bebcb 100644
> --- a/include/tracefs/tracefs.h
> +++ b/include/tracefs/tracefs.h
> @@ -33,6 +33,9 @@ int tracefs_instance_file_write(struct tracefs_instance *instance,
>  char *tracefs_instance_file_read(struct tracefs_instance *instance,
>  				 char *file, int *psize);
>  
> +bool tracefs_file_exist(struct tracefs_instance *instance, char *name);
> +bool tracefs_dir_exist(struct tracefs_instance *instance, char *name);
> +
>  /* events */
>  void tracefs_list_free(char **list);
>  char **tracefs_event_systems(const char *tracing_dir);
> diff --git a/lib/tracefs/tracefs-instance.c b/lib/tracefs/tracefs-instance.c
> index b96ab61c..67123e7c 100644
> --- a/lib/tracefs/tracefs-instance.c
> +++ b/lib/tracefs/tracefs-instance.c
> @@ -13,7 +13,7 @@
>  #include <errno.h>
>  #include <sys/stat.h>
>  #include <fcntl.h>
> -
> +#include <linux/limits.h>
>  #include "tracefs.h"
>  #include "tracefs-local.h"
>  
> @@ -247,3 +247,50 @@ char *tracefs_instance_file_read(struct tracefs_instance *instance,
>  
>  	return buf;
>  }
> +
> +static bool check_file_exist(struct tracefs_instance *instance,
> +			     char *name, bool dir)
> +{
> +	char file[PATH_MAX];
> +	struct stat st;
> +	char *path;
> +	int ret;
> +
> +	path = tracefs_instance_get_dir(instance);
> +	snprintf(file, PATH_MAX, "%s/%s", path, name);
> +	tracefs_put_tracing_file(path);
> +	ret = stat(file, &st);
> +	if (ret < 0)
> +		return false;
> +
> +	if (dir && !S_ISDIR(st.st_mode))
> +		return false;
> +	if (!dir && S_ISDIR(st.st_mode))
> +		return false;
> +
> +	return true;

BTW, the above can be converted to just:

	return !dir == !S_ISDIR(st.st_mode);


> +}
> +
> +/**
> + * tracefs_file_exist - Check if a file with given name exists in given instance
> + * @instance: ftrace instance, can be NULL for the top instance
> + * @name: name of the file
> + *
> + * Returns true if the file exists, false otherwise

Probably should comment that this will return false if the "file" exists
but is a directory.

Is there any reason we would not want to return true here if it was a
directory? A directory is just another kind of "file" (in the Unix world).

-- Steve

> + */
> +bool tracefs_file_exist(struct tracefs_instance *instance, char *name)
> +{
> +	return check_file_exist(instance, name, false);
> +}
> +
> +/**
> + * tracefs_dir_exist - Check if a directory with given name exists in given instance
> + * @instance: ftrace instance, can be NULL for the top instance
> + * @name: name of the directory
> + *
> + * Returns true if the directory exists, false otherwise
> + */
> +bool tracefs_dir_exist(struct tracefs_instance *instance, char *name)
> +{
> +	return check_file_exist(instance, name, true);
> +}