libtracefs: Add API to set custom tracing directory

Usually the system's tracing directory is mounted as a subfolder in
/sys/kernel and is auto detected by the tracefs library, using the
information from the "/proc/mounts" file. But there are cases where
/sys and /proc are mounted inside a container on a custom mount point.
In those cases the tracefs library cannot auto detect the system's
tracing directory. That's why a new API is introduced, to handle these
custom use cases:
 int tracefs_set_tracing_dir(char *tracing_dir);

Link: https://lore.kernel.org/linux-trace-devel/20220826054212.146976-1-tz.stoyanov@gmail.com

Signed-off-by: Tzvetomir Stoyanov (VMware) <tz.stoyanov@gmail.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

5 files changed, 64 insertions, 2 deletions

diff --git a/Documentation/libtracefs-files.txt b/Documentation/libtracefs-files.txt
index ab917a6..2a3f544 100644
--- a/Documentation/libtracefs-files.txt
+++ b/Documentation/libtracefs-files.txt
@@ -3,8 +3,8 @@ libtracefs(3)
 
 NAME
 ----
-tracefs_get_tracing_file, tracefs_put_tracing_file, tracefs_tracing_dir, tracefs_debug_dir -
-Find locations of trace directory and files.
+tracefs_get_tracing_file, tracefs_put_tracing_file, tracefs_tracing_dir, tracefs_debug_dir, tracefs_set_tracing_dir -
+Find and set locations of trace directory and files.
 
 SYNOPSIS
 --------
@@ -16,6 +16,7 @@ char pass:[*]*tracefs_get_tracing_file*(const char pass:[*]_name_);
 void *tracefs_put_tracing_file*(char pass:[*]_name_);
 const char pass:[*]*tracefs_tracing_dir*(void);
 const char pass:[*]*tracefs_debug_dir*(void);
+int *tracefs_set_tracing_dir*(char pass:[*]_tracing_dir_)
 --
 
 DESCRIPTION
@@ -23,6 +24,13 @@ DESCRIPTION
 This set of APIs can be used to find the full path of the trace file
 system mount point and trace files in it.
 
+The *tracefs_set_tracing_dir()* function sets a custom location of the
+system's tracing directory mount point. Usually, the library auto detects
+it using the information from the /proc/mounts file. Use this API only if the
+mount point is not standard and cannot be detected by the library. The _tracing_dir_
+argument can be NULL, in that case the custom location is deleted and the library
+auto detection logic is used.
+
 The *tracefs_get_tracing_file()* function returns the full path of the
 file with given _name_ in the trace file system. The function works
 only with files in the tracefs main directory, it is not trace instance
@@ -46,6 +54,8 @@ be freed.
 
 RETURN VALUE
 ------------
+The *tracefs_set_tracing_dir()* function returns 0 on success, -1 otherwise.
+
 The *tracefs_get_tracing_file()* function returns a string or NULL in case
 of an error. The returned string must be freed with *tracefs_put_tracing_file()*.
 
diff --git a/Documentation/libtracefs.txt b/Documentation/libtracefs.txt
index 82cb574..eb2eff4 100644
--- a/Documentation/libtracefs.txt
+++ b/Documentation/libtracefs.txt
@@ -15,6 +15,7 @@ Locations of tracing files and directories:
 	char pass:[*]*tracefs_get_tracing_file*(const char pass:[*]_name_);
 	void *tracefs_put_tracing_file*(char pass:[*]_name_);
 	const char pass:[*]*tracefs_tracing_dir*(void);
+	int *tracefs_set_tracing_dir*(char pass:[*]_tracing_dir_)
 
 Trace instances:
 	struct tracefs_instance pass:[*]*tracefs_instance_create*(const char pass:[*]_name_);
diff --git a/include/tracefs.h b/include/tracefs.h
index 693860d..fd2240a 100644
--- a/include/tracefs.h
+++ b/include/tracefs.h
@@ -16,6 +16,7 @@ void tracefs_put_tracing_file(char *name);
 /* the returned string must *not* be freed */
 const char *tracefs_tracing_dir(void);
 const char *tracefs_debug_dir(void);
+int tracefs_set_tracing_dir(char *tracing_dir);
 
 /* ftrace instances */
 struct tracefs_instance;
diff --git a/src/tracefs-utils.c b/src/tracefs-utils.c
index 22f82af..55a2f0a 100644
--- a/src/tracefs-utils.c
+++ b/src/tracefs-utils.c
@@ -29,6 +29,7 @@
 #define STR(x) _STR(x)
 
 static int log_level = TEP_LOG_CRITICAL;
+static char *custom_tracing_dir;
 
 /**
  * tracefs_set_loglevel - set log level of the library
@@ -165,6 +166,32 @@ __hidden char *trace_find_tracing_dir(bool debugfs)
 }
 
 /**
+ * tracefs_set_tracing_dir - Set location of the tracing directory
+ * @tracing_dir: full path to the system's tracing directory mount point.
+ *
+ * Set the location to the system's tracing directory. This API should be used
+ * to set a custom location of the tracing directory. There is no need to call
+ * it if the location is standard, in that case the library will auto detect it.
+ *
+ * Returns 0 on success, -1 otherwise.
+ */
+int tracefs_set_tracing_dir(char *tracing_dir)
+{
+	if (custom_tracing_dir) {
+		free(custom_tracing_dir);
+		custom_tracing_dir = NULL;
+	}
+
+	if (tracing_dir) {
+		custom_tracing_dir = strdup(tracing_dir);
+		if (!custom_tracing_dir)
+			return -1;
+	}
+
+	return 0;
+}
+
+/**
  * tracefs_tracing_dir - Get tracing directory
  *
  * Returns string containing the full path to the system's tracing directory.
@@ -174,6 +201,9 @@ const char *tracefs_tracing_dir(void)
 {
 	static const char *tracing_dir;
 
+	if (custom_tracing_dir)
+		return custom_tracing_dir;
+
 	if (tracing_dir)
 		return tracing_dir;
 
diff --git a/utest/tracefs-utest.c b/utest/tracefs-utest.c
index 3f63837..6f083db 100644
--- a/utest/tracefs-utest.c
+++ b/utest/tracefs-utest.c
@@ -11,6 +11,7 @@
 #include <time.h>
 #include <dirent.h>
 #include <ftw.h>
+#include <libgen.h>
 
 #include <CUnit/CUnit.h>
 #include <CUnit/Basic.h>
@@ -1701,8 +1702,11 @@ void del_trace_dir(char *dir)
 
 static void test_custom_trace_dir(void)
 {
+	char *tdir = "/tmp/custom_tracefs";
 	struct tracefs_instance *instance;
 	char *dname = copy_trace_dir();
+	const char *trace_dir;
+	char *tfile;
 
 	instance = tracefs_instance_alloc(dname, NULL);
 	CU_TEST(instance != NULL);
@@ -1717,6 +1721,22 @@ static void test_custom_trace_dir(void)
 	tracefs_instance_free(instance);
 	del_trace_dir(dname);
 	free(dname);
+
+	trace_dir = tracefs_tracing_dir();
+	CU_TEST(trace_dir != NULL);
+	CU_TEST(tracefs_set_tracing_dir(tdir) == 0);
+	CU_TEST(strcmp(tdir, tracefs_tracing_dir()) == 0);
+	tfile = tracefs_get_tracing_file("trace");
+	CU_TEST(tfile != NULL);
+	CU_TEST(strcmp(tdir, dirname(tfile)) == 0);
+	free(tfile);
+
+	CU_TEST(tracefs_set_tracing_dir(NULL) == 0);
+	CU_TEST(strcmp(trace_dir, tracefs_tracing_dir()) == 0);
+	tfile = tracefs_get_tracing_file("trace");
+	CU_TEST(tfile != NULL);
+	CU_TEST(strcmp(trace_dir, dirname(tfile)) == 0);
+	free(tfile);
 }
 
 static int test_suite_destroy(void)