[PATCH bpf-next] libbpf: configure log verbosity with env variable

[Mykyta Yatsenko mykyta.yatsenko5@gmail.com]

From: Mykyta Yatsenko <yatsenko@meta.com>

Configure logging verbosity by setting LIBBPF_LOG_LEVEL environment
variable, which is applied only to default logger. Once user set their
custom logging callback, it is up to them to handle filtering.

Signed-off-by: Mykyta Yatsenko <yatsenko@meta.com>
---
 Documentation/bpf/libbpf/libbpf_overview.rst |  7 ++++++
 tools/lib/bpf/libbpf.c                       | 24 +++++++++++++++++++-
 tools/lib/bpf/libbpf.h                       |  5 +++-
 3 files changed, 34 insertions(+), 2 deletions(-)

diff --git a/Documentation/bpf/libbpf/libbpf_overview.rst b/Documentation/bpf/libbpf/libbpf_overview.rst
index f36a2d4ffea2..982dfd71a13d 100644
--- a/Documentation/bpf/libbpf/libbpf_overview.rst
+++ b/Documentation/bpf/libbpf/libbpf_overview.rst
@@ -219,6 +219,13 @@ compilation and skeleton generation. Using Libbpf-rs will make building user
 space part of the BPF application easier. Note that the BPF program themselves
 must still be written in plain C.
 
+libbpf logging
+==============
+
+By default, libbpf logs informational and warning messages to stderr. The verbosity of these
+messages can be controlled by setting the environment variable LIBBPF_LOG_LEVEL to either warn,
+info, or debug. A custom log callback can be set using ``libbpf_set_print()``.
+
 Additional Documentation
 ========================
 
diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index 5401f2df463d..d0465ca74afc 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -229,7 +229,29 @@ static const char * const prog_type_name[] = {
 static int __base_pr(enum libbpf_print_level level, const char *format,
 		     va_list args)
 {
-	if (level == LIBBPF_DEBUG)
+	static enum libbpf_print_level min_level = LIBBPF_INFO;
+	static const char *env_var = "LIBBPF_LOG_LEVEL";
+	static bool initialized;
+
+	if (!initialized) {
+		char *verbosity;
+
+		initialized = true;
+		verbosity = getenv(env_var);
+		if (verbosity) {
+			if (strcasecmp(verbosity, "warn") == 0)
+				min_level = LIBBPF_WARN;
+			else if (strcasecmp(verbosity, "debug") == 0)
+				min_level = LIBBPF_DEBUG;
+			else if (strcasecmp(verbosity, "info") == 0)
+				min_level = LIBBPF_INFO;
+			else
+				fprintf(stderr, "Unexpected value of %s env variable\n", env_var);
+		}
+	}
+
+	/* if too verbose, skip logging  */
+	if (level > min_level)
 		return 0;
 
 	return vfprintf(stderr, format, args);
diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
index c3f77d9260fe..26e4e35528c5 100644
--- a/tools/lib/bpf/libbpf.h
+++ b/tools/lib/bpf/libbpf.h
@@ -98,7 +98,10 @@ typedef int (*libbpf_print_fn_t)(enum libbpf_print_level level,
 
 /**
  * @brief **libbpf_set_print()** sets user-provided log callback function to
- * be used for libbpf warnings and informational messages.
+ * be used for libbpf warnings and informational messages. If the user callback
+ * is not set, messages are logged to stderr by default. The verbosity of these
+ * messages can be controlled by setting the environment variable
+ * LIBBPF_LOG_LEVEL to either warn, info, or debug.
  * @param fn The log print function. If NULL, libbpf won't print anything.
  * @return Pointer to old print function.
  *
-- 
2.45.0