From: JP Kobryn <inwardvessel@gmail.com>
Date: Sat, 25 Mar 2023 01:08:45 +0000 (-0700)
Subject: libbpf: Ensure print callback usage is thread-safe
X-Git-Tag: v6.6.7~2736^2~129^2~56
X-Git-Url: http://review.tizen.org/git/?a=commitdiff_plain;h=f1cb927cdb6280e0ce283174cc1bf395dffc43d4;p=platform%2Fkernel%2Flinux-starfive.git

libbpf: Ensure print callback usage is thread-safe

This patch prevents races on the print function pointer, allowing the
libbpf_set_print() function to become thread-safe.

Signed-off-by: JP Kobryn <inwardvessel@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20230325010845.46000-1-inwardvessel@gmail.com
---

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index f6a071d..15737d7 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -216,9 +216,10 @@ static libbpf_print_fn_t __libbpf_pr = __base_pr;
 
 libbpf_print_fn_t libbpf_set_print(libbpf_print_fn_t fn)
 {
-	libbpf_print_fn_t old_print_fn = __libbpf_pr;
+	libbpf_print_fn_t old_print_fn;
+
+	old_print_fn = __atomic_exchange_n(&__libbpf_pr, fn, __ATOMIC_RELAXED);
 
-	__libbpf_pr = fn;
 	return old_print_fn;
 }
 
@@ -227,8 +228,10 @@ void libbpf_print(enum libbpf_print_level level, const char *format, ...)
 {
 	va_list args;
 	int old_errno;
+	libbpf_print_fn_t print_fn;
 
-	if (!__libbpf_pr)
+	print_fn = __atomic_load_n(&__libbpf_pr, __ATOMIC_RELAXED);
+	if (!print_fn)
 		return;
 
 	old_errno = errno;
diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
index 1615e55..0b73623 100644
--- a/tools/lib/bpf/libbpf.h
+++ b/tools/lib/bpf/libbpf.h
@@ -101,6 +101,8 @@ typedef int (*libbpf_print_fn_t)(enum libbpf_print_level level,
  * be used for libbpf warnings and informational messages.
  * @param fn The log print function. If NULL, libbpf won't print anything.
  * @return Pointer to old print function.
+ *
+ * This function is thread-safe.
  */
 LIBBPF_API libbpf_print_fn_t libbpf_set_print(libbpf_print_fn_t fn);