linux-kernel - [PATCH] kcov: don't instrument lib/find

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Message-Id: <20240722223726.194658-1-andrey.konovalov@linux.dev>
Date: Tue, 23 Jul 2024 00:37:26 +0200
From: andrey.konovalov@...ux.dev
To: Dmitry Vyukov <dvyukov@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Cc: Andrey Konovalov <andreyknvl@...il.com>,
	Aleksandr Nogikh <nogikh@...gle.com>,
	Marco Elver <elver@...gle.com>,
	Alexander Potapenko <glider@...gle.com>,
	kasan-dev@...glegroups.com,
	linux-mm@...ck.org,
	Yury Norov <yury.norov@...il.com>,
	Rasmus Villemoes <linux@...musvillemoes.dk>,
	linux-kernel@...r.kernel.org
Subject: [PATCH] kcov: don't instrument lib/find_bit.c

From: Andrey Konovalov <andreyknvl@...il.com>

This file produces large amounts of flaky coverage not useful for the
KCOV's intended use case (guiding the fuzzing process).

Signed-off-by: Andrey Konovalov <andreyknvl@...il.com>

---

I noticed this while running one of the syzkaller's programs.

In one run of the program, the number of KCOV entries amounts to ~300k,
with the top ones:

 117285 /home/user/src/lib/find_bit.c:137 (discriminator 10)
 116752 /home/user/src/lib/find_bit.c:137 (discriminator 3)
   2455 /home/user/src/lib/vsprintf.c:2559
   2033 /home/user/src/fs/kernfs/dir.c:317
   1662 /home/user/src/fs/kernfs/kernfs-internal.h:72
   ...

In another run (that triggers exactly the same behavior in the kernel),
the amount of entries drops to ~110k:

   7141 /home/user/src/lib/find_bit.c:137 (discriminator 10)
   7110 /home/user/src/lib/find_bit.c:137 (discriminator 3)
   2455 /home/user/src/lib/vsprintf.c:2559
   2033 /home/user/src/fs/kernfs/dir.c:317
   1662 /home/user/src/fs/kernfs/kernfs-internal.h:72
    ...

With this patch applied, the amount of KCOV entries for the same program
remains somewhat stable at ~100k.
---
 lib/Makefile | 1 +
 1 file changed, 1 insertion(+)

diff --git a/lib/Makefile b/lib/Makefile
index 322bb127b4dc..0fde1c360f32 100644
--- a/lib/Makefile
+++ b/lib/Makefile
@@ -14,6 +14,7 @@ KCOV_INSTRUMENT_list_debug.o := n
 KCOV_INSTRUMENT_debugobjects.o := n
 KCOV_INSTRUMENT_dynamic_debug.o := n
 KCOV_INSTRUMENT_fault-inject.o := n
+KCOV_INSTRUMENT_find_bit.o := n
 
 # string.o implements standard library functions like memset/memcpy etc.
 # Use -ffreestanding to ensure that the compiler does not try to "optimize"
-- 
2.25.1