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Wed, 18 Mar 2026 09:27:13 -0700 (PDT) From: Jann Horn Date: Wed, 18 Mar 2026 17:27:01 +0100 Subject: [PATCH v2 4/4] kcov: introduce extended PC coverage collection mode Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable Message-Id: <20260318-kcov-extrecord-v2-4-2522da6fcd3f@google.com> References: <20260318-kcov-extrecord-v2-0-2522da6fcd3f@google.com> In-Reply-To: <20260318-kcov-extrecord-v2-0-2522da6fcd3f@google.com> To: Dmitry Vyukov , Andrey Konovalov , Alexander Potapenko Cc: Nathan Chancellor , Nick Desaulniers , Bill Wendling , Justin Stitt , linux-kernel@vger.kernel.org, kasan-dev@googlegroups.com, llvm@lists.linux.dev, Jann Horn X-Mailer: b4 0.15-dev X-Developer-Signature: v=1; a=ed25519-sha256; t=1773851224; l=5681; i=jannh@google.com; s=20240730; h=from:subject:message-id; bh=52RZwge2HiHzFPyKFZk5GPDAZYf3/NA1Sia8icDS9Tc=; b=tLZgiv8MkBfdhi/W3flmRalK3OU9W4IRmqGFBLRSj8kDcwSh7Nei0AFtOSJwz3H/1nqGUg3dn dDZNtgFXJmxB6aSRsbo3EwV8RxFRYsCND/pdn97snaexZB7ADSu8WBA X-Developer-Key: i=jannh@google.com; a=ed25519; pk=AljNtGOzXeF6khBXDJVVvwSEkVDGnnZZYqfWhP1V+C8= This is the second half of CONFIG_KCOV_EXT_RECORDS. Introduce a new KCOV mode KCOV_TRACE_PC_EXT which replaces the upper 8 bits of recorded instruction pointers with metadata. For now, userspace can use this metadata to distinguish three types of records: - function entry - function exit - normal basic block inside the function Internally, this new mode is represented as a variant of KCOV_MODE_TRACE_PC, distinguished with the flag KCOV_EXT_FORMAT. Store this flag as part of the mode in task_struct::kcov_mode and in kcov::mode to avoid having to pass it around separately everywhere. Signed-off-by: Jann Horn --- include/linux/kcov.h | 7 +++++++ include/uapi/linux/kcov.h | 12 ++++++++++++ kernel/kcov.c | 39 ++++++++++++++++++++++++++++++++++----- 3 files changed, 53 insertions(+), 5 deletions(-) diff --git a/include/linux/kcov.h b/include/linux/kcov.h index e5502d674029..455302b1cd1c 100644 --- a/include/linux/kcov.h +++ b/include/linux/kcov.h @@ -25,8 +25,15 @@ enum kcov_mode { KCOV_MODE_REMOTE =3D 4, }; =20 +/* + * Modifier for KCOV_MODE_TRACE_PC to record function entry/exit marked wi= th + * metadata bits. + */ +#define KCOV_EXT_FORMAT (1 << 29) #define KCOV_IN_CTXSW (1 << 30) =20 +#define KCOV_MODE_TRACE_PC_EXT (KCOV_MODE_TRACE_PC | KCOV_EXT_FORMAT) + void kcov_task_init(struct task_struct *t); void kcov_task_exit(struct task_struct *t); =20 diff --git a/include/uapi/linux/kcov.h b/include/uapi/linux/kcov.h index ed95dba9fa37..8d8a233bd61f 100644 --- a/include/uapi/linux/kcov.h +++ b/include/uapi/linux/kcov.h @@ -35,8 +35,20 @@ enum { KCOV_TRACE_PC =3D 0, /* Collecting comparison operands mode. */ KCOV_TRACE_CMP =3D 1, + /* + * Extended PC coverage collection mode. + * In this mode, the top byte of the PC is replaced with flag bits + * (KCOV_RECORDFLAG_*). + */ + KCOV_TRACE_PC_EXT =3D 2, }; =20 +#define KCOV_RECORD_IP_MASK 0x00ffffffffffffff +#define KCOV_RECORDFLAG_TYPEMASK 0xf000000000000000 +#define KCOV_RECORDFLAG_TYPE_NORMAL 0xf000000000000000 +#define KCOV_RECORDFLAG_TYPE_ENTRY 0x0000000000000000 +#define KCOV_RECORDFLAG_TYPE_EXIT 0x1000000000000000 + /* * The format for the types of collected comparisons. * diff --git a/kernel/kcov.c b/kernel/kcov.c index 7edb39e18bfe..3cd4ee4cc310 100644 --- a/kernel/kcov.c +++ b/kernel/kcov.c @@ -55,7 +55,12 @@ struct kcov { refcount_t refcount; /* The lock protects mode, size, area and t. */ spinlock_t lock; - enum kcov_mode mode __guarded_by(&lock); + /* + * Mode, consists of: + * - enum kcov_mode + * - flag KCOV_EXT_FORMAT + */ + unsigned int mode __guarded_by(&lock); /* Size of arena (in long's). */ unsigned int size __guarded_by(&lock); /* Coverage buffer shared with user space. */ @@ -232,8 +237,14 @@ void notrace __sanitizer_cov_trace_pc(void) { struct task_struct *cur =3D current; =20 - if (READ_ONCE(cur->kcov_mode) !=3D KCOV_MODE_TRACE_PC) + if ((READ_ONCE(cur->kcov_mode) & ~KCOV_EXT_FORMAT) !=3D KCOV_MODE_TRACE_P= C) return; + /* + * No bitops are needed here for setting the record type because + * KCOV_RECORDFLAG_TYPE_NORMAL has the high bits set. + * This relies on userspace not caring about the rest of the top byte + * for KCOV_RECORDFLAG_TYPE_NORMAL records. + */ kcov_add_pc_record(cur, canonicalize_ip(_RET_IP_)); } EXPORT_SYMBOL(__sanitizer_cov_trace_pc); @@ -249,12 +260,28 @@ void notrace __sanitizer_cov_trace_pc_entry(void) * This hook replaces __sanitizer_cov_trace_pc() for the function entry * basic block; it should still emit a record even in classic kcov mode. */ - if (kcov_mode !=3D KCOV_MODE_TRACE_PC) + if ((kcov_mode & ~KCOV_EXT_FORMAT) !=3D KCOV_MODE_TRACE_PC) return; + if ((kcov_mode & KCOV_EXT_FORMAT) !=3D 0) + record =3D (record & KCOV_RECORD_IP_MASK) | KCOV_RECORDFLAG_TYPE_ENTRY; kcov_add_pc_record(cur, record); } void notrace __sanitizer_cov_trace_pc_exit(void) { + struct task_struct *cur =3D current; + unsigned long record; + + /* + * This hook is not called at the beginning of a basic block; the basic + * block from which the hook was invoked is already covered by a + * preceding hook call. + * So unlike __sanitizer_cov_trace_pc_entry(), this PC should only be + * reported in extended mode, where function exit events are recorded. + */ + if (READ_ONCE(cur->kcov_mode) !=3D KCOV_MODE_TRACE_PC_EXT) + return; + record =3D (canonicalize_ip(_RET_IP_) & KCOV_RECORD_IP_MASK) | KCOV_RECOR= DFLAG_TYPE_EXIT; + kcov_add_pc_record(cur, record); } #endif =20 @@ -377,7 +404,7 @@ EXPORT_SYMBOL(__sanitizer_cov_trace_switch); #endif /* ifdef CONFIG_KCOV_ENABLE_COMPARISONS */ =20 static void kcov_start(struct task_struct *t, struct kcov *kcov, - unsigned int size, void *area, enum kcov_mode mode, + unsigned int size, void *area, unsigned int mode, int sequence) { kcov_debug("t =3D %px, size =3D %u, area =3D %px\n", t, size, area); @@ -577,6 +604,8 @@ static int kcov_get_mode(unsigned long arg) #else return -ENOTSUPP; #endif + else if (arg =3D=3D KCOV_TRACE_PC_EXT) + return IS_ENABLED(CONFIG_KCOV_EXT_RECORDS) ? KCOV_MODE_TRACE_PC_EXT : -E= NOTSUPP; else return -EINVAL; } @@ -1089,7 +1118,7 @@ void kcov_remote_stop(void) * and kcov_remote_stop(), hence the sequence check. */ if (sequence =3D=3D kcov->sequence && kcov->remote) - kcov_move_area(kcov->mode, kcov->area, kcov->size, area); + kcov_move_area(kcov->mode & ~KCOV_EXT_FORMAT, kcov->area, kcov->size, ar= ea); spin_unlock(&kcov->lock); =20 if (in_task()) { --=20 2.53.0.851.ga537e3e6e9-goog