linux-kernel - [PATCH 1/2] kcov: size of arena is now given in bytes.

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Date:   Wed, 16 Nov 2016 22:27:45 +0100
From:   Quentin Casasnovas <quentin.casasnovas@...cle.com>
To:     Dmitry Vyukov <dvyukov@...gle.com>
Cc:     linux-kernel@...r.kernel.org,
        Vegard Nossum <vegard.nossum@...cle.com>,
        Michal Zalewski <lcamtuf@...il.com>,
        Kees Cook <keescook@...omium.org>,
        syzkaller <syzkaller@...glegroups.com>,
        Quentin Casasnovas <quentin.casasnovas@...cle.com>
Subject: [PATCH 1/2] kcov: size of arena is now given in bytes.

We'll introduce a different mode of tracing a-la AFL fixed table and Dmitry
suggests that the code would be simpler with the size expressed in bytes as
opposed unsigned longs.

We only change the kcov::size field, which will be shared between different
modes, but leave the task_struct::kcov_size field expressed in unsigned
long in order to save an unecessary bitshift/division in the hot path when
using KCOV_MODE_TRACE.

Cc: Dmitry Vyukov <dvyukov@...gle.com>
Cc: Michal Zalewski <lcamtuf@...il.com>
Cc: Kees Cook <keescook@...omium.org>
Signed-off-by: Quentin Casasnovas <quentin.casasnovas@...cle.com>
Signed-off-by: Vegard Nossum <vegard.nossum@...cle.com>
---
 kernel/kcov.c | 10 ++++------
 1 file changed, 4 insertions(+), 6 deletions(-)

diff --git a/kernel/kcov.c b/kernel/kcov.c
index 30e6d05aa5a9..c2aa93851f93 100644
--- a/kernel/kcov.c
+++ b/kernel/kcov.c
@@ -32,7 +32,7 @@ struct kcov {
 	/* The lock protects mode, size, area and t. */
 	spinlock_t		lock;
 	enum kcov_mode		mode;
-	/* Size of arena (in long's for KCOV_MODE_TRACE). */
+	/* Size of arena in bytes. */
 	unsigned		size;
 	/* Coverage buffer shared with user space. */
 	void			*area;
@@ -140,7 +140,7 @@ static int kcov_mmap(struct file *filep, struct vm_area_struct *vma)
 		return -ENOMEM;
 
 	spin_lock(&kcov->lock);
-	size = kcov->size * sizeof(unsigned long);
+	size = kcov->size;
 	if (kcov->mode == KCOV_MODE_DISABLED || vma->vm_pgoff != 0 ||
 	    vma->vm_end - vma->vm_start != size) {
 		res = -EINVAL;
@@ -198,13 +198,11 @@ static int kcov_ioctl_locked(struct kcov *kcov, unsigned int cmd,
 			return -EBUSY;
 		/*
 		 * Size must be at least 2 to hold current position and one PC.
-		 * Later we allocate size * sizeof(unsigned long) memory,
-		 * that must not overflow.
 		 */
 		size = arg;
 		if (size < 2 || size > INT_MAX / sizeof(unsigned long))
 			return -EINVAL;
-		kcov->size = size;
+		kcov->size = size * sizeof(unsigned long);
 		kcov->mode = KCOV_MODE_TRACE;
 		return 0;
 	case KCOV_ENABLE:
@@ -223,7 +221,7 @@ static int kcov_ioctl_locked(struct kcov *kcov, unsigned int cmd,
 			return -EBUSY;
 		t = current;
 		/* Cache in task struct for performance. */
-		t->kcov_size = kcov->size;
+		t->kcov_size = kcov->size / sizeof(unsigned long);
 		t->kcov_area = kcov->area;
 		/* See comment in __sanitizer_cov_trace_pc(). */
 		barrier();
-- 
2.10.2