linux-kernel - Re: Possible Memory Leak in KCOV Linux 4.15-rc1

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Date:   Mon, 22 Jan 2018 09:29:58 +0100
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Shankara Pailoor <sp3485@...umbia.edu>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        syzkaller <syzkaller@...glegroups.com>,
        Andrew Zhu Aday <andrew.aday@...umbia.edu>
Subject: Re: Possible Memory Leak in KCOV Linux 4.15-rc1

On Sun, Jan 21, 2018 at 6:54 PM, Shankara Pailoor <sp3485@...umbia.edu> wrote:
> Below is a reproducer.
>
> #define _GNU_SOURCE
> #include <fcntl.h>
> #include <sys/ioctl.h>
> #include <sys/mman.h>
> #include <stdlib.h>
> #include <unistd.h>
>
> #define KCOV_INIT_TRACE _IOR('c', 1, unsigned long)
> #define KCOV_ENABLE _IO('c', 100)
> #define KCOV_DISABLE _IO('c', 101)
> #define COVER_SIZE (16 << 20)
>
>
> void kcov_setup() {
>     unsigned long *cover;
>     int fd;
>
>     fd = open("/sys/kernel/debug/kcov", O_RDWR);
>     if (fd == -1)
>         perror("open"), exit(1);
>     if (ioctl(fd, KCOV_INIT_TRACE, COVER_SIZE))
>         perror("ioctl"), exit(1);
>     cover = (unsigned long*)mmap(NULL,
>                                 COVER_SIZE * sizeof(unsigned long),
>                                 PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
>     if ((void*)cover == MAP_FAILED)
>         perror("mmap"), exit(1);
>     if (ioctl(fd, KCOV_ENABLE, 0))
>         perror("ioctl"), exit(1);
>
> }
>
> void main() {
>     int i;
>     for (i = 0; i < 4; i++)
>         kcov_setup();
>     sleep(10);
> }


Thanks!

I've just mailed a fix for this:
https://groups.google.com/d/msg/syzkaller/Hclbq5Elfs8/J5V_FnC4DgAJ

With that patch the second KCOV_ENABLE return EBUSY.
To make it clear, it won't fix your code per se, because it's a misuse
of the API. You need to either call KCOV_DISABLE first, or reuse the
first kcov descriptor and mapping, you can just write 0 to the first
work of the mapping to reset current coverage (which should be much
faster).