linux-kernel - Re: [PATCH -mm 3/5] fault-inject: make fail-nth read/write interface symmetric

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Message-ID: <CACT4Y+YhP+FhfWebRSh4W+3inif7VFrgWf4xZY+F6r8805UQfQ@mail.gmail.com>
Date:   Fri, 7 Apr 2017 22:37:01 +0200
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Akinobu Mita <akinobu.mita@...il.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH -mm 3/5] fault-inject: make fail-nth read/write interface symmetric

On Thu, Apr 6, 2017 at 4:55 PM, Akinobu Mita <akinobu.mita@...il.com> wrote:
> The read interface for fail-nth looks a bit odd.  Read from this file
> returns "NYYYY..." or "YYYYY..." (this makes me surprise when cat this
> file).  Because there is no EOF condition. The first character indicates
> current->fail_nth is zero or not, and then current->fail_nth is reset
> to zero.
>
> Just returning task->fail_nth value is more natural to understand.
>
> Cc: Dmitry Vyukov <dvyukov@...gle.com>
> Signed-off-by: Akinobu Mita <akinobu.mita@...il.com>
> ---
>  Documentation/fault-injection/fault-injection.txt | 13 +++++++------
>  fs/proc/base.c                                    | 14 ++++++--------
>  2 files changed, 13 insertions(+), 14 deletions(-)
>
> diff --git a/Documentation/fault-injection/fault-injection.txt b/Documentation/fault-injection/fault-injection.txt
> index a321905..370ddcb 100644
> --- a/Documentation/fault-injection/fault-injection.txt
> +++ b/Documentation/fault-injection/fault-injection.txt
> @@ -139,9 +139,9 @@ o proc entries
>  - /proc/self/task/<current-tid>/fail-nth:
>
>         Write to this file of integer N makes N-th call in the task fail.
> -       Read from this file returns a single char 'Y' or 'N'
> -       that says if the fault setup with a previous write to this file was
> -       injected or not, and disables the fault if it wasn't yet injected.
> +       Read from this file returns a integer value. A value of '0' indicates
> +       that the fault setup with a previous write to this file was injected.
> +       A positive integer N indicates that the fault wasn't yet injected.
>         Note that this file enables all types of faults (slab, futex, etc).
>         This setting takes precedence over all other generic debugfs settings
>         like probability, interval, times, etc. But per-capability settings
> @@ -325,13 +325,14 @@ int main()
>                 write(fail_nth, buf, strlen(buf));
>                 res = socketpair(AF_LOCAL, SOCK_STREAM, 0, fds);
>                 err = errno;
> -               read(fail_nth, buf, 1);
> +               pread(fail_nth, buf, sizeof(buf), 0);
>                 if (res == 0) {
>                         close(fds[0]);
>                         close(fds[1]);
>                 }
> -               printf("%d-th fault %c: res=%d/%d\n", i, buf[0], res, err);
> -               if (buf[0] != 'Y')
> +               printf("%d-th fault %c: res=%d/%d\n", i, atoi(buf) ? 'N' : 'Y',
> +                       res, err);
> +               if (atoi(buf))
>                         break;
>         }
>         return 0;
> diff --git a/fs/proc/base.c b/fs/proc/base.c
> index 42c52e2..9d14215 100644
> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
> @@ -1383,7 +1383,8 @@ static ssize_t proc_fail_nth_read(struct file *file, char __user *buf,
>                                   size_t count, loff_t *ppos)
>  {
>         struct task_struct *task;
> -       int err;
> +       char numbuf[PROC_NUMBUF];
> +       ssize_t len;
>
>         task = get_proc_task(file_inode(file));
>         if (!task)
> @@ -1391,13 +1392,10 @@ static ssize_t proc_fail_nth_read(struct file *file, char __user *buf,
>         put_task_struct(task);
>         if (task != current)
>                 return -EPERM;
> -       if (count < 1)
> -               return -EINVAL;
> -       err = put_user((char)(current->fail_nth ? 'N' : 'Y'), buf);
> -       if (err)
> -               return err;
> -       current->fail_nth = 0;
> -       return 1;
> +       len = snprintf(numbuf, sizeof(numbuf), "%u\n", task->fail_nth);

If we allow setting this for non current task, then we need to prevent
data races as the task uses task->fail_nth concurrently. Reads then
should use READ_ONCE and writes in fault-inject.c should use
WRITE_ONCE.

> +       len = simple_read_from_buffer(buf, count, ppos, numbuf, len);
> +
> +       return len;
>  }
>
>  static const struct file_operations proc_fail_nth_operations = {
> --
> 2.7.4
>