Messages in this thread | | | From | Akinobu Mita <> | Date | Sat, 8 Apr 2017 17:25:41 +0900 | Subject | Re: [PATCH -mm 4/5] fault-inject: simplify access check for fail-nth |
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2017-04-08 5:45 GMT+09:00 Dmitry Vyukov <dvyukov@google.com>: > On Thu, Apr 6, 2017 at 4:56 PM, Akinobu Mita <akinobu.mita@gmail.com> wrote: >> The fail-nth file is created with 0666 and the access is permitted if >> and only if the task is current. >> >> This file is owned by the currnet user. So we can create it with 0644 >> and allow the owner to write it. This enables to watch the status of >> task->fail_nth from another processes. >> >> Cc: Dmitry Vyukov <dvyukov@google.com> >> Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com> >> --- >> fs/proc/base.c | 22 ++++++++-------------- >> 1 file changed, 8 insertions(+), 14 deletions(-) >> >> diff --git a/fs/proc/base.c b/fs/proc/base.c >> index 9d14215..14e7b73 100644 >> --- a/fs/proc/base.c >> +++ b/fs/proc/base.c >> @@ -1366,16 +1366,16 @@ static ssize_t proc_fail_nth_write(struct file *file, const char __user *buf, >> int err; >> unsigned int n; >> >> + err = kstrtoint_from_user(buf, count, 0, &n); >> + if (err) >> + return err; >> + >> task = get_proc_task(file_inode(file)); >> if (!task) >> return -ESRCH; >> + task->fail_nth = n; >> put_task_struct(task); >> - if (task != current) >> - return -EPERM; >> - err = kstrtouint_from_user(buf, count, 0, &n); >> - if (err) >> - return err; >> - current->fail_nth = n; >> + >> return count; >> } >> >> @@ -1389,11 +1389,9 @@ static ssize_t proc_fail_nth_read(struct file *file, char __user *buf, >> task = get_proc_task(file_inode(file)); >> if (!task) >> return -ESRCH; >> - put_task_struct(task); >> - if (task != current) >> - return -EPERM; >> len = snprintf(numbuf, sizeof(numbuf), "%u\n", task->fail_nth); >> len = simple_read_from_buffer(buf, count, ppos, numbuf, len); >> + put_task_struct(task); >> >> return len; >> } >> @@ -3358,11 +3356,7 @@ static const struct pid_entry tid_base_stuff[] = { >> #endif >> #ifdef CONFIG_FAULT_INJECTION >> REG("make-it-fail", S_IRUGO|S_IWUSR, proc_fault_inject_operations), >> - /* >> - * Operations on the file check that the task is current, >> - * so we create it with 0666 to support testing under unprivileged user. >> - */ >> - REG("fail-nth", 0666, proc_fail_nth_operations), >> + REG("fail-nth", 0644, proc_fail_nth_operations), > > /\/\/\/\/\/\ > > This breaks us. > Under setuid sandbox test threads can't open the file anymore. And we > can't pre-open the files before dropping privs as new threads can be > created afterwards.
Could you provide a working example for this? Because I'm not sure I understand the problem you described here.
If we omit resetting tsk->fail_nth in dup_task_struct(), tsk->fail_nth is inherited from parent to child process. So the parent process can pre-open and set fail-nth file and reset parent's own ->fail_nth after fork by writing 0 to fail-nth file. Does that fix your problem?
> I think the root cause of all these problems (permissions, parsing, > serialization, broken cat, symmetry) is that we are trying to fit a > programmatic API into reads and writes on textual files. We don't need > symmetry, we don't need read+write to reset injection, we don't need > parsing and serialization, it does not make sense to do this of > non-current task, it definitely does not make sense to cat this, etc. > > What do you think of 2 ioctls on /sys/kernel/debug/fail_nth?
I think the misc device is suitable than debugfs file for ioctl only knob. But I prefer read/write interface than ioctl if possible.
>> #endif >> #ifdef CONFIG_TASK_IO_ACCOUNTING >> ONE("io", S_IRUSR, proc_tid_io_accounting), >> -- >> 2.7.4 >>
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