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SubjectRe: [PATCH 4/4][PoC][RFC] Allow to trace fd usage with rlimit-events
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Hi,

On 10/19/2017 01:05 AM, Al Viro wrote:
> On Wed, Oct 18, 2017 at 10:32:30PM +0200, Krzysztof Opasiak wrote:
>
>> @@ -417,7 +417,7 @@ static int task_get_unused_fd_flags(struct binder_proc *proc, int flags)
>> rlim_cur = task_rlimit(proc->tsk, RLIMIT_NOFILE);
>> unlock_task_sighand(proc->tsk, &irqs);
>>
>> - return __alloc_fd(files, 0, rlim_cur, flags);
>> + return __alloc_fd(proc->tsk, 0, rlim_cur, flags);
>
> Who said that proc->files will remain equal to proc->tsk->files?
>
>> -static void __put_unused_fd(struct files_struct *files, unsigned int fd)
>> +static void __put_unused_fd(struct task_struct *owner, unsigned int fd)
>> {
>> + struct files_struct *files = owner->files;
>> struct fdtable *fdt = files_fdtable(files);
>> __clear_open_fd(fd, fdt);
>> if (fd < files->next_fd)
>> files->next_fd = fd;
>> +
>> + if (rlimit_noti_watch_active(owner, RLIMIT_NOFILE)) {
>> + unsigned int count;
>> +
>> + count = count_open_fds(fdt);
>> + rlimit_noti_res_changed(owner, RLIMIT_NOFILE, count + 1, count);
>> + }
>> }
>
> [... and similar for other __...fd() primitives]
> This is blatantly wrong - you *CAN'T* modify files_struct unless it's
> a) yours (i.e. current->files) or
> b) you've had its refcount incremented for you by some process that
> did, at the time, have current->files pointing to it.
>
> There is a reason why binder keeps ->files explicitly, rather than going through
> ->tsk->files.

Your are perfectly right! Thank you very much for catching this.

To be honest, initially I just added the struct task_struct ptr to the
argument list keeping that in mind. Then when I was cleaning up patches
before sending I found this to look a litlle bit odd and forgot that I
did this on purpose because tsk->files can be reassigned and that's why
I removed the files param.

I'll fix this for v2. Thanks once again.

Best regards,
--
Krzysztof Opasiak
Samsung R&D Institute Poland
Samsung Electronics

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