linux-kernel - Re: [PATCH 11/13] proc: readdir /proc/*/task

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Date:   Tue, 28 Aug 2018 12:36:22 +0000
From:   "Ahmed S. Darwish" <darwish.07@...il.com>
To:     Alexey Dobriyan <adobriyan@...il.com>
Cc:     akpm@...ux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 11/13] proc: readdir /proc/*/task

On Tue, Aug 28, 2018 at 02:15:01AM +0300, Alexey Dobriyan wrote:
> ---
>  fs/proc/base.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
>

Missing description and S-o-b. Further comments below..

> diff --git a/fs/proc/base.c b/fs/proc/base.c
> index 33f444721965..668e465c86b3 100644
> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
> @@ -3549,11 +3549,11 @@ static int proc_task_readdir(struct file *file, struct dir_context *ctx)
>  	for (task = first_tid(proc_pid(inode), tid, ctx->pos - 2, ns);
>  	     task;
>  	     task = next_tid(task), ctx->pos++) {
> -		char name[10 + 1];
> -		unsigned int len;
> +		char name[10], *p = name + sizeof(name);
> +

Multiple issues:

- len should be 11, as was in the original code
  (0xffffffff = 4294967295, 10 letters)

- while we're at it, let's use a constant for the '11' instead of
  mysterious magic numbers

- 'p' is clearly overflowing the stack here

>  		tid = task_pid_nr_ns(task, ns);
> -		len = snprintf(name, sizeof(name), "%u", tid);
> -		if (!proc_fill_cache(file, ctx, name, len,
> +		p = _print_integer_u32(p, tid);
> +		if (!proc_fill_cache(file, ctx, p, name + sizeof(name) - p,

You're replacing snprintf() code __that did proper len checking__
with code that does not. That's not good.

I can't see how the fourth proc_fill_cache() parameter, ``name +
sizeof(name)'' safely ever replace the original 'len' parameter.
It's a pointer value .. (!)

Overall this looks like a broken patch submitted by mistake.

Thanks,

-- 
Darwish
http://darwish.chasingpointers.com