linux-kernel - Re: [PATCH][v2] proc: use vmalloc for our kernel buffer

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Date:   Thu, 13 Aug 2020 18:31:55 +0100
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Josef Bacik <josef@...icpanda.com>
Cc:     Christoph Hellwig <hch@....de>, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, kernel-team@...com,
        willy@...radead.org
Subject: Re: [PATCH][v2] proc: use vmalloc for our kernel buffer

On Thu, Aug 13, 2020 at 01:19:18PM -0400, Josef Bacik wrote:

> > in sunrpc proc_dodebug() turns into
> > 		left -= snprintf(buffer, left, "0x%04x\n",
					 ^^^^
					 left + 1, that is.

> > 				 *(unsigned int *) table->data);
> > and that's not the only example.
> > 
> 
> We wouldn't even need the extra +1 part, since we're only copying in how
> much the user wants anyway, we could just go ahead and convert this to
> 
> left -= snprintf(buffer, left, "0x%04x\n", *(unsigned int *) table->data);
> 
> and be fine, right?  Or am I misunderstanding what you're looking for?  Thanks,

snprintf() always produces a NUL-terminated string.  And if you are passing 7 as
len, you want 0xf0ad\n to be copied to user.  For that you need 8 passed to
snprintf, and 8-byte buffer given to it.