Re: [PATCH 1/1] fs: strncmp() for user space buffers

[Al Viro <viro@ZenIV.linux.org.uk>]

On Sun, Feb 28, 2016 at 03:39:08PM -0800, Amber Thrall wrote:
> Apologies for the confusing name, struggled to find an appropriate name
> while staying consistent with the naming schemes of
> simple_read/write_to_buffer() functions, as it based off of them. I'd
> love to hear alternative names.
> 
> I saw possible uses for this proposed function being an easy way to
> interact with debugfs, via their write file operation. For
> example in the function xenvif_write_io_ring() the string "kick" is
> checked for against a user space buffer.

TBH, that caller leaves an impression of rather... poor API - "any write
of no more than 32 bytes that starts with 'k' 'i' 'c' 'k' is OK (and
everything beyond first 4 characters is ignored), anything else is
rejected, in some cases with whining into syslog, in some - quietly".
I don't know if encouraging stuff like that is a good idea...

In any case, you've ended up open-coding kmemdup_user() + strncmp() + kfree();
the problem with combining those into a single helper is that calling
conventions will be very error-prone - you have zero/positive/negative for
passing strncmp() result *and* you need to report errors somehow.