linux-kernel - Re: [PATCH v2] seq_file: Unconditionally use vmalloc for buffer

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Message-ID: <YFCn4ERBMGoqxvUU@zeniv-ca.linux.org.uk>
Date:   Tue, 16 Mar 2021 12:43:12 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Kees Cook <keescook@...omium.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Lee Duncan <lduncan@...e.com>, Chris Leech <cleech@...hat.com>,
        Adam Nichols <adam@...mm-co.com>, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH v2] seq_file: Unconditionally use vmalloc for buffer

On Tue, Mar 16, 2021 at 08:24:50AM +0100, Greg Kroah-Hartman wrote:

> > Completely agreed. seq_get_buf() should be totally ripped out.
> > Unfortunately, this is going to be a long road because of sysfs's ATTR
> > stuff, there are something like 5000 callers, and the entire API was
> > designed to avoid refactoring all those callers from
> > sysfs_kf_seq_show().
> 
> What is wrong with the sysfs ATTR stuff?  That should make it so that we
> do not have to change any caller for any specific change like this, why
> can't sysfs or kernfs handle it automatically?

Hard to tell, since that would require _finding_ the sodding ->show()
instances first.  Good luck with that, seeing that most of those appear
to come from templates-done-with-cpp...

AFAICS, Kees wants to protect against ->show() instances stomping beyond
the page size.  What I don't get is what do you get from using seq_file
if you insist on doing raw access to the buffer rather than using
seq_printf() and friends.  What's the point?