linux-kernel - Re: [PATCH 3/9] procfs: add proc_read_from_buffer() and pid_entry

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Message-ID: <CALCETrV12jeZagHBjFnEvJo9wu=sengNa0s2feK8Z-2mjqVWwg@mail.gmail.com>
Date:	Mon, 26 May 2014 10:59:10 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Djalal Harouni <tixxdz@...ndz.org>
Cc:	Kees Cook <keescook@...omium.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Alexey Dobriyan <adobriyan@...il.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Linux FS Devel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH 3/9] procfs: add proc_read_from_buffer() and
 pid_entry_read() helpers

On Mon, May 26, 2014 at 10:41 AM, Djalal Harouni <tixxdz@...ndz.org> wrote:
> On Mon, May 26, 2014 at 10:01:20AM -0700, Andy Lutomirski wrote:
>> On Mon, May 26, 2014 at 6:27 AM, Djalal Harouni <tixxdz@...ndz.org> wrote:
>> > This patch is preparation, it adds a couple of helpers to read data and
>> > to get the cached permission checks during that ->read().
>> >
>> > Currently INF entries share the same code, they do not implement
>> > specific ->open(), only ->read() coupled with callback calls. Doing
>> > permission checks during ->open() will not work and will only disturb
>> > the INF entries that do not need permission checks. Yes not all the INF
>> > entries need checks, the ones that need protections are listed below:
>> > /proc/<pid>/wchan
>> > /proc/<pid>/syscall
>> > /proc/<pid>/{auxv,io}  (will be handled in next patches).
>> >
>> > So to have proper permission checks convert this INF entries to REG
>> > entries and use their open() and read() handlers to implement these
>> > checks. To achieve this we add the following helpers:
>> >
>> > * proc_read_from_buffer() a wrapper around simple_read_from_buffer(), it
>> >   makes sure that count <= PROC_BLOCK_SIZE (3*1024)
>> >
>> > * pid_entry_read(): it will get a free page and pass it to the specific
>> >   /proc/<pid>/$entry handler (callback). The handler is of the same
>> >   types of the previous INF handlers, it will only receive an extra
>> >   "permitted" argument that contains the cached permission check that
>> >   was performed during ->open().
>> >
>> >   The handler is of type:
>> >   typedef int (*proc_read_fn_t)(char *page,
>> >                                struct task_struct *task, int permitted);
>>
>> [...]
>>
>> This strikes me as *way* too complicated.  Why not just change
>> proc_info_file_operations to *always* cache may_ptrace on open?
> Not all the INF entries need permission checks during open:
> The one that need it:
> /proc/<pid>/{wchan|syscall}  converted in this series.
> /proc/<pid>/{auxv|io}  (will be converted in next series)
>
> The ones that do not need it:
> /proc/<pid>/{limite|cmdline|shedstat|oom_score|...}
>
> There is no reason to do it for these entries, and if you do it you will
> also have to passe the cached permission checks, so I don't think that
> you want to modify all the INF handlers especially the ones that do not
> need it to take an extra argument.

...but you don't have to pass the checks, because nothing will care
whether you passed them.

> Then you will also modify this:
> in file fs/proc/internal.h the:
> union proc_op {
>         ...
>         int (*proc_read)(struct task_struct *task, char *page);
>         ...
>  }
>
> And then you will follow in all other places and handlers... and modify
> the definition of INF entries... It's much more complicated...
>

But the final result will be much less messy and have less duplicated
code.  Your patch seems like it duplicates a bunch of code and I
suspect that Al Viro will dislike it if he reads it.

--Andy
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