linux-kernel - Re: [PATCH try #3] proc: fix PAGE_SIZE limit of /proc/$PID/cmdline

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Date:	Fri, 10 Apr 2015 14:01:32 -0400
From:	Jarod Wilson <jarod@...hat.com>
To:	Alexey Dobriyan <adobriyan@...il.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Miklos Szeredi <miklos@...redi.hu>,
	Zefan Li <lizefan@...wei.com>, Oleg Nesterov <oleg@...hat.com>
Subject: Re: [PATCH try #3] proc: fix PAGE_SIZE limit of /proc/$PID/cmdline

On Fri, Apr 10, 2015 at 05:13:29PM +0300, Alexey Dobriyan wrote:
> /proc/$PID/cmdline truncates output at PAGE_SIZE. It is easy to see with
> 
> 	$ cat /proc/self/cmdline $(seq 1037) 2>/dev/null
> 
> However, command line size was never limited to PAGE_SIZE but to 128 KB and
> relatively recently limitation was removed altogether.
> 
> People noticed and are asking questions:
> http://stackoverflow.com/questions/199130/how-do-i-increase-the-proc-pid-cmdline-4096-byte-limit
> 
> seq file interface is not OK, because it kmalloc's for whole output and
> open + read(, 1) + sleep will pin arbitrary amounts of kernel memory.
> To not do that, limit must be imposed which is incompatible with
> arbitrary sized command lines.
> 
> I apologize for hairy code, but this it direct consequence of command line
> layout in memory and hacks to support things like "init [3]".
> 
> The loops are "unrolled" otherwise it is either macros which hide
> control flow or functions with 7-8 arguments with equal line count.

That definitely qualifies as hairy. How big of a problem is it really in
practice if we continued using seq_file though? This only happens when
someone actually accesses /proc/$PID/cmdline, no? And if they're doing
that, they probably want that info, so is it so terrible if memory is held
on to for a bit? We're only talking about a few kB. That said, properly
walking the entire cmdline without having to specify an arbitrary limit
ahead of time does sound slightly more end-user-friendly. I'll give this
patch a spin here.

-- 
Jarod Wilson
jarod@...hat.com

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