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

[Alexey Dobriyan <adobriyan@gmail.com>]

On Fri, Apr 10, 2015 at 02:01:32PM -0400, Jarod Wilson wrote:
> 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.

Well, it's 8 MB at least because of kmalloc and more when it starts
to vmalloc, so either you increase but keep the limit, or allow
to pin semi-arbitrary amount of kernel memory IF you want to stay
with seqfile. My patch requires just 1 page plus whatever g_u_p
requires.