linux-kernel.vger.kernel.org archive mirror
 help / color / mirror / Atom feed
From: Andrew Morton <akpm@linux-foundation.org>
To: Randy Dunlap <rdunlap@infradead.org>
Cc: linux-kernel@vger.kernel.org, "Jens Axboe" <axboe@kernel.dk>,
	"Toralf Förster" <toralf.foerster@gmx.de>,
	linux-mm@kvack.org
Subject: Re: [RFC PATCH 2/2] mm: readahead: handle LARGE input to get_init_ra_size()
Date: Tue, 22 Dec 2020 17:35:33 -0800	[thread overview]
Message-ID: <20201222173533.c9e28416835d7487b0e28cda@linux-foundation.org> (raw)
In-Reply-To: <20201220211051.1416-1-rdunlap@infradead.org>

On Sun, 20 Dec 2020 13:10:51 -0800 Randy Dunlap <rdunlap@infradead.org> wrote:

> Add a test to detect if the input ra request size has its high order
> bit set (is negative when tested as a signed long). This would be a
> really Huge readahead.
> 
> If so, WARN() with the value and a stack trace so that we can see
> where this is happening and then make further corrections later.
> Then adjust the size value so that it is not so Huge (although
> this may not be needed).

What motivates this change?  Is there any reason to think this can
happen?

Also, everything in there *should* be unsigned, because a negative
readahead is semantically nonsensical.  Is our handling of this
inherently unsigned quantity incorrect somewhere?

> --- linux-5.10.1.orig/mm/readahead.c
> +++ linux-5.10.1/mm/readahead.c
> 
> ...
>
> @@ -303,14 +304,21 @@ void force_page_cache_ra(struct readahea
>  }
>  
>  /*
> - * Set the initial window size, round to next power of 2 and square
> + * Set the initial window size, round to next power of 2
>   * for small size, x 4 for medium, and x 2 for large
>   * for 128k (32 page) max ra
>   * 1-8 page = 32k initial, > 8 page = 128k initial
>   */
>  static unsigned long get_init_ra_size(unsigned long size, unsigned long max)
>  {
> -	unsigned long newsize = roundup_pow_of_two(size);
> +	unsigned long newsize;
> +
> +	if ((signed long)size < 0) { /* high bit is set: ultra-large ra req */
> +		WARN_ONCE(1, "%s: size=0x%lx\n", __func__, size);
> +		size = -size;	/* really only need to flip the high/sign bit */
> +	}
> +
> +	newsize = roundup_pow_of_two(size);

Is there any way in which userspace can deliberately trigger warning?
Via sys_readadhead() or procfs tuning or whatever?

I guess that permitting a user-triggerable WARN_ONCE() isn't a huuuuge
problem - it isn't a DoS if it only triggers a single time.  It does
permit the malicious user to disable future valid warnings, but I don't
see what incentive there would be for this.  But still, it seems
desirable to avoid it.


  reply	other threads:[~2020-12-23  1:36 UTC|newest]

Thread overview: 7+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-12-20 21:10 Randy Dunlap
2020-12-23  1:35 ` Andrew Morton [this message]
2020-12-23  1:50   ` Randy Dunlap
2020-12-29 18:01     ` Toralf Förster
2020-12-29 18:11       ` Randy Dunlap
2020-12-29 20:00         ` Randy Dunlap
2020-12-29 20:46           ` Toralf Förster

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20201222173533.c9e28416835d7487b0e28cda@linux-foundation.org \
    --to=akpm@linux-foundation.org \
    --cc=axboe@kernel.dk \
    --cc=linux-kernel@vger.kernel.org \
    --cc=linux-mm@kvack.org \
    --cc=rdunlap@infradead.org \
    --cc=toralf.foerster@gmx.de \
    --subject='Re: [RFC PATCH 2/2] mm: readahead: handle LARGE input to get_init_ra_size()' \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).