linux-kernel - Re: [PATCH] lib/lzo: Avoid output overruns when compressing

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Message-ID: <bnvklfvsoh34663ttsboec6aidxmu2cib32okb4vyi2iitdqme@lbvte54od7gi>
Date: Fri, 28 Feb 2025 22:55:35 +0900
From: Sergey Senozhatsky <senozhatsky@...omium.org>
To: Ard Biesheuvel <ardb@...nel.org>
Cc: Sergey Senozhatsky <senozhatsky@...omium.org>, 
	David Sterba <dsterba@...e.cz>, Herbert Xu <herbert@...dor.apana.org.au>, 
	Linux Crypto Mailing List <linux-crypto@...r.kernel.org>, Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, 
	Nitin Gupta <nitingupta910@...il.com>, Richard Purdie <rpurdie@...nedhand.com>, 
	Andrew Morton <akpm@...ux-foundation.org>, Linus Torvalds <torvalds@...ux-foundation.org>, 
	"Markus F.X.J. Oberhumer" <markus@...rhumer.com>, Dave Rodgman <dave.rodgman@....com>
Subject: Re: [PATCH] lib/lzo: Avoid output overruns when compressing

On (25/02/28 13:43), Ard Biesheuvel wrote:
> On Fri, 28 Feb 2025 at 06:24, Sergey Senozhatsky
> <senozhatsky@...omium.org> wrote:
> >
> > On (25/02/26 14:00), David Sterba wrote:
> > > What strikes me as alarming that you insert about 20 branches into a
> > > realtime compression algorithm, where everything is basically a hot
> > > path.  Branches that almost never happen, and never if the output buffer
> > > is big enough.
> > >
> > > Please drop the patch.
> >
> > David, just for educational purposes, there's only safe variant of lzo
> > decompression, which seems to be doing a lot of NEED_OP (HAVE_OP) adding
> > branches and so on, basically what Herbert is adding to the compression
> > path.  So my question is - why NEED_OP (if (!HAVE_OP(x)) goto output_overrun)
> > is a no go for compression, but appears to be fine for decompression?
> >
> 
> Because compression has a bounded worst case (compressing data with
> LZO can actually increase the size but only by a limited amount),
> whereas decompressing a small input could produce gigabytes of output.

One can argue that sometimes we know the upper bound.  E.g. zswap
and zram always compress physical pages, so decompression cannot
result in a bigger (than the original physical page) data.