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SubjectRe: [RFC] LZO de/compression support - take 3
Hi Satyam,

Thanks for you comments.

On 5/24/07, Satyam Sharma <satyam.sharma@gmail.com> wrote:
> On 5/23/07, Nitin Gupta <nitingupta910@gmail.com> wrote:
> > diff --git a/include/linux/lzo1x.h b/include/linux/lzo1x.h
> > [...]
> > +/* Size of temp buffer (workmem) required by lzo1x_compress */
> > +#define LZO1X_WORKMEM_SIZE ((size_t) (16384L * sizeof(unsigned char *)))
> > +
> > +/*
> > + * This required 'workmem' of size LZO1X_WORKMEM_SIZE
> > + */
> > +int lzo1x_compress(const unsigned char *src, size_t src_len,
> > + unsigned char *dst, size_t *dst_len,
> > + void *workmem);
>
> Just defining and exporting LZO1X_WORKMEM_SIZE may not be enough
> to guarantee that users _will_ pass in workmem of size exactly that much.
>
> If this workmem is really merely a temp buffer required by lzo1x_compress(),
> I'd suggest you rename lzo1x_compress() in lzo1x_compress.c to
> __lzo1x_compress(), and implement a lzo1x_compress() wrapper for the
> user that handles the allocation (and subsequent free'ing) of this temp
> buffer itself.
>

I did not include such wrapper since allocation method will depend on
particular scenario (like kmalloc vs. vmalloc and flags GFP_KERNEL vs
GFP_ATOMIC etc...). But still maybe we can have "default" wrapper that
does, say vmalloc(GFP_KERNEL)/vfree and let others with specific
requirement have their own wrappers.

> [ I vaguely remember discussing something of this sort with Richard
> when he had submitted his patchset too, perhaps you can look into
> his implementation to see how he's managing this ... ]
>

ok.

> > +/*
> > + * This decompressor expects valid compressed data.
> > + *
> > + * If the compressed data gets corrupted somehow (e.g. transmission
> > + * via an erroneous channel, disk errors, ...) it will probably crash
> > + * your application because absolutely no additional checks are done.
> ^^^^^^^^^^^
>
> Whoa! "your application" here is _kernel code_ and not a userspace
> program ... "crashing" it is something we could do without :-)
>

Firstly, will change s/your application/kernel. "crash" also seems a
bit vague in this sense...

> > + */
> > +int lzo1x_decompress(const unsigned char *src, size_t src_len,
> > + unsigned char *dst, size_t *dst_len);
> > +
> > +
> > +/*
> > + * The `safe' decompressor. Somewhat slower.
> > + *
> > + * This decompressor will catch all compressed data violations and
> > + * return an error code in this case.
> > + */
> > +int lzo1x_decompress_safe(const unsigned char *src, size_t src_len,
> > + unsigned char *dst, size_t *dst_len);
> > +#endif
>
> I just read the follow-ups to this, so perhaps we /can/ use the unsafe
> versions in certain situations. But I agree with Michael's suggestion
> to rename _safe to decompress and decompress to _unsafe ...

Ok. I will do this.

>
> > diff --git a/lib/lzo1x/Makefile b/lib/lzo1x/Makefile
> > [...]
> > +#
> > +# When compiling this module out of tree, do 'make prepare_lzo'
> > +# before compiling as usual
> > +#
> > +obj-$(CONFIG_LZO1X) += lzo1x.o
> > +CFLAGS_lzo1x_decompress_safe.o += -DLZO1X_DECOMPRESS_SAFE
> > +lzo1x-objs := lzo1x_compress.o lzo1x_decompress.o lzo1x_decompress_safe.o
> > +
> > +prepare_lzo:
> > + @ln -sf lzo1x_decompress.c lzo1x_decompress_safe.c
> > +
>
> ... ah, so that's why the master Makefile changes.
>
> Hmmm, perhaps you could extract the common stuff between the
> _safe and _unsafe versions out into a separate function and then
> reuse it from _safe and _unsafe wrappers? In any case, this kind
> of Makefile jugglery (even in the master Makefile) just to avoid the
> above doesn't seem quite right ...

Ok. I will look into this.

>
> > diff --git a/lib/lzo1x/lzo1x_decompress.c b/lib/lzo1x/lzo1x_decompress.c
> > [...]
> > +#if defined(LZO1X_DECOMPRESS_SAFE)
> > +input_overrun:
> > + *out_len = (size_t)(op - out);
> > + return LZO_E_INPUT_OVERRUN;
> > +
> > +output_overrun:
> > + *out_len = (size_t)(op - out);
> > + return LZO_E_OUTPUT_OVERRUN;
> > +
> > +lookbehind_overrun:
> > + *out_len = (size_t)(op - out);
> > + return LZO_E_LOOKBEHIND_OVERRUN;
> > +#endif
> > +}
> > +
> > +EXPORT_SYMBOL(lzo1x_decompress);
>
> Ok, so this is all there is between _safe and _unsafe, it seems ...
>

No. The code has macros like NEED_IP, NEED_OP etc. at many places
which are no-op for 'unsafe' version.

> > diff --git a/lib/lzo1x/lzo1x_int.h b/lib/lzo1x/lzo1x_int.h
> > [...]
> > +/* Macros for 'safe' decompression */
> > +#ifdef LZO1X_DECOMPRESS_SAFE
> > +
> > +#define lzo1x_decompress lzo1x_decompress_safe
> > +#define TEST_IP (ip < ip_end)
> > +#define NEED_IP(x) \
> > + if ((size_t)(ip_end - ip) < (size_t)(x)) goto input_overrun
> > +#define NEED_OP(x) \
> > + if ((size_t)(op_end - op) < (size_t)(x)) goto output_overrun
> > +#define TEST_LB(m_pos) \
> > + if (m_pos < out || m_pos >= op) goto lookbehind_overrun
> > +#define HAVE_TEST_IP
> > +#define HAVE_ANY_OP
> > +
> > +#else /* !LZO1X_DECOMPRESS_SAFE */
> > +
> > +#define TEST_IP 1
> > +#define TEST_LB(x) ((void) 0)
> > +#define NEED_IP(x) ((void) 0)
> > +#define NEED_OP(x) ((void) 0)
> > +#undef HAVE_TEST_IP
> > +#undef HAVE_ANY_OP
> > +
> > +#endif /* LZO1X_DECOMPRESS_SAFE */
>
> ... ugh. Yes, extracting the common stuff between the _safe and _unsafe
> variants in a common low-level __lzo1x_decompress kind of function
> definitely looks doable. The low-level function could simply take an extra
> argument (say, set by the _safe and _unsafe wrappers) that tells it
> whether it is being called as safe or unsafe ... helps us get rid of the
> disruptions to all the Makefiles above and these #ifdef's ugliness ..

Hmm..I will try to get this done.

>
> BTW, it'd be really cool if Richard and yourself could get together and
> pool your energies / efforts to develop a common / same patchset for this.
> (I wonder how different your implementations are, actually, and if there
> are any significant performance disparities, especially.) I really like your
> work, as it clears up the major gripe I had with Richard's patchset -- the
> ugliness (and monstrosity) of it.

I am really looking forward to co-operating with Richard regarding
this - although our approach for this porting is quite different but I
hope we can get around this. Duplication sucks! :)

> But he's also worked up the glue code for
> cryptoapi / jffs2 etc for this, so no point duplicating his efforts.
>
> Satyam
>

Sure. I am not going to duplicate these.

Cheers,
Nitin
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