On Thu, Dec 03, 2020 at 01:42:03AM +0000, Nick Terrell wrote:
>
>
> > On Dec 2, 2020, at 5:16 PM, Michał Mirosław <mirq-linux@xxxxxxxxxxxx> wrote:
> >
> > On Wed, Dec 02, 2020 at 12:32:40PM -0800, Nick Terrell wrote:
> >> From: Nick Terrell <terrelln@xxxxxx>
> >>
> >> This patch:
> >> - Moves `include/linux/zstd.h` -> `lib/zstd/zstd.h`
> >> - Adds a new API in `include/linux/zstd.h` that is functionally
> >> equivalent to the in-use subset of the current API. Functions are
> >> renamed to avoid symbol collisions with zstd, to make it clear it is
> >> not the upstream zstd API, and to follow the kernel style guide.
> >> - Updates all callers to use the new API.
> >>
> >> There are no functional changes in this patch. Since there are no
> >> functional change, I felt it was okay to update all the callers in a
> >> single patch, since once the API is approved, the callers are
> >> mechanically changed.
> > [...]
> >> --- a/lib/decompress_unzstd.c
> >> +++ b/lib/decompress_unzstd.c
> > [...]
> >> static int INIT handle_zstd_error(size_t ret, void (*error)(char *x))
> >> {
> >> - const int err = ZSTD_getErrorCode(ret);
> >> -
> >> - if (!ZSTD_isError(ret))
> >> + if (!zstd_is_error(ret))
> >> return 0;
> >>
> >> - switch (err) {
> >> - case ZSTD_error_memory_allocation:
> >> - error("ZSTD decompressor ran out of memory");
> >> - break;
> >> - case ZSTD_error_prefix_unknown:
> >> - error("Input is not in the ZSTD format (wrong magic bytes)");
> >> - break;
> >> - case ZSTD_error_dstSize_tooSmall:
> >> - case ZSTD_error_corruption_detected:
> >> - case ZSTD_error_checksum_wrong:
> >> - error("ZSTD-compressed data is corrupt");
> >> - break;
> >> - default:
> >> - error("ZSTD-compressed data is probably corrupt");
> >> - break;
> >> - }
> >> + error("ZSTD decompression failed");
> >> return -1;
> >> }
> >
> > This looses diagnostics specificity - is this intended? At least the
> > out-of-memory condition seems useful to distinguish.
>
> Good point. The zstd API no longer exposes the error code enum,
> but it does expose zstd_get_error_name() which can be used here.
> I was thinking that the string needed to be static for some reason, but
> that is not the case. I will make that change.
>
> >> +size_t zstd_compress_stream(zstd_cstream *cstream,
> >> + struct zstd_out_buffer *output, struct zstd_in_buffer *input)
> >> +{
> >> + ZSTD_outBuffer o;
> >> + ZSTD_inBuffer i;
> >> + size_t ret;
> >> +
> >> + memcpy(&o, output, sizeof(o));
> >> + memcpy(&i, input, sizeof(i));
> >> + ret = ZSTD_compressStream(cstream, &o, &i);
> >> + memcpy(output, &o, sizeof(o));
> >> + memcpy(input, &i, sizeof(i));
> >> + return ret;
> >> +}
> >
> > Is all this copying necessary? How is it different from type-punning by
> > direct pointer cast?
>
> If breaking strict aliasing and type-punning by pointer casing is okay, then
> we can do that here. These memcpys will be negligible for performance, but
> type-punning would be more succinct if allowed.
Ah, this might break LTO builds due to strict aliasing violation.
So I would suggest to just #define the ZSTD names to kernel ones
for the library code. Unless there is a cleaner solution...
Best Regards
Michał Mirosław
