[illumos-Developer] Ugh... xdr_{float,double} problems

[Jason King]

On Sun, Jan 30, 2011 at 2:52 PM, Garrett D'Amore <garrett at nexenta.com> wrote:
> General review feedback:
>
> No spacing between # and CPP directives, please.

Fixed.

>
> I'm a bit worried about pragma redefine_extname -- what happens to that
> on gcc or Studio?  Do they both support it?

They should -- it's used extensively throughout the code already
(stdio, sockets, etc. about 150 times), studio definitely does, if gcc
doesn't there would be much bigger problems :)

>
> I'm thinking #error might be better here.... it would force people
> working on alternate processor ports to adapt the code...

In that case, should we just ditch the portable implementation altogether?

>
>        - Garrett
>
> On Sun, 2011-01-30 at 14:27 -0600, Jason King wrote:
>> Yeah, basically I'm a dumbass and shouldn't try to bugfix during weeks
>> where I'm sleep deprived (my apologies to everyone for that).  Dr.
>> Grabowski was right
>>
>> I see the problem, and was able to get access to a S10 (sparc & x86)
>> box to obtain a known good XDR bytestream (of floats and doubles) to
>> test against so the test can be more definitive.
>>
>> I have a webrev of the fix here: http://cr.illumos.org/view/kc6gu36g/
>> The #pragma redefine_extname bit was a suggestion from Gordon (just to
>> help people porting to other platforms out).
>>
>> I really need to run errands now but will be back later to post the
>> test code (short version, I take the known good stream from s10
>> x86/sparc, decode it, compare values, then encode the values to
>> another stream and compare vs. trying to 'guess' the correct values in
>> the previous test), and take care of the rest.
>>
>> On Sat, Jan 29, 2011 at 10:05 PM, Garrett D'Amore <garrett at nexenta.com> wrote:
>> > Any update on this?  I've not had a chance yet to backout your change..
>> > I'm not sure what the right thing to do here is... I'd rather have a
>> > "real" fix.
>> >
>> >        - Garrett
>> >
>> > On Fri, 2011-01-28 at 12:10 -0600, Jason King wrote:
>> >> Ok, however things will still be broken, my fix just breaks (or more
>> >> precisely, breaks everyone's workarounds) faster.  I can probably have
>> >> a true fix later tonight as well.
>> >>
>> >> On Fri, Jan 28, 2011 at 10:58 AM, Garrett D'Amore <garrett at nexenta.com> wrote:
>> >> > Ok, I'll backout the changes.  Can you please file a bug?  I probably won't
>> >> > get around to backing this change out until tonight as I'm traveling and on
>> >> > limited battery power.
>> >> >
>> >> >    - Garrett
>> >> >
>> >> > On 01/28/11 08:48 AM, Jason King wrote:
>> >> >>
>> >> >> After lengthy discussions, and even more digging... it looks like
>> >> >> there's a bit of a very-long standing mess with the C xdr API.
>> >> >>
>> >> >> The problem is, the OTW XDR format for 32&  64 bit doubles is
>> >> >> identical on x86, amd64, sparc, and sparcv9.  In theory, the code
>> >> >> should then just merely copy the values out unchanged.
>> >> >> However, that really happened was on little-endian platforms, the byte
>> >> >> values for fp values were being (incorrectly) byteswapped by the
>> >> >> library.  Instead of fixing the library, it appears the existing
>> >> >> software that actually uses fp with xdr workaround the bug on those
>> >> >> platforms.  Of course the recent putback broke the workaround.
>> >> >>
>> >> >> Essentially:
>> >> >>
>> >> >> 1. code is released with broken xdr_{float,double}
>> >> >> 2. code is written w/ workarounds
>> >> >> 3. code is updated, and __amd64 define is (incorrectly) changed to
>> >> >> amd64.  This causes xdr_{float,double} to revert to a slower, platform
>> >> >> independent version that correctly (minus rouding errors) writes the
>> >> >> value into the stream (without swapping).
>> >> >> 4. My fix correctly writes fp values unchanged to XDR stream, but also
>> >> >> breaks any software with workarounds.  The test cases were written to
>> >> >> the RFC&  man pages (since I couldn't really find anything out there
>> >> >> that uses
>> >> >>
>> >> >> Given the pervasiveness of this, we probably need to do something similar
>> >> >> to:
>> >> >>
>> >> >> 1. Revert the behavior to the traditional broken behavior
>> >> >> 2. Update the man pages to document this behavior
>> >> >> 3. Add a #define to the rpc/xdr.h header file (XDR_FLOAT_NO_SWAP ?) to
>> >> >> allow applications to select the correct behavior when defined (and
>> >> >> document this in the manpage).
>> >> >>
>> >> >> _______________________________________________
>> >> >> Developer mailing list
>> >> >> Developer at lists.illumos.org
>> >> >> http://lists.illumos.org/m/listinfo/developer
>> >> >>
>> >> >
>> >> >
>> >> > _______________________________________________
>> >> > Developer mailing list
>> >> > Developer at lists.illumos.org
>> >> > http://lists.illumos.org/m/listinfo/developer
>> >> >
>> >
>> >
>> >
>
>
>