[TUHS] link() syscall implementation

Oliver Lehmann lehmann at ans-netz.de
Sun Nov 15 07:42:05 AEST 2015

Random832 <random832 at fastmail.com> wrote:

> Oliver Lehmann <lehmann at ans-netz.de> writes:
>>         ldl        rr2,rr8(#4)
>>         ldl        rr4,rr2
>>         and        r4,#32512
>>         ldl        _u+78,rr4
> This looks like it could be:
>   u.u_dirp.l = uap->linkname;
>   u.u_dirp.left &= 0x7f00;

Good guess, but the optimizer didn't optimized this "away" so this
generates literally what is done in C:

         ldl     rr2,rr8(#4)
         ldl     _u+78,rr2
         ld      r2,_u+78
         and     r2,#32512
         ld      _u+78,r2

>> does the same but  with more instructions and of course not 1:1
>> binary "the same" ;)
>> wonder why
>>   u.u_dirp.l = (caddr_t)(((long)uap->linkname) & 0x7F00FFFF);
>> didn't worked.
> Well, what code did it generate? Are longs in the same byte order as
> pointers (could it have needed to be FFFF7F00)?

Should be the same byte order, yes. It generates:

0530 3582  0004     584         ldl     rr2,rr8(#4)
0536 0702  7f00                 and     r2,#32512
04d2 5d02  8000*    585         ldl     _u+78,rr2
04d6 004e*

OK - it is not temporarily copied to rr4, but directly modfied in
rr2 and then assigned to u.u_dirp. It should be "the same" - But
  - ln(1) does not work with this code. Instead of creating a
hardlink, it creates a new empty file (different inode).

And to be honest - who would write such a code in the first place
    u.u_dirp.l = (caddr_t)(((long)uap->linkname) & 0x7F00FFFF);
Feels to crazy for me as someone would really write such a code.
There are so many "better" way (readability) someone would come
up with than this.... I/We just didn't figured out the better
way yet ;)

Another possibility could be, that the sys2.o was generated with
an earlier version if the assembler and not recompiled with the
"newer"/shipped one and that it might be impossible to generate
this code now. But all sccs tags in the assembler are before the
sccs tag in sys2.o....

Regards, Oliver

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