[TUHS] link() syscall implementation
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:
>> 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
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....
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