[TUHS] Introduction

Jose R. Valverde jrvalverde at cnb.csic.es
Tue Jun 24 00:18:01 AEST 2008

> The (right now) remaining question is here:
> 	http://pofo.de/P8000/problems.php

My guess on this:

> I've some functions where the asm code looks as follows:
> 0530 3582  0004     584         ldl     rr2,rr8(#4)
> 0534 9424                       ldl     rr4,rr2
> 0536 0704  7f00     585         and     r4,#32512
> 04d2 5d04  8000*    586         ldl     _u+78,rr4
> 04d6 004e*
> This means, an unsigned long value stored in rr8 at position 4 gets loaded into rr2, then into rr4 and then ANDed with 7F00FFFF (r4 are the first 2 bytes of rr4). After the operation is done, the result gets loaded into the address the external reference _u is stored + 78 bytes. The C code I tried to produce out of this information is:

May be there is an additional cast being done. On prf.c you have a
similar AND:

                s=(char *)(*(long *)adx & 0x7F00FFFF);

As you can see there is a double indirection. My guess is that the
AND is done to clear some segmentation information, say to ensure the
datsegment of the program, possibly as a
security measure against a user process providing a pointer crafted
to point to an invalid address. The raw -unsafe- code would have
looked like
		s = (char *) *adx;

So, the address pointed to by adx, which is a char * is first cast 
into long *, then ANDed to clear those bits, then assigned. That would
mean that char* would then be restricted in this system to fit within
that 0x7F00FFFF mask. If that is so, then the original code in sys2.c 
for link()

		u.u_dirp.l = (caddr_t) ((long) uap->linkname);

was recoded to ensure that the (void) int* it got from uap was cleaned
before actual use:

	u.u_dirp.l = (caddr_t) (*((long *)(uap->linkname &0x7F00FFFF)))

uap->linkname is a re-interpretation (as per the struct cast) of the
data stored in u.u_ap, but u.u_ap is an (int*), a generic pointer that
might point to anything (a char* as expected or anything else). Then,
this would explain why you see other register usage in other similar
situations like in rdwr() after assignment of uap->cbuf (another char*)

Could you try that or some such? It would be used then whenever a
char * is to be retrieved through a generic int (void) pointer.

	These opinions are mine and only mine. Hey man, I saw them first!

			    José R. Valiigencia Artificial cuando falta la Natural
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