Good day,
I got some plan9 binaries to work on Linux (8c, 8l, cat, sed, cal and
a few more were tested), but all others are failing at exactly the
same instruction:
plock+0x31 MOVL 0x30(CX), CX
(which is called whenever malloc is used). For some context:
acid: asm(plock+0x20)
plock+0x20 JEQ plock+0x27(SB)
plock+0x22 CALL abort(SB)
plock+0x27 MOVL pv+0xc(SP), AX
plock+0x2b MOVL _tos(SB), CX
plock+0x31 MOVL 0x30(CX), CX
plock+0x32 DECL CX
plock+0x33 XORB CL, 0xc4830448(CX)
plock+0x39 ADCB AL,BL
The line of interest is plock+0x2b: (_tos(SB), CX); which is supposed
to store a value at CX. However, when the binary is run in Linux, CX
becomes 0 after that instruction, so plock+0x31 becomes (MOVL 0x30,
CX) resulting in a segfault as 0x30 is an invalid address in the
process address space (it starts only at 0x1000).
acid: asm(_tos)
_tos ADDB AL, 0x0(AX)
I defined a TEXT section for _tos, in the 'Hello Assembly' program
discussed earlier, and used _tos after printing Hello on the screen.
acid tells me CX becomes 0 after that instruction, which is exactly
what happens on Linux too:
DATA string<>+0(SB)/8, $"Plan9\n\z\z"
GLOBL string<>+0(SB), $8
TEXT _main+0(SB), 1, $0
MOVL $string<>+0(SB), 4(SP)
MOVL $8, AX
MOVL _tos(SB), CX
MOVL 0x30(CX), CX
INT $64
TEXT _tos+0(SB), 1, $0
ADDB AL, 0x0(AX)
8.out: 1831: suicide: sys: trap: fault read addr=0x30 pc=0x00001033
What is _tos supposed to do, and why does it set CX to different
values for some plan9 binaries, but not in a standalone assembled
program and on linux (in both cases, CX is set to 0)?
Thanks in advance for your help!
Regards,
Anant
