Hi!
We continue to play with qemu, and it turned out that due to some
unknow issue the kernel built with stack overflow detection traps inside
error path out of mcount. The problem with that the following code in
arch/sparc/lib/mcount.S if we happen to jump at line 68 we have
%g3 = 0x4000 and then we assign that to stack pointer.
----
arch/sparc/lib/mcount.S:
66 ldx [%g3 + %g1], %g7
67 cmp %sp, %g7
68 bleu,pt %xcc, 2f
69 sethi %hi(THREAD_SIZE), %g3
70 add %g7, %g3, %g7
71 cmp %sp, %g7
72 blu,pn %xcc, 1f
73 nop
74 /* If we are already on ovstack, don't hop onto it
75 * again, we are already trying to output the stack overflow
76 * message.
77 */
78 sethi %hi(ovstack), %g7 ! cant
move to panic stack fast enough
79 or %g7, %lo(ovstack), %g7
80 add %g7, OVSTACKSIZE, %g3
81 sub %g3, STACK_BIAS + 192, %g3
82 sub %g7, STACK_BIAS, %g7
83 cmp %sp, %g7
84 blu,pn %xcc, 2f
85 cmp %sp, %g3
86 bleu,pn %xcc, 1f
87 nop
88 2: mov %g3, %sp
89 sethi %hi(panicstring), %g3
90 call prom_printf
91 or %g3, %lo(panicstring), %o0
92 call prom_halt
93 nop
----
A few steps later we take misaligned trap inside prom_printf
while accessing stack frame, please see qemu trace below.
Here is qemu trace:
----
IN: _mcount
0x0000000000545b34: or %g3, 0xb0, %o0 ! 0x70f8b0
IN: prom_printf
0x000000000053fa80: save %sp, -192, %sp
IN: prom_printf
0x000000000053fa84: add %fp, 0x887, %g1
IN: prom_printf
0x000000000053fa88: sethi %hi(0x788c00), %l0
IN: prom_printf
0x000000000053fa8c: stx %i1, [ %fp + 0x887 ]
107007: Unaligned Memory Access (v=0034) pc=000000000053fa8c
npc=000000000053fa90 SP=0000000000003f40
----
I wonder if mcount ever works on sparc64, if it does work indeed I'd
like to know what I'm missing there.
This is running on qemu-system-sparc64 so there is an opportunity to
have bug in emulation, but still misaligned stack pointer is evident.
--
Kind regards,
Igor V. Kovalenko
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