* Randolph Chung <randolph@xxxxxxxxx>:
>> +unsigned long return_address(unsigned int level)
>> +{
>> + struct unwind_frame_info info;
>> + struct pt_regs r;
>> + unsigned long sp;
>> +
>> + /* initialize unwind info */
>> + asm volatile ("copy %%r30, %0" : "=r"(sp));
>> + memset(&r, 0, sizeof(struct pt_regs));
>> + r.iaoq[0] = (unsigned long) current_text_addr();
>> + r.gr[2] = (unsigned long) __builtin_return_address(0);
>> + r.gr[30] = sp;
>> + unwind_frame_init(&info, current, &r);
>> +
>> + /* unwind stack */
>> + ++level;
>> + do {
>> + if (unwind_once(&info) < 0 || info.ip == 0)
>> + return 0;
>> + if (!__kernel_text_address(info.ip)) {
>> + return 0;
>> + }
>> + } while (info.ip && level--);
>> +
>> + return info.ip;
>> +}
>> +
>
> Can you show an objdump of this function once it is compiled? I suspect
> the stack pointer initialization here is not reliable.
> Ideally unwind_frame_init is called with the frame address in gr[30].
> With a big struct like pt_regs on the stack, the sp initialization might
> be quite far from the actual frame address.
>
> The unwind_once() code uses like heuristics to try to recover from
> inaccurate stack pointers (by aligning and stepping the frame 64 bytes
> at a time) but that is really a brute force guess.
>
> I realize I used a similar construct in traps.c, but even there I think
> it doesn't work reliably.
Hi Randolph,
I agree, that those kind of implementations may not always be reliable.
Nevertheless, I started off with a copy of your code, but it didn't
worked at all. I assume due to some compiler-trickery to include the
static local functions in other functions it does work in traps.c
nevertheless.
I did sucessfully tested the patch I submitted here, both in 32- and 64-bit.
Below is the disassembly of the code which you asked for...
Helge
Disassembly of section .text.return_address:
00000000 <return_address>:
0: 08 03 02 41 copy r3,r1
4: 6b c2 3f d9 stw rp,-14(sp)
8: 08 1e 02 43 copy sp,r3
c: 6f c1 04 80 stw,ma r1,240(sp)
10: 68 67 04 00 stw r7,200(r3)
14: 08 02 02 47 copy rp,r7
18: 68 66 04 08 stw r6,204(r3)
1c: 08 1a 02 46 copy r26,r6
20: 68 65 04 10 stw r5,208(r3)
24: 68 64 04 18 stw r4,20c(r3)
28: 08 1e 02 44 copy sp,r4
2c: 34 65 00 50 ldo 28(r3),r5
30: 34 19 00 00 ldi 0,r25
34: 08 05 02 5a copy r5,r26
38: e8 40 00 00 b,l 40 <return_address+0x40>,rp
3c: 34 18 03 b0 ldi 1d8,r24
40: 68 67 00 60 stw r7,30(r3)
44: eb 80 40 00 blr r0,ret0
48: 08 00 02 40 nop
4c: 68 64 01 40 stw r4,a0(r3)
50: 68 7c 03 a0 stw ret0,1d0(r3)
54: 03 c0 08 bc mfctl tr6,ret0
58: 34 64 00 10 ldo 8(r3),r4
5c: 0f 80 10 99 ldw 0(ret0),r25
60: 08 04 02 5a copy r4,r26
64: e8 40 00 00 b,l 6c <return_address+0x6c>,rp
68: 08 05 02 58 copy r5,r24
6c: 08 04 02 5a copy r4,r26
70: e8 40 00 00 b,l 78 <return_address+0x78>,rp
74: 34 c6 00 02 ldo 1(r6),r6
78: 8f 80 60 80 cmpib,> 0,ret0,c0 <return_address+0xc0>
7c: 34 1c 00 00 ldi 0,ret0
80: 48 7c 00 20 ldw 10(r3),ret0
84: 87 80 20 68 cmpib,= 0,ret0,c0 <return_address+0xc0>
88: 08 1c 02 5a copy ret0,r26
8c: e8 40 00 00 b,l 94 <return_address+0x94>,rp
90: 08 00 02 40 nop
94: 87 80 20 40 cmpib,= 0,ret0,bc <return_address+0xbc>
98: 48 7c 00 20 ldw 10(r3),ret0
9c: 87 80 20 40 cmpib,= 0,ret0,c4 <return_address+0xc4>
a0: 48 62 3f d9 ldw -14(r3),rp
a4: 84 c0 20 30 cmpib,= 0,r6,c4 <return_address+0xc4>
a8: 08 04 02 5a copy r4,r26
ac: e8 40 00 00 b,l b4 <return_address+0xb4>,rp
b0: 34 c6 3f ff ldo -1(r6),r6
b4: 87 80 7f 95 cmpib,<= 0,ret0,84 <return_address+0x84>
b8: 48 7c 00 20 ldw 10(r3),ret0
bc: 34 1c 00 00 ldi 0,ret0
c0: 48 62 3f d9 ldw -14(r3),rp
c4: 48 67 04 00 ldw 200(r3),r7
c8: 48 66 04 08 ldw 204(r3),r6
cc: 48 65 04 10 ldw 208(r3),r5
d0: 48 64 04 18 ldw 20c(r3),r4
d4: 34 7e 00 80 ldo 40(r3),sp
d8: e8 40 c0 00 bv r0(rp)
dc: 4f c3 3f 81 ldw,mb -40(sp),r3
--
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