Re: core dump analysis, was Re: stack smashing detected

[Finn Thain <fthain@linux-m68k.org>]

On 31 Mar 2023, I wrote,

> ...
> 
> So %a3 was a pointer into stack frame 6??
> 
> (gdb) x/z $a3
> 0xefee1068:     0xc00e0172
> 
> Clearly 0xd000c38e != 0xc00e0172 (that is, %fp@(-4) != %a3@) but did the 
> canary value change? It rather looks like the canary pointer is wrong...
> 

So the question is, why is %a3 wrong? We know it was wrong because %a3 and 
%fp got saved on the stack when __libc_message was called:

(gdb) info frame 5
Stack frame at 0xefee1044:
 pc = 0xc009b328 in __libc_message (../sysdeps/posix/libc_fatal.c:155); 
    saved pc = 0xc012a3c2
 called by frame at 0xefee1060, caller of frame at 0xefee0fd0
 source language c.
 Arglist at 0xefee103c, args: action=<optimized out>, fmt=<optimized out>
 Locals at 0xefee103c, Previous frame's sp is 0xefee1044
 Saved registers:
  d2 at 0xefee100c, d3 at 0xefee1010, d4 at 0xefee1014, d5 at 0xefee1018,
  d6 at 0xefee101c, d7 at 0xefee1020, a2 at 0xefee1024, a3 at 0xefee1028,
  a4 at 0xefee102c, a5 at 0xefee1030, fp at 0xefee103c, pc at 0xefee1040
(gdb) x/z 0xefee1028
0xefee1028:     0xefee1068
(gdb) x/z 0xefee1068
0xefee1068:     0xc00e0172
(gdb) x/z 0xefee103c
0xefee103c:     0xefee10d8
(gdb) x/z 0xefee10d8-4
0xefee10d4:     0x00ac3dbe

That value for %fp@(-4) is the correct canary value (see below) showing 
that there's no buffer overrun in __wait3, and also showing that %a3 was 
wrong when the canary was to be validated.

> Another way to find the value of %a3 during __wait3() execution is to look 
> at its initialization: moveal %a5@(108),%a3. And we can see from 'info 
> frame' above that __stack_chk_fail() saved %a5 at 0xefee1064.
> 
> (gdb) x/4z 0xefee1060
> 0xefee1060:     0xc0182c5e      0xc0198000      0xc00e0172      0xd001e718
> (gdb) x/z *0xefee1064+108
> 0xc019806c:     Cannot access memory at address 0xc019806c
> 

That was a mistake on my part. It should have been,

(gdb) x/z 0xefee1064
0xefee1064:     0xc0198000
(gdb) x/z 0xc0198000+108
0xc019806c:     0xc0023e8c

So that's the value %a3 was given at __wait3+24, and it looks fine:

(gdb) x/z 0xc0023e8c
0xc0023e8c <__stack_chk_guard>: 0x00ac3dbe

... the expected value for %a3 at __wait3+70 is <stack_chk_guard> as this 
is used to validate the canary at %fp-4.

So, in summary, the canary validation failed in this case not because the 
canary got clobbered but because %a3 got clobbered, somewhere between 
__wait3+24 and __wait3+70 (below).

The call to __GI___wait4_time64 causes %a3 to be saved to and restored 
from the stack, so stack corruption seems to be a strong possibility to 
explain the change in %a3.

But if that's what happened, I'd expect __GI___wait4_time64 to report 
stack smashing, not __wait3... And it just begs the question, what then 
caused the corruption? Was it the wait4 syscall? Was it another thread? 
And why is it so rare?

(gdb) disass __wait3
Dump of assembler code for function __wait3:
   0xc00e0070 <+0>:     linkw %fp,#-96
   0xc00e0074 <+4>:     moveml %a2-%a3/%a5,%sp@-
   0xc00e0078 <+8>:     lea %pc@(0xc0198000),%a5
   0xc00e0080 <+16>:    movel %fp@(8),%d0
   0xc00e0084 <+20>:    moveal %fp@(16),%a2
   0xc00e0088 <+24>:    moveal %a5@(108),%a3
   0xc00e008c <+28>:    movel %a3@,%fp@(-4)
   0xc00e0090 <+32>:    tstl %a2
   0xc00e0092 <+34>:    beqw 0xc00e0152 <__wait3+226>
   0xc00e0096 <+38>:    pea %fp@(-92)
   0xc00e009a <+42>:    movel %fp@(12),%sp@-
   0xc00e009e <+46>:    movel %d0,%sp@-
   0xc00e00a0 <+48>:    pea 0xffffffff
   0xc00e00a4 <+52>:    bsrl 0xc00e0174 <__GI___wait4_time64>
   0xc00e00aa <+58>:    lea %sp@(16),%sp
   0xc00e00ae <+62>:    tstl %d0
   0xc00e00b0 <+64>:    bgts 0xc00e00c8 <__wait3+88>
   0xc00e00b2 <+66>:    moveal %fp@(-4),%a0
   0xc00e00b6 <+70>:    movel %a3@,%d1
   0xc00e00b8 <+72>:    cmpl %a0,%d1
   0xc00e00ba <+74>:    bnew 0xc00e016c <__wait3+252>
   0xc00e00be <+78>:    moveml %fp@(-108),%a2-%a3/%a5
   0xc00e00c4 <+84>:    unlk %fp
   0xc00e00c6 <+86>:    rts
   0xc00e00c8 <+88>:    pea 0x44
   0xc00e00cc <+92>:    clrl %sp@-
   0xc00e00ce <+94>:    pea %a2@(4)
   0xc00e00d2 <+98>:    movel %d0,%fp@(-96)
   0xc00e00d6 <+102>:   bsrl 0xc00b8850 <__GI_memset>
   0xc00e00dc <+108>:   movel %fp@(-88),%a2@
   0xc00e00e0 <+112>:   movel %fp@(-80),%a2@(4)
   0xc00e00e6 <+118>:   movel %fp@(-72),%a2@(8)
   0xc00e00ec <+124>:   movel %fp@(-64),%a2@(12)
   0xc00e00f2 <+130>:   movel %fp@(-60),%a2@(16)
   0xc00e00f8 <+136>:   movel %fp@(-56),%a2@(20)
   0xc00e00fe <+142>:   movel %fp@(-52),%a2@(24)
   0xc00e0104 <+148>:   movel %fp@(-48),%a2@(28)
   0xc00e010a <+154>:   movel %fp@(-44),%a2@(32)
   0xc00e0110 <+160>:   movel %fp@(-40),%a2@(36)
   0xc00e0116 <+166>:   movel %fp@(-36),%a2@(40)
   0xc00e011c <+172>:   movel %fp@(-32),%a2@(44)
   0xc00e0122 <+178>:   movel %fp@(-28),%a2@(48)
   0xc00e0128 <+184>:   movel %fp@(-24),%a2@(52)
   0xc00e012e <+190>:   movel %fp@(-20),%a2@(56)
   0xc00e0134 <+196>:   movel %fp@(-16),%a2@(60)
   0xc00e013a <+202>:   movel %fp@(-12),%a2@(64)
   0xc00e0140 <+208>:   movel %fp@(-8),%a2@(68)
   0xc00e0146 <+214>:   lea %sp@(12),%sp
   0xc00e014a <+218>:   movel %fp@(-96),%d0
   0xc00e014e <+222>:   braw 0xc00e00b2 <__wait3+66>
   0xc00e0152 <+226>:   clrl %sp@-
   0xc00e0154 <+228>:   movel %fp@(12),%sp@-
   0xc00e0158 <+232>:   movel %d0,%sp@-
   0xc00e015a <+234>:   pea 0xffffffff
   0xc00e015e <+238>:   bsrl 0xc00e0174 <__GI___wait4_time64>
   0xc00e0164 <+244>:   lea %sp@(16),%sp
   0xc00e0168 <+248>:   braw 0xc00e00b2 <__wait3+66>
   0xc00e016c <+252>:   bsrl 0xc012a38c <__stack_chk_fail>
End of assembler dump.
(gdb) disass __GI___wait4_time64
Dump of assembler code for function __GI___wait4_time64:
   0xc00e0174 <+0>:     lea %sp@(-80),%sp
   0xc00e0178 <+4>:     moveml %d2-%d5/%a2-%a3/%a5,%sp@-
   0xc00e017c <+8>:     lea %pc@(0xc0198000),%a5
   0xc00e0184 <+16>:    movel %sp@(116),%d2
   0xc00e0188 <+20>:    moveal %sp@(124),%a2
   0xc00e018c <+24>:    moveal %a5@(108),%a3
   0xc00e0190 <+28>:    movel %a3@,%sp@(104)
   0xc00e0194 <+32>:    bsrl 0xc0052e2c <__m68k_read_tp@plt>
   0xc00e019a <+38>:    movel %a0@(-29920),%d4
   0xc00e019e <+42>:    bnew 0xc00e026c <__GI___wait4_time64+248>
   0xc00e01a2 <+46>:    tstl %a2
   0xc00e01a4 <+48>:    beqs 0xc00e01aa <__GI___wait4_time64+54>
   0xc00e01a6 <+50>:    moveq #32,%d4
   0xc00e01a8 <+52>:    addl %sp,%d4
   0xc00e01aa <+54>:    movel %sp@(120),%d3
   0xc00e01ae <+58>:    movel %sp@(112),%d1
   0xc00e01b2 <+62>:    moveq #114,%d0
   0xc00e01b4 <+64>:    trap #0
   0xc00e01b6 <+66>:    cmpil #-4096,%d0
   0xc00e01bc <+72>:    bhiw 0xc00e02a6 <__GI___wait4_time64+306>
   0xc00e01c0 <+76>:    tstl %d0
   0xc00e01c2 <+78>:    blew 0xc00e0256 <__GI___wait4_time64+226>
   0xc00e01c6 <+82>:    tstl %a2
   0xc00e01c8 <+84>:    beqw 0xc00e0256 <__GI___wait4_time64+226>
   0xc00e01cc <+88>:    movel %sp@(32),%a2@(4)
   0xc00e01d2 <+94>:    smi %d1
   0xc00e01d4 <+96>:    extbl %d1
   0xc00e01d6 <+98>:    movel %d1,%a2@
   0xc00e01d8 <+100>:   movel %sp@(36),%a2@(12)
   0xc00e01de <+106>:   smi %d1
   0xc00e01e0 <+108>:   extbl %d1
   0xc00e01e2 <+110>:   movel %d1,%a2@(8)
   0xc00e01e6 <+114>:   movel %sp@(40),%a2@(20)
   0xc00e01ec <+120>:   smi %d1
   0xc00e01ee <+122>:   extbl %d1
   0xc00e01f0 <+124>:   movel %d1,%a2@(16)
   0xc00e01f4 <+128>:   movel %sp@(44),%a2@(28)
   0xc00e01fa <+134>:   smi %d1
   0xc00e01fc <+136>:   extbl %d1
   0xc00e01fe <+138>:   movel %d1,%a2@(24)
   0xc00e0202 <+142>:   movel %sp@(48),%a2@(32)
   0xc00e0208 <+148>:   movel %sp@(52),%a2@(36)
   0xc00e020e <+154>:   movel %sp@(56),%a2@(40)
   0xc00e0214 <+160>:   movel %sp@(60),%a2@(44)
   0xc00e021a <+166>:   movel %sp@(64),%a2@(48)
   0xc00e0220 <+172>:   movel %sp@(68),%a2@(52)
   0xc00e0226 <+178>:   movel %sp@(72),%a2@(56)
   0xc00e022c <+184>:   movel %sp@(76),%a2@(60)
   0xc00e0232 <+190>:   movel %sp@(80),%a2@(64)
   0xc00e0238 <+196>:   movel %sp@(84),%a2@(68)
   0xc00e023e <+202>:   movel %sp@(88),%a2@(72)
   0xc00e0244 <+208>:   movel %sp@(92),%a2@(76)
   0xc00e024a <+214>:   movel %sp@(96),%a2@(80)
   0xc00e0250 <+220>:   movel %sp@(100),%a2@(84)
   0xc00e0256 <+226>:   moveal %sp@(104),%a0
   0xc00e025a <+230>:   movel %a3@,%d1
   0xc00e025c <+232>:   cmpl %a0,%d1
   0xc00e025e <+234>:   bnew 0xc00e02f2 <__GI___wait4_time64+382>
   0xc00e0262 <+238>:   moveml %sp@+,%d2-%d5/%a2-%a3/%a5
   0xc00e0266 <+242>:   lea %sp@(80),%sp
   0xc00e026a <+246>:   rts
   0xc00e026c <+248>:   bsrl 0xc00a1b88 <__GI___pthread_enable_asynccancel>
   0xc00e0272 <+254>:   movel %d0,%d5
   0xc00e0274 <+256>:   tstl %a2
   0xc00e0276 <+258>:   beqs 0xc00e02c4 <__GI___wait4_time64+336>
   0xc00e0278 <+260>:   moveq #32,%d4
   0xc00e027a <+262>:   addl %sp,%d4
   0xc00e027c <+264>:   movel %sp@(120),%d3
   0xc00e0280 <+268>:   movel %sp@(112),%d1
   0xc00e0284 <+272>:   moveq #114,%d0
   0xc00e0286 <+274>:   trap #0
   0xc00e0288 <+276>:   cmpil #-4096,%d0
   0xc00e028e <+282>:   bhis 0xc00e02c8 <__GI___wait4_time64+340>
   0xc00e0290 <+284>:   movel %d5,%sp@-
   0xc00e0292 <+286>:   movel %d0,%sp@(32)
   0xc00e0296 <+290>:   bsrl 0xc00a1bea <__GI___pthread_disable_asynccancel>
   0xc00e029c <+296>:   addql #4,%sp
   0xc00e029e <+298>:   movel %sp@(28),%d0
   0xc00e02a2 <+302>:   braw 0xc00e01c0 <__GI___wait4_time64+76>
   0xc00e02a6 <+306>:   movel %d0,%sp@(28)
   0xc00e02aa <+310>:   bsrl 0xc0052e2c <__m68k_read_tp@plt>
   0xc00e02b0 <+316>:   addal %a5@(2cf8),%a0
   0xc00e02b8 <+324>:   movel %sp@(28),%d0
   0xc00e02bc <+328>:   negl %d0
   0xc00e02be <+330>:   movel %d0,%a0@
   0xc00e02c0 <+332>:   moveq #-1,%d0
   0xc00e02c2 <+334>:   bras 0xc00e0256 <__GI___wait4_time64+226>
   0xc00e02c4 <+336>:   clrl %d4
   0xc00e02c6 <+338>:   bras 0xc00e027c <__GI___wait4_time64+264>
   0xc00e02c8 <+340>:   movel %d0,%sp@(28)
   0xc00e02cc <+344>:   bsrl 0xc0052e2c <__m68k_read_tp@plt>
   0xc00e02d2 <+350>:   addal %a5@(2cf8),%a0
   0xc00e02da <+358>:   movel %sp@(28),%d0
   0xc00e02de <+362>:   negl %d0
   0xc00e02e0 <+364>:   movel %d0,%a0@
   0xc00e02e2 <+366>:   movel %d5,%sp@-
   0xc00e02e4 <+368>:   bsrl 0xc00a1bea <__GI___pthread_disable_asynccancel>
   0xc00e02ea <+374>:   addql #4,%sp
   0xc00e02ec <+376>:   moveq #-1,%d0
   0xc00e02ee <+378>:   braw 0xc00e0256 <__GI___wait4_time64+226>
   0xc00e02f2 <+382>:   bsrl 0xc012a38c <__stack_chk_fail>
End of assembler dump.