GDB - see what have accessed the address/break on it

I am trying to overwrite the function vtable:

Here it is done:

Before memcpy

(gdb) x/100x 0xb7993150
0xb7993150:     0xb6eae130      0xb6eae130      0x41414141      0x41414141
0xb7993160:     0x41414141      0x41414141      0x41414141      0x41414141
0xb7993170:     0x00000000      0x00000000      0x00000030      0x00000023
0xb7993180:     0x6c707061      0x74616369      0x2f6e6f69      0x6574636f
0xb7993190:     0x74732d74      0x6d616572      0x00000000      0x0000001b
0xb79931a0:     0x00000001      0x00000001      0xb7993078      0x00000000
0xb79931b0:     0x00000000      0x0000002b      0xb6727688      0xb79931e0
0xb79931c0:     0x00000000      0xb6727678      0xb79931f8      0xb6727678
0xb79931d0:     0xb7993208      0x00000000      0x00000000      0x0000001b
0xb79931e0:     0x00000001      0x00000001      0xb79931b8      0x00000000
0xb79931f0:     0x66657463      0x00000013      0x00000000      0xb79932c8
0xb7993200:     0xb7993218      0x00000013      0x00000000      0xb79932d8
0xb7993210:     0xb79932d8      0x00000013      0xb666ee65      0xb79931f8
0xb7993220:     0xb7993228      0x00000013      0xb66b6971      0xb7993218
0xb7993230:     0xb7993238      0x00000013      0xb6693c11      0xb7993228
0xb7993240:     0xb7993248      0x00000013      0xb669ba79      0xb7993238

after memcpy

0xb7993150:     0x7d000000      0x67337874      0x41414141      0x41414141
0xb7993160:     0x41414141      0x41414141      0x41414141      0x41414141
0xb7993170:     0x99999999      0x42424242      0x42424242      0x42424242
0xb7993180:     0x42424242      0x42424242      0x42424242      0x42424242
0xb7993190:     0x42424242      0x42424242      0x42424242      0x42424242
0xb79931a0:     0x42424242      0x42424242      0x42424242      0x42424242
0xb79931b0:     0x42424242      0x42424242      0x42424242      0x46454443
0xb79931c0:     0x4a494847      0x0000204b      0xe8919cb3      0xb67276b6
0xb79931d0:     0xb7993208      0x00000000      0x00000000      0x0000001b
0xb79931e0:     0x00000001      0x00000001      0xb79931b8      0x00000000
0xb79931f0:     0x66657463      0x00000013      0x00000000      0xb79932c8
0xb7993200:     0xb7993218      0x00000013      0x00000000      0xb79932d8
0xb7993210:     0xb79932d8      0x00000013      0xb666ee65      0xb79931f8
0xb7993220:     0xb7993228      0x00000013      0xb66b6971      0xb7993218
0xb7993230:     0xb7993238      0x00000013      0xb6693c11      0xb7993228
0xb7993240:     0xb7993248      0x00000013      0xb669ba79      0xb7993238
0xb7993250:     0xb7993258      0x00000013      0xb666990d      0xb7993248
0xb7993260:     0xb7993268      0x00000013      0xb665ad61      0xb7993258
0xb7993270:     0xb7993278      0x00000013      0xb66c83b5      0xb7993268
0xb7993280:     0xb7993288      0x00000013      0xb666adad      0xb7993278

How Can I stop/make breakpoint when 0x99999999 is accessed/read as a vtable pointer?

Will just

b *0xb7993170 

work?

I am trying to overwrite the function vtable:

There is no such thing. Perhaps you mean class vtable?

Will just b *0xb7993170 work?

No. b *0xb7993170 will only work IF 0xb7993170 contains executable code. If it actually contains a vtable (i.e. a pointer to code), you'll want:

awatch *(int**)0xb7993170

Or you can just continue the binary and let it crash when it tries to dereference 0x42424242.

P.S. Given that 0xb7993170 contained 0 before the memcpy, it was certainly not a function pointer (but could still be part of vtable).

You need to set a watch point on the actual pointer location (i.e. 0xb7993170 will not work, you need 0xb7993178 or something like that).