Valgrind stack misses a function completely - c++

Question

i have two c files:
a.c
void main(){
...
getvtable()->function();
}
the vtable is pointing to a function that is located in b.c:
void function(){
malloc(42);
}
now if i trace the program in valgrind I get the following:
==29994== 4,155 bytes in 831 blocks are definitely lost in loss record 26 of 28
==29994== at 0x402CB7A: malloc (in /usr/lib/valgrind/vgpreload_memcheck-x86-linux.so)
==29994== by 0x40A24D2: (below main) (libc-start.c:226)
so the call to function is completely ommited on the stack! How is it possible? In case I use GDB, a correct stack including "function" is shown.
Debug symbols are included, Linux, 32-bit.
Upd:
Answering the first question, I get the following output when debugging valgrind's GDB server. The breakpoint is not coming, while it comes when i debug directly with GDB.
stasik#gemini:~$ gdb -q
(gdb) set confirm off
(gdb) target remote | vgdb
Remote debugging using | vgdb
relaying data between gdb and process 11665
[Switching to Thread 11665]
0x040011d0 in ?? ()
(gdb) file /home/stasik/leak.so
Reading symbols from /home/stasik/leak.so...done.
(gdb) break function
Breakpoint 1 at 0x110c: file ../../source/leakclass.c, line 32.
(gdb) commands
Type commands for breakpoint(s) 1, one per line.
End with a line saying just "end".
>silent
>end
(gdb) continue
Continuing.
Program received signal SIGTRAP, Trace/breakpoint trap.
0x0404efcb in ?? ()
(gdb) source thread-frames.py
Stack level 0, frame at 0x42348a0:
eip = 0x404efcb; saved eip 0x4f2f544c
called by frame at 0x42348a4
Arglist at 0x4234898, args:
Locals at 0x4234898, Previous frame's sp is 0x42348a0
Saved registers:
ebp at 0x4234898, eip at 0x423489c
Stack level 1, frame at 0x42348a4:
eip = 0x4f2f544c; saved eip 0x6e492056
called by frame at 0x42348a8, caller of frame at 0x42348a0
Arglist at 0x423489c, args:
Locals at 0x423489c, Previous frame's sp is 0x42348a4
Saved registers:
eip at 0x42348a0
Stack level 2, frame at 0x42348a8:
eip = 0x6e492056; saved eip 0x205d6f66
called by frame at 0x42348ac, caller of frame at 0x42348a4
Arglist at 0x42348a0, args:
Locals at 0x42348a0, Previous frame's sp is 0x42348a8
Saved registers:
eip at 0x42348a4
Stack level 3, frame at 0x42348ac:
eip = 0x205d6f66; saved eip 0x61746144
---Type <return> to continue, or q <return> to quit---
called by frame at 0x42348b0, caller of frame at 0x42348a8
Arglist at 0x42348a4, args:
Locals at 0x42348a4, Previous frame's sp is 0x42348ac
Saved registers:
eip at 0x42348a8
Stack level 4, frame at 0x42348b0:
eip = 0x61746144; saved eip 0x65736162
called by frame at 0x42348b4, caller of frame at 0x42348ac
Arglist at 0x42348a8, args:
Locals at 0x42348a8, Previous frame's sp is 0x42348b0
Saved registers:
eip at 0x42348ac
Stack level 5, frame at 0x42348b4:
eip = 0x65736162; saved eip 0x70616d20
called by frame at 0x42348b8, caller of frame at 0x42348b0
Arglist at 0x42348ac, args:
Locals at 0x42348ac, Previous frame's sp is 0x42348b4
Saved registers:
eip at 0x42348b0
Stack level 6, frame at 0x42348b8:
eip = 0x70616d20; saved eip 0x2e646570
called by frame at 0x42348bc, caller of frame at 0x42348b4
Arglist at 0x42348b0, args:
---Type <return> to continue, or q <return> to quit---
Locals at 0x42348b0, Previous frame's sp is 0x42348b8
Saved registers:
eip at 0x42348b4
Stack level 7, frame at 0x42348bc:
eip = 0x2e646570; saved eip 0x0
called by frame at 0x42348c0, caller of frame at 0x42348b8
Arglist at 0x42348b4, args:
Locals at 0x42348b4, Previous frame's sp is 0x42348bc
Saved registers:
eip at 0x42348b8
Stack level 8, frame at 0x42348c0:
eip = 0x0; saved eip 0x0
caller of frame at 0x42348bc
Arglist at 0x42348b8, args:
Locals at 0x42348b8, Previous frame's sp is 0x42348c0
Saved registers:
eip at 0x42348bc
(gdb) continue
Continuing.
Program received signal SIGTRAP, Trace/breakpoint trap.
0x0404efcb in ?? ()
(gdb) continue
Continuing.

Answer 1

I see two possible reasons:
Valgrind is using a different stack unwind method than GDB
The address space layout is different while running your program under the two environments and you're only hitting stack corruption under Valgrind.
We can gain more insight by using Valgrind's builtin gdbserver.
Save this Python snippet to thread-frames.py
import gdb
f = gdb.newest_frame()
while f is not None:
f.select()
gdb.execute('info frame')
f = f.older()
t.gdb
set confirm off
file MY-PROGRAM
break function
commands
silent
end
run
source thread-frames.py
quit
v.gdb
set confirm off
target remote | vgdb
file MY-PROGRAM
break function
commands
silent
end
continue
source thread-frames.py
quit
(Change MY-PROGRAM, function in the scripts above and the commands below as required)
Get details about the stack frames under GDB:
$ gdb -q -x t.gdb
Breakpoint 1 at 0x80484a2: file valgrind-unwind.c, line 6.
Stack level 0, frame at 0xbffff2f0:
eip = 0x80484a2 in function (valgrind-unwind.c:6); saved eip 0x8048384
called by frame at 0xbffff310
source language c.
Arglist at 0xbffff2e8, args:
Locals at 0xbffff2e8, Previous frame's sp is 0xbffff2f0
Saved registers:
ebp at 0xbffff2e8, eip at 0xbffff2ec
Stack level 1, frame at 0xbffff310:
eip = 0x8048384 in main (valgrind-unwind.c:17); saved eip 0xb7e33963
caller of frame at 0xbffff2f0
source language c.
Arglist at 0xbffff2f8, args:
Locals at 0xbffff2f8, Previous frame's sp is 0xbffff310
Saved registers:
ebp at 0xbffff2f8, eip at 0xbffff30c
Get the same data under Valgrind:
$ valgrind --vgdb=full --vgdb-error=0 ./MY-PROGRAM
In another shell:
$ gdb -q -x v.gdb
relaying data between gdb and process 574
0x04001020 in ?? ()
Breakpoint 1 at 0x80484a2: file valgrind-unwind.c, line 6.
Stack level 0, frame at 0xbe88e2c0:
eip = 0x80484a2 in function (valgrind-unwind.c:6); saved eip 0x8048384
called by frame at 0xbe88e2e0
source language c.
Arglist at 0xbe88e2b8, args:
Locals at 0xbe88e2b8, Previous frame's sp is 0xbe88e2c0
Saved registers:
ebp at 0xbe88e2b8, eip at 0xbe88e2bc
Stack level 1, frame at 0xbe88e2e0:
eip = 0x8048384 in main (valgrind-unwind.c:17); saved eip 0x4051963
caller of frame at 0xbe88e2c0
source language c.
Arglist at 0xbe88e2c8, args:
Locals at 0xbe88e2c8, Previous frame's sp is 0xbe88e2e0
Saved registers:
ebp at 0xbe88e2c8, eip at 0xbe88e2dc
If GDB can successfully unwind the stack while connecting to "valgrind --gdb" then it's a problem with Valgrind's stack unwind algorithm. You can inspect the "info frame" output carefully for inline and tail call frames or some other reason that could throw Valgrind off. Otherwise it's probably stack corruption.

Answer 2

Ok, compiling all .so parts and the main program with an explicit -O0 seems to solve the problem. It seems that some of the optimizations of the 'core' program that was loading the .so (so was always compiled unoptimized) was breaking the stack.

Answer 3

This is Tail-call optimization in action.
The function function calls malloc as the last thing it does. The compiler sees this and kills the stack frame for function before it calls malloc. The advantage is that when malloc returns it returns directly to whichever function called function. I.e. it avoids malloc returning to function only to hit yet another return instruction.
In this case the optimization has prevented an unnecessary jump and made stack usage slightly more efficient, which is nice, but in the case of a recursive tail call then this optimization is a huge win as it turns a recursion into something more like iteration.
As you've discovered already, disabling optimization makes debugging much easier. If you want to debug optimized code (for performance testing, perhaps), then, as #Zang MingJie already said, you can disable this one optimization with -fno-optimize-sibling-calls.