Whenever GDB steps in functions, instead of showing correct arguments in frame info, it prints garbage data
gcc (Ubuntu 9.3.0-17ubuntu1~20.04) 9.3.0
$ gcc tc -g #no other flags are used
GNU gdb (Ubuntu 9.2-0ubuntu1~20.04) 9.2
The program (tc):
#include<stdio.h>
void foo(int v){
printf(" BAR = %d\n", v);
}
int main(){
int a = 8;
foo(a);
a = 33;
foo(a);
foo(85);
}
GDB output:
Breakpoint 1, main () at t.c:7
7 int main(){
(gdb) step
8 int a = 8;
(gdb) step
9 foo(a);
(gdb) step
foo (v=21845) at t.c:3
3 void foo(int v){
(gdb) finish
Run till exit from #0 foo (v=21845) at t.c:3
BAR = 8
main () at t.c:10
10 a = 33;
(gdb) s
11 foo(a);
(gdb)
foo (v=8) at t.c:3
3 void foo(int v){
(gdb) fin
Run till exit from #0 foo (v=8) at t.c:3
BAR = 33
main () at t.c:12
12 foo(85);
(gdb) s
foo (v=33) at t.c:3
3 void foo(int v){
(gdb) fin
Run till exit from #0 foo (v=33) at t.c:3
BAR = 85
0x00005555555551a9 in main () at t.c:12
12 foo(85);
(gdb) s
13 }
But after performing a step after stepping in functions, arguments are written with correct data:
Breakpoint 1, main () at t.c:7
7 int main(){
(gdb) s
8 int a = 8;
(gdb)
9 foo(a);
(gdb)
foo (v=21845) at t.c:3
3 void foo(int v){
(gdb) info args
v = 21845
(gdb) s
4 printf(" BAR = %d\n", v);
(gdb) info args
v = 8
Is there any way to fix this so GDB show correct arguments of functions?
Looking at the disassembly, gdb is stopped at the first instruction of function foo
, before the function prologue (which sets up the stack and frame pointers) has been run:
(gdb) step
9 foo(a);
(gdb) step
foo (v=21845) at t.c:3
3 void foo(int v){
(gdb) disas
Dump of assembler code for function foo:
=> 0x0000555555555149 <+0>: endbr64
0x000055555555514d <+4>: push %rbp
0x000055555555514e <+5>: mov %rsp,%rbp
0x0000555555555151 <+8>: sub $0x10,%rsp
0x0000555555555155 <+12>: mov %edi,-0x4(%rbp)
0x0000555555555158 <+15>: mov -0x4(%rbp),%eax
0x000055555555515b <+18>: mov %eax,%esi
0x000055555555515d <+20>: lea 0xea0(%rip),%rdi # 0x555555556004
0x0000555555555164 <+27>: mov $0x0,%eax
0x0000555555555169 <+32>: callq 0x555555555050 <printf@plt>
0x000055555555516e <+37>: nop
0x000055555555516f <+38>: leaveq
0x0000555555555170 <+39>: retq
End of assembler dump.
Gdb's step command normally steps over a function's prologue , that is, it stops the program after the prologue has run. Here, gdb apparently doesn't recognize the instruction endbr64
as being part of any known prologue.
We can see that &v
is beyond the bounds of the current stack frame:
(gdb) p &v
$1 = (int *) 0x7fffffffe3fc
(gdb) i r rbp rsp
rbp 0x7fffffffe420
rsp 0x7fffffffe408
Since the new stack frame hasn't been set up yet, gdb will read a garbage value for v
.
Stepping a few more instructions will set up the stack frame and spill v
from %edi
to -0x4(%rbp)
:
(gdb) stepi
=> 0x000055555555514d <foo+4>: push %rbp
(gdb) stepi
=> 0x000055555555514e <foo+5>: mov %rsp,%rbp
(gdb) stepi
=> 0x0000555555555151 <foo+8>: sub $0x10,%rsp
(gdb) stepi
=> 0x0000555555555155 <foo+12>: mov %edi,-0x4(%rbp)
(gdb) stepi
4 printf(" BAR = %d\n", v);
=> 0x0000555555555158 <foo+15>: mov -0x4(%rbp),%eax
Verify that &v
is now within the stack frame, and examine v
's value:
(gdb) p &v
$2 = (int *) 0x7fffffffe3fc
(gdb) i r rbp rsp
rbp 0x7fffffffe400
rsp 0x7fffffffe3f0
(gdb) p v
$3 = 8
Gcc emits endbr64
when given the -fcf-protection
option, which has been the default in Ubuntu's gcc since version 19.10 .
If you compile your program with -fcf-protection=none
, gdb can recognize and run the prologue before stopping, and it will show the correct value of v
:
(gdb) step
9 foo(a);
(gdb) step
foo (v=8) at t.c:4
4 printf(" BAR = %d\n", v);
(gdb) disas
Dump of assembler code for function foo:
0x0000555555555139 <+0>: push %rbp
0x000055555555513a <+1>: mov %rsp,%rbp
0x000055555555513d <+4>: sub $0x10,%rsp
0x0000555555555141 <+8>: mov %edi,-0x4(%rbp)
=> 0x0000555555555144 <+11>: mov -0x4(%rbp),%eax
0x0000555555555147 <+14>: mov %eax,%esi
0x0000555555555149 <+16>: lea 0xeb4(%rip),%rdi # 0x555555556004
0x0000555555555150 <+23>: mov $0x0,%eax
0x0000555555555155 <+28>: callq 0x555555555030 <printf@plt>
0x000055555555515a <+33>: nop
0x000055555555515b <+34>: leaveq
0x000055555555515c <+35>: retq
End of assembler dump.
It looks like support for the endbr
instructions was added to gdb in March 2020 , so things should be fine if you can use gdb 10.1 or later:
$ ~/gdb10.1/bin/gdb -q t
...
(gdb) step
9 foo(a);
(gdb) step
foo (v=8) at t.c:4
4 printf(" BAR = %d\n", v);
(gdb) disas
Dump of assembler code for function foo:
0x0000555555555149 <+0>: endbr64
0x000055555555514d <+4>: push %rbp
0x000055555555514e <+5>: mov %rsp,%rbp
0x0000555555555151 <+8>: sub $0x10,%rsp
0x0000555555555155 <+12>: mov %edi,-0x4(%rbp)
=> 0x0000555555555158 <+15>: mov -0x4(%rbp),%eax
0x000055555555515b <+18>: mov %eax,%esi
0x000055555555515d <+20>: lea 0xea0(%rip),%rdi # 0x555555556004
0x0000555555555164 <+27>: mov $0x0,%eax
0x0000555555555169 <+32>: call 0x555555555050 <printf@plt>
0x000055555555516e <+37>: nop
0x000055555555516f <+38>: leave
0x0000555555555170 <+39>: ret
End of assembler dump.
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