stack-sizes

stack-sizes

Utility that parses stack sizes section from elf objects and displays the preallocated stack size of each function.

Usage

Compile a program with clang and the -fstack-size-section flag:

#include <stdio.h>
#include <stdint.h>

void foo(uint8_t ind) {
  char a[256] = {0};

  scanf("%c\n", &(a[ind]));
}

int main(void) {
  uint8_t bar;

  scanf("%hhu", &bar);
  foo(bar);
  return 0;
}

$ clang -Wall -Wextra -fstack-size-section t.c -o t
$ go run stack-sizes.go -f t
0x401140        foo: 296 bytes
0x4011b0        main: 24 bytes

$ objdump -d -Mintel t | grep -A 10 "<foo>:"
0000000000401140 <foo>:
  401140:       55                      push   rbp       ; 8
  401141:       48 89 e5                mov    rbp,rsp
  401144:       48 81 ec 20 01 00 00    sub    rsp,0x120 ; 288
  40114b:       40 88 f8                mov    al,dil
  40114e:       31 f6                   xor    esi,esi
  401150:       88 45 ff                mov    BYTE PTR [rbp-0x1],al
  401153:       48 8d 8d f0 fe ff ff    lea    rcx,[rbp-0x110]
  40115a:       48 89 ca                mov    rdx,rcx
  40115d:       48 89 d7                mov    rdi,rdx
  401160:       ba 00 01 00 00          mov    edx,0x100
$ objdump -d -Mintel t | grep -A 10 "<main>:"
00000000004011b0 <main>:
  4011b0:       55                      push   rbp      ; 8
  4011b1:       48 89 e5                mov    rbp,rsp
  4011b4:       48 83 ec 10             sub    rsp,0x10 ; 16
  4011b8:       c7 45 fc 00 00 00 00    mov    DWORD PTR [rbp-0x4],0x0
  4011bf:       48 bf 08 20 40 00 00    movabs rdi,0x402008
  4011c6:       00 00 00 
  4011c9:       48 8d 75 fb             lea    rsi,[rbp-0x5]
  4011cd:       b0 00                   mov    al,0x0
  4011cf:       e8 6c fe ff ff          call   401040 <__isoc99_scanf@plt>
  4011d4:       0f b6 7d fb             movzx  edi,BYTE PTR [rbp-0x5]

From the llvm docs:

A section containing metadata on function stack sizes will be emitted when TargetLoweringObjectFile::StackSizesSection is not null, and TargetOptions::EmitStackSizeSection is set (-stack-size-section). The section will contain an array of pairs of function symbol values (pointer size) and stack sizes (unsigned LEB128). The stack size values only include the space allocated in the function prologue. Functions with dynamic stack allocations are not included

Which is triggered by using the -stack-size-section in llc:

Emit the .stack_sizes section which contains stack size metadata. The section contains an array of pairs of function symbol values (pointer size) and stack sizes (unsigned LEB128). The stack size values only include the space allocated in the function prologue. Functions with dynamic stack allocations are not included.

You might also be interested in using the -Wframe-larger-than clang argument, which will warn you if one of your stack frames is bigger than the specified amount of bytes:

$ clang -Wall -Wextra -Wframe-larger-than=100 t.c -o t
t.c:4:6: warning: stack frame size of 296 bytes in function 'foo' [-Wframe-larger-than=]
void foo(uint8_t ind) {
     ^
1 warning generated.