Access build-id at runtime

Question

Access build-id at runtime

I am trying to figure out how to access the build-id generated by the linker at runtime.

From this page, https://linux.die.net/man/1/ld

When I build a test program like:

% gcc test.c -o test -Wl,--build-id=sha1

I can see that the build ID is present in the binary:

% readelf -n test

Displaying notes found in: .note.gnu.build-id
  Owner                 Data size   Description
  GNU                  0x00000014   NT_GNU_BUILD_ID (unique build ID bitstring)
    Build ID: 85aa97bd52ddc4dc2a704949c2545a3a9c69c6db

I would like to print this at run-time.

EDIT: Assume you can't access the elf file from which the running process was loaded (permissions, embedded/no filesystem, etc).

EDIT: the accepted answer works, but the linker does not necessarily have to place the variable at the end of the section. If there were a way to get a pointer to the start of the section, that would be more reliable.

c

gcc

linker

elf

linker-scripts

asked on Stack Overflow Apr 11, 2019 by

Todd Freed • edited Apr 20, 2019 by

Todd Freed

2 Answers

Figured it out. Here is a working example,

#include <stdio.h>

//
// variable must have an initializer
//  https://gcc.gnu.org/onlinedocs/gcc-3.3.1/gcc/Variable-Attributes.html
//
// the linker places this immediately after the section data
// 
char build_id __attribute__((section(".note.gnu.build-id"))) = '!';

int main(int argc, char ** argv)
{
  const char * s;

  s = &build_id;

  // section data is 20 bytes in size
  s -= 20;

  // sha1 data continues for 20 bytes
  printf("  > Build ID: ");
  int x;
  for(x = 0; x < 20; x++) {
    printf("%02hhx", s[x]);
  }

  printf(" <\n");

  return 0;
}

When I run this, I get output that matches readelf,

0 % gcc -g main.c -o test -Wl,--build-id=sha1 && readelf -n test | tail -n 5 && ./test
Displaying notes found in: .note.gnu.build-id
  Owner                 Data size       Description
  GNU                  0x00000014       NT_GNU_BUILD_ID (unique build ID bitstring)
    Build ID: c5eca2cb08f4f5a31bb695955c7ebd2722ca10e9
  > Build ID: c5eca2cb08f4f5a31bb695955c7ebd2722ca10e9 <

answered on Stack Overflow Apr 13, 2019 by

Todd Freed

One possibility is to use linker scripts to get the address of the .note.gnu.build-id section:

#include <stdio.h>

// These will be set by the linker script
extern char build_id_start;
extern char build_id_end;

int main(int argc, char **argv) {
  const char *s;

  s = &build_id_start;

  // skip over header (16 bytes)
  s += 16;

  printf("  > Build ID: ");
  for (; s < &build_id_end; s++) {
    printf("%02hhx", *s);
  }

  printf(" <\n");

  return 0;
}

In the linker script, the symbols build_id_start and build_id_end are defined:

build_id_start = ADDR(.note.gnu.build-id);
build_id_end = ADDR(.note.gnu.build-id) + SIZEOF(.note.gnu.build-id);

The code then can be compiled and run:

gcc build-id.c linker-script.ld -o test && readelf -n test | grep NT_GNU_BUILD_ID -A1 && ./test
  GNU                  0x00000014   NT_GNU_BUILD_ID (unique build ID bitstring)
    Build ID: 7e87ff227443c8f2d5c8e2340638a2ec77d008a1
  > Build ID: 7e87ff227443c8f2d5c8e2340638a2ec77d008a1 <

answered on Stack Overflow Dec 28, 2019 by

Steffen Kieß

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