Bug 234523 - elf_aux_info() doesn't expose all AT_* vectors
Summary: elf_aux_info() doesn't expose all AT_* vectors
Status: Closed FIXED
Alias: None
Product: Base System
Classification: Unclassified
Component: bin (show other bugs)
Version: CURRENT
Hardware: Any Any
: --- Affects Only Me
Assignee: Mitchell Horne
URL:
Keywords:
Depends on:
Blocks:
 
Reported: 2018-12-31 11:20 UTC by Jan Beich
Modified: 2020-06-02 17:15 UTC (History)
2 users (show)

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Description Jan Beich freebsd_committer 2018-12-31 11:20:12 UTC
KERN_PROC_AUXV allows accessing arbitrary AT_* vectors but libc exposes only few via elf_aux_info(3). For example, one may want to rely on libc caching AT_EXECPATH instead of retrieving KERN_PROC_PATHNAME directly.

  $ cat elf_aux_info.c
  #include <sys/auxv.h>
  #include <limits.h>
  #include <stdio.h>

  int main()
  {
      char exe[PATH_MAX + 1] = {0};
      elf_aux_info(AT_EXECPATH, &exe, sizeof(exe));
      printf("%s\n", exe);
      return 0;
  }

  $ make elf_aux_info
  $ ./elf_aux_info

  $

vs.

  $ cat getauxval_naive.c
  #include <sys/param.h>
  #include <sys/sysctl.h>
  #include <elf.h>
  #include <errno.h>
  #include <unistd.h>
  #include <stdio.h>

  static unsigned long getauxval(unsigned long type) {
    Elf_Auxinfo auxv[AT_COUNT];
    size_t len = sizeof(auxv);
    int mib[] = {
      CTL_KERN,
      KERN_PROC,
      KERN_PROC_AUXV,
      getpid(),
    };

    if (sysctl(mib, nitems(mib), auxv, &len, NULL, 0) != -1) {
      for (size_t i = 0; i < nitems(auxv); i++)
	if ((unsigned long)auxv[i].a_type == type)
	  return auxv[i].a_un.a_val;

      errno = ENOENT;
    }
    return 0;
  }

  int main()
  {
      printf("%s\n", (char *)getauxval(AT_EXECPATH));
      return 0;
  }

  $ make getauxval_naive
  $ ./getauxval_naive
  /home/foo/getauxval_naive
  $
Comment 1 Mitchell Horne freebsd_committer 2020-06-02 17:15:45 UTC
Hey Jan,

I self-assigned this a while ago but never saw it through to completion. From the patches I produced, kib@ expressed that he did not want to expose arbitrary AT_* vectors without an intended use-case. I think what he said was reasonable; some of these vectors have no use or meaning outside of rtld.

For the specific case of exposing AT_EXECPATH, it was handled by r354694. I'm marking this PR as fixed, with the understanding that another can be opened in the future if some other use-case requires one of the remaining unexposed vectors.