[TUHS] /bin/true (was basic tools / Universal Unix)

Dan Cross crossd at gmail.com
Wed Nov 29 04:26:54 AEST 2017

On Tue, Nov 28, 2017 at 12:56 PM, Warner Losh <imp at bsdimp.com> wrote:
> Ah, the tyranny of  static analysis tools... _exit(0) should be marked
> such that the tools know it does not return. This means the /*NOTREACHED*/
> isn't needed. And since there's no real exit path out of  main, the return
> (0) is equally bogus (because main can't return). Yet lint and other tools
> have ushered in this insanity.

Hmm; in what way can main() not return? Surely this is true if `_exit(0)`
is called as this calls the exit system call, which cannot -- by definition
-- return. But main() itself can return to whatever calls it (usually
`start`, I'd imagine). For that matter, I'm not aware of any prohibition
against calling `main()` recursively.

: tempest; cat r.c
#include <stdio.h>

main(int argc, char *argv[])
if (argc > 1) {
argc--; argv++;
printf("%s", argv[0]);
if (argc > 1) printf(" ");
return main(argc, argv);
return 0;
: tempest; make r
cc     r.c   -o r
: tempest; ./r hi from Dan
hi from Dan
: tempest;

This is sort of an admittedly weird way to write `echo`, but it seems to
work ok.

I'm glad to see these days that these sorts of lame false positives have
> been eliminated...


Then again _exit(0) is a useless optimization. It saves three closes for
> files that are bound to be closed at image tear down. If it really is that
> important (absent data, my gut tells me it isn't), then this should be
> written in assembler. FreeBSD/amd64 would be something like:
> #include <sys/syscall.h>
> #include <machine/asm.h>
> ENTRY(_start)
> xor %r10, %r10
> mov $SYS_exit, %eax
> syscall
> END(_start)
> This some small hacks to each arch's SYS.h in libc, this could even be
> smaller and MI :). this is tiny:
> -rwxrwxr-x  1 imp  imp  672 Nov 28 10:18 true
>   text   data   bss   dec   hex   filename
>     10      0     0    10   0xa   true

This is much smaller than the binary for the assembler program I posted for
macOS earlier in this thread: the result there was much larger (but due to
the requirement to have a non-empty data segment in the executable; this
ends up being page-aligned and filled with zeros).

Contrast that with FreeBSD's /usr/bin/true:
> -r-xr-xr-x  1 root  wheel  4624 Nov 20 11:56 /usr/bin/true
>   text   data   bss    dec     hex   filename
>   1540    481     8   2029   0x7ed   /usr/bin/true
> which is little more than return(0), but also has a fair amount of
> copyright and SCCS data.

Is the copyright data actually present in the object file? I see some RCS
$Id$ strings (in the guise of $FreeBSD:$ stuff) but no copyright strings in
/usr/bin/true on my system.

        - Dan C.
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