[Toybox] FYI: kernel builds rely on GNU expr extensions

[Rob Landley]

On 5/1/19 3:23 PM, enh via Toybox wrote:
> so you know how i said i wasn't going to use anything in pending for
> the build? turns out i wasn't paying enough attention and accidentally
> used expr...

Eh, if you're deploying pending on the device using it in the build isn't that
much more of a lift...

ALso, $(( )) is expr the same way [ ] is test, so I've gotta finish and promote
it soon to make toysh work...

> i'm reverting that now (but keeping it for the device), but folks
> building their kernels from within an Android tree hit this. (we
> recommend that you _don't_ build your kernel that way, and we don't do
> that ourselves, but realistically we're never going to stop everyone
> from doing it. and i know "build the kernel with toybox" is
> interesting to you anyway.)

It's explicitly one of my goals, to make that supportable, yes. :)

> so, yeah, the kernel build uses some GNU extensions. `index` looks
> like it's there (though perhaps only for tests?), and the specific bug
> report i got was about GNU unary + which the info page describes thus:



>    Strings are not quoted for ‘expr’ itself, though you may need to
>   quote them to protect characters with special meaning to the shell,
>   e.g., spaces.  However, regardless of whether it is quoted, a string
>   operand should not be a parenthesis or any of ‘expr’’s operators like
>   ‘+’, so you cannot safely pass an arbitrary string ‘$str’ to expr merely
>   by quoting it to the shell.  One way to work around this is to use the
>   GNU extension ‘+’, (e.g., ‘+ "$str" = foo’); a more portable way is to
>   use ‘" $str"’ and to adjust the rest of the expression to take the
>   leading space into account (e.g., ‘" $str" = " foo"’).

That was confusing so I looked at the man page:

       + TOKEN
              interpret TOKEN as a string, even if it is a
              keyword like 'match' or an operator like '/'

  $ expr + match
  match
  $ expr 1+2
  1+2
  $ expr 1 +2
  expr: syntax error
  $ expr 1 + 2
  3

Seems straightforward enough. And now I remember why implementing $(( )) with
common plumbing with expr is still on my todo list. :)

> though i'm honestly not convinced this is their real problem. the
> specific example they give is this from scripts/Makefile.lib in the
> kernel:
> 
> size_append = printf $(shell \
> dec_size=0; \
> for F in $1; do \
>     fsize=$$($(CONFIG_SHELL) $(srctree)/scripts/file-size.sh $$F); \
>     dec_size=$$(expr $$dec_size + $$fsize); \
> done; \
> ...
> 
> but given the `dec_size=0` line, i think they're confused and whatever
> failure they're seeing is coming from somewhere else...

Fun. I'll throw it on the todo pile and shuffle "expr" near the top of the
promotion heap...

Thanks,

Rob