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

[enh]

(fwiw, with an instrumented build of the 4.19 Android common kernel, i
only see `expr 4 * 65536 + 019 * 256 + 037`. so i don't know what
they're _really_ doing to hit this.)

On Thu, May 2, 2019 at 9:15 AM Rob Landley <rob at landley.net> wrote:
>
> 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
> _______________________________________________
> Toybox mailing list
> Toybox at lists.landley.net
> http://lists.landley.net/listinfo.cgi/toybox-landley.net