Re: [RFC] builtin functions and `-ffreestanding -nostartfies` with static binaries

[Segher Boessenkool <segher@kernel.crashing.org>]

Hi!

On Fri, Jan 10, 2020 at 09:50:48PM +0530, Siddhesh Poyarekar wrote:
> Statically built independent programs that implement their own program
> entry points (i.e. -ffreestanding -nostartfiles) and call __builtin_*
> functions break when the builtin function in question is implemented as
> an IFUNC in glibc and the builtin results in a glibc call instead of
> some inline code.
> 
> This happens because the startup code where ifuncs are resolved never
> gets executed (since glibc's startup code is never executed) and hence
> the PLT jumps fail.  The bug report talks about this as an aarch64
> problem but I've been able to reproduce the problem on x86_64 as well.
> One just needs to make sure that the __builtin_foo call results in a
> glibc call.

-ffreestanding means you might not have any of the C standard library,
and -nostartfiles means you do not do any of the standard initialisation.
Why then would you expect any ifunc to work?

> I spent some time thinking about this and while it's trivial to fix by
> disabling ifuncs for static glibc, I wanted a solution that wasn't such
> a big hammer.  The other alternative I could think of is to have an
> exported alias (called __builtin_strlen for example instead of strlen)
> of a default implementation of the builtin function in glibc that gcc
> generates a call to if freestanding && nostartfiles && static.
> 
> Any thoughts or other ideas on how this could be implemented?

Why do you not want the startfiles, but do want their effects?


Segher