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Re: [RFC] builtin functions and `-ffreestanding -nostartfies` with static binaries
- From: Segher Boessenkool <segher at kernel dot crashing dot org>
- To: Siddhesh Poyarekar <siddhesh at gotplt dot org>
- Cc: gcc at gcc dot gnu dot org, GLIBC Devel <libc-alpha at sourceware dot org>
- Date: Sat, 11 Jan 2020 07:25:09 -0600
- Subject: Re: [RFC] builtin functions and `-ffreestanding -nostartfies` with static binaries
- References: <email@example.com>
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?