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

[Siddhesh Poyarekar <siddhesh at gotplt dot org>]

On 10/01/20 10:25 pm, Alexander Monakov wrote:
> On Fri, 10 Jan 2020, Siddhesh Poyarekar wrote:
> 
>> 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.
> 
> In the Linaro bugreport you mention,
> 
>> Basically, IFUNCs and freestanding don't mix.
> 
> but really any libc (Glibc included) and -nostartfiles don't mix: stdio
> won't be initialized, TLS won't be setup, and pretty much all other
> libc-internal datastructures won't be properly setup. Almost no libc functions
> are callable, because for example if they try to access 'errno', they crash.
> 
> Looking at the opening comment of the failing kselftest source:
> 
>  * This program tries to be as small as possible itself, to
>  * avoid perturbing the system memory utilization with its
>  * own execution.  It also attempts to have as few dependencies
>  * on kernel features as possible.
>  *
>  * It should be statically linked, with startup libs avoided.
>  * It uses no library calls, and only the following 3 syscalls:
>  *   sysinfo(), write(), and _exit()
> 
> so in fact allowing it to link with libc strlen would be contrary to its intent.
> The fix is simple: add -nodefaultlibs next to -nostartfiles in its Makefile, and
> write a trivial loop in place of __builtin_strlen.

That's a valid point.  I'll recommend dropping __builtin_strlen from
that test.

Thanks,
Siddhesh