Re: Extending clone_args for clone3()

[Yury Khrustalev <yury.khrustalev@arm.com>]

Hi everyone,

A gentle ping :)

On Mon, May 19, 2025 at 03:06:29PM +0100, Yury Khrustalev wrote:
> Hi,
> 
> I'm working on an RFC patch for Glibc to make use of the newly added
> shadow_stack_token field in struct clone_args in [1] on arm64 targets.
> 
> I encountered the following problem. Glibc might be built with newer
> version of struct clone_args than the currently running kernel. In
> this case, we may attempt to use a non-zero value in the new field
> in args (and pass size bigger than expected by the kernel) and the
> kernel will reject the syscall with E2BIG error.
> 
> This seems to be due to a fail-early approach. The unexpected non-
> zero values beyond what's supported by the kernel may indicate that
> userspace expects something to happen (and may even have allocated
> some resources). So it's better to indicate a problem rather than
> silently ignore this and have userspace encounter an error later.
> 
> However, it creates difficulty with using extended "versions" of
> the clone3 syscall. AFAIK, there is no way to ask kernel about
> the supported size of struct clone_args except for making syscalls
> with decreasing value of size until we stop getting E2BIG.
> 
> This seems fragile and may call for writing cumbersome code. In essence,
> we will have to have clone30(), clone31(), clone32()... wrappers which
> probably defeats the point of why clone3 was added:
> 
> 
>   if (clone32_supported && clone32(...) == -1 && errno == E2BIG)
>     {
>       clone32_supported = false;
>       /* ... */
>     }
>   else if (clone31_supported && clone31(...) == -1 && errno == E2BIG)
>     {
>       clone12_supported = false;
>       /* ... */
>     }
>  ...
> 
> Is there a neat way to work around this? What was the idea for extending
> clone_args in practice?
> 
> I suppose we can't rely on kernel version because support for extended
> clone_args can be backported. In any case, we'd have to do a syscall
> for this (it would probably be great to have kernel version in auxv).
> 
> I appreciate any advice here.
> 
> Thanks,
> Yury
> 
> 
> [1]: https://lore.kernel.org/all/20250416-clone3-shadow-stack-v16-0-2ffc9ca3917b@kernel.org/
> 

Kind regards,
Yury