Re: [PATCH v3 1/2] aarch64: Use generic argv adjustment in ld.so [BZ #23293]

[Florian Weimer <fweimer@redhat.com>]

* Szabolcs Nagy via Libc-alpha:

> When an executable is invoked as
>
>   ./ld.so [ld.so-args] ./exe [exe-args]
>
> then the argv is adujusted in ld.so before calling the entry point of
> the executable so ld.so args are not visible to it.  On most targets
> this requires moving argv, env and auxv on the stack to ensure correct
> stack alignment at the entry point.  This had several issues:
>
> - The code for this adjustment on the stack is written in asm as part
>   of the target specific ld.so _start code which is hard to maintain.
>
> - The adjustment is done after _dl_start returns, where it's too late
>   to update GLRO(dl_auxv), as it is already readonly, so it points to
>   memory that was clobbered by the adjustment. This is bug 23293.
>
> - _environ is also wrong in ld.so after the adjustment, but it is
>   likely not used after _dl_start returns so this is not user visible.
>
> - _dl_argv was updated, but for this it was moved out of relro, which
>   changes security properties across targets unnecessarily.
>
> This patch introduces a generic _dl_start_args_adjust function that
> handles the argument adjustments after ld.so processed its own args
> and before relro protection is applied.  It sets _dl_skip_args to 0 so
> the existing adjustment in asm is not invoked.  Each target has to
> opt-in to use this new adjustment since some targets don't need it.
> Once all targets are updated, _dl_argv declaration can be simplified.
>
> A new _dl_start_argptr was introduced because the original sp is not
> passed to dl_main which now has to do the adjustments.
>
> --
> v2:
> - use p != NULL, and a_type != AT_NULL
> - remove the confusing paragraph from the commit message.

Looks good to me.

Reviewed-by: Florian Weimer <fweimer@redhat.com>

Thanks,
Florian