arm64: signal: Ensure signal delivery failure is recoverable

arm64: signal: Ensure signal delivery failure is recoverable

Commit eaf62ce1563b ("arm64/signal: Set up and restore the GCS
context for signal handlers") introduced a potential failure point
at the end of setup_return(). This is unfortunate as it is too late
to deliver a SIGSEGV: if that SIGSEGV is handled, the subsequent
sigreturn will end up returning to the original handler, which is
not the intention (since we failed to deliver that signal).

Make sure this does not happen by calling gcs_signal_entry()
at the very beginning of setup_return(), and add a comment just
after to discourage error cases being introduced from that point
onwards.

While at it, also take care of copy_siginfo_to_user(): since it may
fail, we shouldn't be calling it after setup_return() either. Call
it before setup_return() instead, and move the setting of X1/X2
inside setup_return() where it belongs (after the "point of no
failure").

Background: the first part of setup_rt_frame(), including
setup_sigframe(), has no impact on the execution of the interrupted
thread. The signal frame is written to the stack, but the stack
pointer remains unchanged. Failure at this stage can be recovered by
a SIGSEGV handler, and sigreturn will restore the original context,
at the point where the original signal occurred. On the other hand,
once setup_return() has updated registers including SP, the thread's
control flow has been modified and we must deliver the original
signal.

Fixes: eaf62ce1563b ("arm64/signal: Set up and restore the GCS context for signal handlers")
Signed-off-by: Kevin Brodsky <kevin.brodsky@arm.com>
Reviewed-by: Dave Martin <Dave.Martin@arm.com>
Reviewed-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20241210160940.2031997-1-kevin.brodsky@arm.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>