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For AArch64 a restricted ptrace call will result in an error when
trying to access PAC keys. Malicious code could derive the key via
the system call and potentially defeat any PAC control-flow
integrity checks. The ptrace call to set registers, PTRACE_SETREGSET,
is already blocked, so they cannot be changed.

Change-Id: I6954e3b038912871a964d413a305c69f8a57d543
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2438130Reviewed-by: Robert Sesek <rsesek@chromium.org>
Reviewed-by: Matthew Denton <mpdenton@chromium.org>
Commit-Queue: Richard Townsend <richard.townsend@arm.com>
Commit-Queue: Robert Sesek <rsesek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#813031}