Re: [PATCH v6 08/13] arm64: expose user PAC bit positions via ptrace

On 12/7/18 12:39 PM, Kristina Martsenko wrote:

From: Mark Rutland <mark.rutland@arm.com>

When pointer authentication is in use, data/instruction pointers have a
number of PAC bits inserted into them. The number and position of these
bits depends on the configured TCR_ELx.TxSZ and whether tagging is
enabled. ARMv8.3 allows tagging to differ for instruction and data
pointers.

For userspace debuggers to unwind the stack and/or to follow pointer
chains, they need to be able to remove the PAC bits before attempting to
use a pointer.

This patch adds a new structure with masks describing the location of
the PAC bits in userspace instruction and data pointers (i.e. those
addressable via TTBR0), which userspace can query via PTRACE_GETREGSET.
By clearing these bits from pointers (and replacing them with the value
of bit 55), userspace can acquire the PAC-less versions.

This new regset is exposed when the kernel is built with (user) pointer
authentication support, and the address authentication feature is
enabled. Otherwise, the regset is hidden.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Kristina Martsenko <redacted>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Ramana Radhakrishnan <redacted>
Cc: Will Deacon <redacted>
---
 arch/arm64/include/asm/pointer_auth.h |  8 ++++++++
 arch/arm64/include/uapi/asm/ptrace.h  |  7 +++++++
 arch/arm64/kernel/ptrace.c            | 38 +++++++++++++++++++++++++++++++++++
 include/uapi/linux/elf.h              |  1 +
 4 files changed, 54 insertions(+)

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>


r~


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