In case of high register pressure in PIC mode, address of the stack
protector's guard can be spilled on ARM targets as shown in PR85434,
thus allowing an attacker to control what the canary would be compared
against. ARM does lack stack_protect_set and stack_protect_test insn
patterns, defining them does not help as the address is expanded
regularly and the patterns only deal with the copy and test of the
guard with the canary.

This problem does not occur for x86 targets because the PIC access and
the test can be done in the same instruction. Aarch64 is exempt too
because PIC access insn pattern are mov of UNSPEC which prevents it from
the second access in the epilogue being CSEd in cse_local pass with the
first access in the prologue.

The approach followed here is to create new "combined" set and test
standard pattern names that take the unexpanded guard and do the set or
test. This allows the target to use an opaque pattern (eg. using UNSPEC)
to hide the individual instructions being generated to the compiler and
split the pattern into generic load, compare and branch instruction
after register allocator, therefore avoiding any spilling. This is here
implemented for the ARM targets. For targets not implementing these new
standard pattern names, the existing stack_protect_set and
stack_protect_test pattern names are used.

To be able to split PIC access after register allocation, the functions
had to be augmented to force a new PIC register load and to control
which register it loads into. This is because sharing the PIC register
between prologue and epilogue could lead to spilling due to CSE again
which an attacker could use to control what the canary gets compared
against.

ChangeLog entries are as follows:

*** gcc/ChangeLog ***

2018-07-05  Thomas Preud'homme  <thomas.preudhomme@linaro.org>

    PR target/85434
    * target-insns.def (stack_protect_combined_set): Define new standard
    pattern name.
    (stack_protect_combined_test): Likewise.
    * cfgexpand.c (stack_protect_prologue): Try new
    stack_protect_combined_set pattern first.
    * function.c (stack_protect_epilogue): Try new
    stack_protect_combined_test pattern first.
    * config/arm/arm.c (require_pic_register): Add pic_reg and compute_now
    parameters to control which register to use as PIC register and force
    reloading PIC register respectively.
    (legitimize_pic_address): Expose above new parameters in prototype and
    adapt recursive calls accordingly.
    (arm_legitimize_address): Adapt to new legitimize_pic_address
    prototype.
    (thumb_legitimize_address): Likewise.
    (arm_emit_call_insn): Adapt to new require_pic_register prototype.
    * config/arm/arm-protos.h (legitimize_pic_address): Adapt to prototype
    change.
    * config/arm/arm.md (movsi expander): Adapt to legitimize_pic_address
    prototype change.
    (stack_protect_combined_set): New insn_and_split pattern.
    (stack_protect_set): New insn pattern.
    (stack_protect_combined_test): New insn_and_split pattern.
    (stack_protect_test): New insn pattern.
    * config/arm/unspecs.md (UNSPEC_SP_SET): New unspec.
    (UNSPEC_SP_TEST): Likewise.
    * doc/md.texi (stack_protect_combined_set): Document new standard
    pattern name.
    (stack_protect_set): Clarify that the operand for guard's address is
    legal.
    (stack_protect_combined_test): Document new standard pattern name.
    (stack_protect_test): Clarify that the operand for guard's address is
    legal.

*** gcc/testsuite/ChangeLog ***

2018-07-05  Thomas Preud'homme  <thomas.preudhomme@linaro.org>

    PR target/85434
    * gcc.target/arm/pr85434.c: New test.

Testing: Bootstrapped on ARM in both Arm and Thumb-2 mode as well as on
Aarch64. Testsuite shows no regression on these 3 variants either both
with default flags and with -fstack-protector-all.

Is this ok for trunk? If yes, would this be acceptable as a backport to
GCC 6, 7 and 8 provided that no regression is found?

Best regards,

Thomas