Richard Earnshaw <richard.earns...@foss.arm.com> writes:
> On 14/09/2022 15:20, Andrea Corallo via Gcc-patches wrote:
>> Hi all,
>>
>> this patch enables address return signature and verification based on
>> Armv8.1-M Pointer Authentication [1].
>>
>> To sign the return address, we use the PAC R12, LR, SP instruction
>> upon function entry. This is signing LR using SP and storing the
>> result in R12. R12 will be pushed into the stack.
>>
>> During function epilogue R12 will be popped and AUT R12, LR, SP will
>> be used to verify that the content of LR is still valid before return.
>>
>> Here an example of PAC instrumented function prologue and epilogue:
>>
>> void foo (void);
>>
>> int main()
>> {
>> foo ();
>> return 0;
>> }
>>
>> Compiled with '-march=armv8.1-m.main -mbranch-protection=pac-ret
>> -mthumb' translates into:
>>
>> main:
>> pac ip, lr, sp
>> push {r3, r7, ip, lr}
>> add r7, sp, #0
>> bl foo
>> movs r3, #0
>> mov r0, r3
>> pop {r3, r7, ip, lr}
>> aut ip, lr, sp
>> bx lr
>>
>> The patch also takes care of generating a PACBTI instruction in place
>> of the sequence BTI+PAC when Branch Target Identification is enabled
>> contextually.
>>
>> Ex. the previous example compiled with '-march=armv8.1-m.main
>> -mbranch-protection=pac-ret+bti -mthumb' translates into:
>>
>> main:
>> pacbti ip, lr, sp
>> push {r3, r7, ip, lr}
>> add r7, sp, #0
>> bl foo
>> movs r3, #0
>> mov r0, r3
>> pop {r3, r7, ip, lr}
>> aut ip, lr, sp
>> bx lr
>>
>> As part of previous upstream suggestions a test for varargs has been
>> added and '-mtpcs-frame' is deemed being incompatible with this return
>> signing address feature being introduced.
>>
>> [1]
>> <https://community.arm.com/developer/ip-products/processors/b/processors-ip-blog/posts/armv8-1-m-pointer-authentication-and-branch-target-identification-extension>
>>
>> gcc/Changelog
>>
>> 2021-11-03 Andrea Corallo <andrea.cora...@arm.com>
>>
>> * config/arm/arm.c: (arm_compute_frame_layout)
>> (arm_expand_prologue, thumb2_expand_return, arm_expand_epilogue)
>> (arm_conditional_register_usage): Update for pac codegen.
>> (arm_current_function_pac_enabled_p): New function.
>> * config/arm/arm.md (pac_ip_lr_sp, pacbti_ip_lr_sp, aut_ip_lr_sp):
>> Add new patterns.
>> * config/arm/unspecs.md (UNSPEC_PAC_IP_LR_SP)
>> (UNSPEC_PACBTI_IP_LR_SP, UNSPEC_AUT_IP_LR_SP): Add unspecs.
>>
>> gcc/testsuite/Changelog
>>
>> 2021-11-03 Andrea Corallo <andrea.cora...@arm.com>
>>
>> * gcc.target/arm/pac.h : New file.
>> * gcc.target/arm/pac-1.c : New test case.
>> * gcc.target/arm/pac-2.c : Likewise.
>> * gcc.target/arm/pac-3.c : Likewise.
>> * gcc.target/arm/pac-4.c : Likewise.
>> * gcc.target/arm/pac-5.c : Likewise.
>> * gcc.target/arm/pac-6.c : Likewise.
>> * gcc.target/arm/pac-7.c : Likewise.
>> * gcc.target/arm/pac-8.c : Likewise.
>>
>
> + if (arm_current_function_pac_enabled_p () && !(arm_arch7 &&
> arm_arch_cmse))
> + error ("This architecture does not support branch protection
> instructions");
>
> This test feels wrong. What does having cmse give us? I suspect you
> want a test that ensures we have at least v8-m.main so that the NOP
> instructions are correctly defined as NOPs (or, in this case, PACBTI
> instructions) rather than unpredictable; but if that's the case then I
> think you really want to write the test that way here (perhaps in a
> macro) and then move this test into that so that it becomes
> self-documenting - but don't we have a v8-m.main test anyway?
Yep
> + if (arm_current_function_pac_enabled_p ())
> + {
> + gcc_assert (!(saved_regs_mask & (1 << PC_REGNUM)));
> + arm_emit_multi_reg_pop (saved_regs_mask);
> + emit_insn (gen_aut_nop ());
> + emit_jump_insn (simple_return_rtx);
> + }
>
> The assert is using indents that are just spaces, but the other lines
> use tabs. Please use tabs everywhere rather than mixing like this.
Ack.
> +/* Return TRUE if return address signing mechanism is enabled. */
> +bool
> +arm_current_function_pac_enabled_p (void)
> +{
> + return aarch_ra_sign_scope == AARCH_FUNCTION_ALL
> + || (aarch_ra_sign_scope == AARCH_FUNCTION_NON_LEAF
> + && !crtl->is_leaf);
> +}
>
> This is a case where you should use parenthesis around the expression so
> that the continuation lines are correctly indented.
Ack.
> @@ -11518,7 +11518,7 @@ (define_expand "prologue"
> arm_expand_prologue ();
> else
> thumb1_expand_prologue ();
> - DONE;
> + DONE;
> "
> )
>
> Although this is a trivial cleanup, it has nothing to do with this
> patch. Please remove.
Okay.
> + "arm_arch7 && arm_arch_cmse"
>
> See my comments earlier about this test; the same applies here.
>
> + (unspec:SI [(reg:SI SP_REGNUM) (reg:SI LR_REGNUM)]
> + UNSPEC_PAC_NOP))]
> +
> Again you have a mix of lines indented with tabs and lines indented with
> just spaces. Similarly with pacbti_nop and aut_nop.
>
> Do you have a test for the nested functions case (I can't see it, but
> perhaps I've missed it somewhere)?
We have gcc/testsuite/gcc.target/arm/pac-7.c added by this patch.
> R.
Andrea
