On 2/14/24 3:55 PM, Richard Earnshaw (lists) wrote:
> On 14/02/2024 09:20, Tejas Belagod wrote:
>> On 2/7/24 11:41 PM, Richard Earnshaw (lists) wrote:
>>> On 07/02/2024 07:59, Tejas Belagod wrote:
>>>> This patch fixes a bug that causes indirect calls in PAC-enabled functions
>>>> to be tailcalled incorrectly when all argument registers R0-R3 are used.
>>>>
>>>> Tested on arm-none-eabi for armv8.1-m.main. OK for trunk?
>>>>
>>>> 2024-02-07  Tejas Belagod  <tejas.belagod@arm.com>
>>>>
>>>>      PR target/113780
>>>>      * gcc/config/arm.cc (arm_function_ok_for_sibcall): Don't allow tailcalls
>>>>        for indirect calls with 4 or more arguments in pac-enabled functions.
>>>>
>>>>      * gcc.target/arm/pac-sibcall.c: New.
>>>> ---
>>>>    gcc/config/arm/arm.cc                      | 12 ++++++++----
>>>>    gcc/testsuite/gcc.target/arm/pac-sibcall.c | 11 +++++++++++
>>>>    2 files changed, 19 insertions(+), 4 deletions(-)
>>>>    create mode 100644 gcc/testsuite/gcc.target/arm/pac-sibcall.c
>>>>
>>>> diff --git a/gcc/config/arm/arm.cc b/gcc/config/arm/arm.cc
>>>> index c44047c377a..c1f8286a4d4 100644
>>>> --- a/gcc/config/arm/arm.cc
>>>> +++ b/gcc/config/arm/arm.cc
>>>> @@ -7980,10 +7980,14 @@ arm_function_ok_for_sibcall (tree decl, tree exp)
>>>>          && DECL_WEAK (decl))
>>>>        return false;
>>>>    -  /* We cannot do a tailcall for an indirect call by descriptor if all the
>>>> -     argument registers are used because the only register left to load the
>>>> -     address is IP and it will already contain the static chain.  */
>>>> -  if (!decl && CALL_EXPR_BY_DESCRIPTOR (exp) && !flag_trampolines)
>>>> +  /* We cannot do a tailcall for an indirect call by descriptor or for an
>>>> +     indirect call in a pac-enabled function if all the argument registers
>>>> +     are used because the only register left to load the address is IP and
>>>> +     it will already contain the static chain or the PAC signature in the
>>>> +     case of PAC-enabled functions.  */
>>>
>>> This comment is becoming a bit unwieldy.  I suggest restructuring it as:
>>>
>>> We cannot tailcall an indirect call by descriptor if all the call-clobbered
>>> general registers are live (r0-r3 and ip).  This can happen when:
>>>     - IP contains the static chain, or
>>>     - IP is needed for validating the PAC signature.
>>>
>>>
>>>> +  if (!decl
>>>> +      && ((CALL_EXPR_BY_DESCRIPTOR (exp) && !flag_trampolines)
>>>> +      || arm_current_function_pac_enabled_p()))
>>>>        {
>>>>          tree fntype = TREE_TYPE (TREE_TYPE (CALL_EXPR_FN (exp)));
>>>>          CUMULATIVE_ARGS cum;
>>>> diff --git a/gcc/testsuite/gcc.target/arm/pac-sibcall.c b/gcc/testsuite/gcc.target/arm/pac-sibcall.c
>>>> new file mode 100644
>>>> index 00000000000..c57bf7a952c
>>>> --- /dev/null
>>>> +++ b/gcc/testsuite/gcc.target/arm/pac-sibcall.c
>>>> @@ -0,0 +1,11 @@
>>>> +/* Testing return address signing.  */
>>>> +/* { dg-do compile } */
>>>> +/* { dg-require-effective-target mbranch_protection_ok } */
>>>> +/* { dg-options " -mcpu=cortex-m85 -mbranch-protection=pac-ret+leaf -O2" } */
>>>
>>> No, you can't just add options like this, you need to first check that they won't result in conflicts with other options on the command line.  See https://gcc.gnu.org/pipermail/gcc-patches/2024-January/644077.html for an example of how to handle this.
>>>
>> Thanks for the review, Richard. Respin attached.
>>
>> Thanks,
>> Tejas.
>>
>>>> +
>>>> +void fail(void (*f)(int, int, int, int))
>>>> +{
>>>> +  f(1, 2, 3, 4);
>>>> +}
>>>> +
>>>> +/* { dg-final { scan-assembler-not "bx\tip\t@ indirect register sibling call" } } */
>>>
>>> R.
>>>
> +++ b/gcc/testsuite/gcc.target/arm/pac-sibcall.c
> @@ -0,0 +1,14 @@
> +/* If all call-clobbered general registers are live (r0-r3, ip), disable
> +   indirect tail-call for a PAC-enabled function.  */
> +
> +/* { dg-do compile } */
> +/* { dg-require-effective-target mbranch_protection_ok } */
> This only checks if -mbranch-protection can work with the existing architecture/cpu; not with the flags you're about to add below.  You should check for arm_arch_v8_1m_main_pacbti_ok instead; then you can assume that -mbranch-protection can be added.
> 

Indeed! Thanks for catching that.

> +/* { dg-add-options arm_arch_v8_1m_main_pacbti } */
> +/* { dg-additional-options "-mbranch-protection=pac-ret+leaf -O2" } */
> 
> Otherwise this is OK if you fix the above.
> 

Thanks Richard. Respin attached. Will apply.

Thanks,
Tejas.

> R.