[PATCH v2 2/8] newlib: libc: strcmp M-profile PACBTI-enablement
Victor L. Do Nascimento
victor.donascimento@arm.com
Fri Aug 5 10:51:47 GMT 2022
Richard Earnshaw <Richard.Earnshaw@foss.arm.com> writes:
> On 03/08/2022 16:35, Victor Do Nascimento wrote:
>> Add function prologue/epilogue to conditionally add BTI landing pads
>> and/or PAC code generation & authentication instructions depending on
>> compilation flags.
>> This patch enables PACBTI for all relevant variants of strcmp:
>> * Newlib for armv8.1-m.main+pacbti
>> * Newlib for armv8.1-m.main+pacbti+mve
>> * Newlib-nano
>> ---
>> newlib/libc/machine/arm/strcmp-arm-tiny.S | 7 +++-
>> newlib/libc/machine/arm/strcmp-armv7.S | 45 +++++++++++++++++------
>> newlib/libc/machine/arm/strcmp-armv7m.S | 36 +++++++++++++++---
>> 3 files changed, 71 insertions(+), 17 deletions(-)
>> diff --git a/newlib/libc/machine/arm/strcmp-arm-tiny.S
>> b/newlib/libc/machine/arm/strcmp-arm-tiny.S
>> index 607a41daf..8085eb4df 100644
>> --- a/newlib/libc/machine/arm/strcmp-arm-tiny.S
>> +++ b/newlib/libc/machine/arm/strcmp-arm-tiny.S
>> @@ -29,10 +29,14 @@
>> /* Tiny version of strcmp in ARM state. Used only when optimizing
>> for size. Also supports Thumb-2. */
>> +#include "pacbti.h"
>> +
>> .syntax unified
>> def_fn strcmp
>> + .fnstart
>> .cfi_sections .debug_frame
>> .cfi_startproc
>> + pacbti_prologue
>
> See comment on patch1 about the names of these macros.
>
>> 1:
>> ldrb r2, [r0], #1
>> ldrb r3, [r1], #1
>> @@ -42,6 +46,7 @@ def_fn strcmp
>> beq 1b
>> 2:
>> subs r0, r2, r3
>> - bx lr
>> + pacbti_epilogue
>> .cfi_endproc
>> + .fnend
>> .size strcmp, . - strcmp
>> diff --git a/newlib/libc/machine/arm/strcmp-armv7.S b/newlib/libc/machine/arm/strcmp-armv7.S
>> index 2f93bfb73..584c89f7e 100644
>> --- a/newlib/libc/machine/arm/strcmp-armv7.S
>> +++ b/newlib/libc/machine/arm/strcmp-armv7.S
>> @@ -45,6 +45,8 @@
>> .thumb
>> .syntax unified
>> +#include "pacbti.h"
>> +
>> /* Parameters and result. */
>> #define src1 r0
>> #define src2 r1
>> @@ -91,8 +93,9 @@
>> ldrd r4, r5, [sp], #16
>> .cfi_restore 4
>> .cfi_restore 5
>> + .cfi_adjust_cfa_offset -16
>
> This looks like it's an omission from the original file and thus
> should be separated out as a separate patch.
>
>> sub result, result, r1, lsr #24
>> - bx lr
>> + pacbti_epilogue
>> #else
>> /* To use the big-endian trick we'd have to reverse all three words.
>> that's slower than this approach. */
>> @@ -112,22 +115,28 @@
>> ldrd r4, r5, [sp], #16
>> .cfi_restore 4
>> .cfi_restore 5
>> + .cfi_adjust_cfa_offset -16
>> sub result, result, r1
>> - bx lr
>> + pacbti_epilogue
>> #endif
>> .endm
>> +
>> .text
>> .p2align 5
>> + .fnstart
>> + .cfi_sections .debug_frame
>> + .cfi_startproc
>> .Lstrcmp_start_addr:
>> #ifndef STRCMP_NO_PRECHECK
>> .Lfastpath_exit:
>> sub r0, r2, r3
>> - bx lr
>> + pacbti_epilogue
>> nop
>> #endif
>> def_fn strcmp
>> + pacbti_prologue
>> #ifndef STRCMP_NO_PRECHECK
>> ldrb r2, [src1]
>> ldrb r3, [src2]
>> @@ -136,16 +145,26 @@ def_fn strcmp
>> cmpcs r2, r3
>> bne .Lfastpath_exit
>> #endif
>> - .cfi_sections .debug_frame
>> - .cfi_startproc
>> strd r4, r5, [sp, #-16]!
>> - .cfi_def_cfa_offset 16
>> + .save {r4, r5}
>
> Hmm, I've just remembered that leaf functions cannot throw exceptions
> (the EHABI only supports synchronous exceptions), so I don't think we
> need .save directives at all. Instead, such functions should be
> marked with ".cantunwind". You still want the dwarf unwind
> information as that's for the debugger, it's just the '.save'
> directives that aren't needed.
>
>> + .cfi_adjust_cfa_offset 16
>> +#if __HAVE_PAC_LEAF
>> + .cfi_offset 4, -20
>> + .cfi_offset 5, -16
>> +#else
>> .cfi_offset 4, -16
>> .cfi_offset 5, -12
>> +#endif /* __HAVE_PAC_LEAF */
>
> Ugh! This is not the right way to do this. Instead (see comments on
> patch 1) use a macro that defines the additional adjustment to use in
> various contexts, something like
>
> .cfi_offset 4, -(16+PAC_UNALIGNED_CFI_ADJ)
>
>> orr tmp1, src1, src2
>> strd r6, r7, [sp, #8]
>> + .save {r6, r7}
>> +#if __HAVE_PAC_LEAF
>> + .cfi_offset 6, -12
>> + .cfi_offset 7, -8
>> +#else
>> .cfi_offset 6, -8
>> .cfi_offset 7, -4
>> +#endif /* __HAVE_PAC_LEAF */
>> mvn const_m1, #0
>> lsl r2, tmp1, #29
>> cbz r2, .Lloop_aligned8
>> @@ -270,7 +289,6 @@ def_fn strcmp
>> ldr data1, [src1], #4
>> beq .Laligned_m2
>> bcs .Laligned_m1
>> -
>> #ifdef STRCMP_NO_PRECHECK
>> ldrb data2, [src2, #1]
>> uxtb tmp1, data1, ror #BYTE1_OFFSET
>> @@ -314,7 +332,8 @@ def_fn strcmp
>> mov result, tmp1
>> ldr r4, [sp], #16
>> .cfi_restore 4
>> - bx lr
>> + .cfi_adjust_cfa_offset -16
>> + pacbti_epilogue
>> #ifndef STRCMP_NO_PRECHECK
>> .Laligned_m1:
>> @@ -364,8 +383,9 @@ def_fn strcmp
>> /* R6/7 Not used in this sequence. */
>> .cfi_restore 6
>> .cfi_restore 7
>> + .cfi_adjust_cfa_offset -16
>> neg result, result
>> - bx lr
>> + pacbti_epilogue
>> 6:
>> .cfi_restore_state
>> @@ -441,7 +461,8 @@ def_fn strcmp
>> /* R6/7 not used in this sequence. */
>> .cfi_restore 6
>> .cfi_restore 7
>> - bx lr
>> + .cfi_adjust_cfa_offset -16
>> + pacbti_epilogue
>> .Lstrcmp_tail:
>> .cfi_restore_state
>> @@ -463,7 +484,9 @@ def_fn strcmp
>> /* R6/7 not used in this sequence. */
>> .cfi_restore 6
>> .cfi_restore 7
>> + .cfi_adjust_cfa_offset -16
>> sub result, result, data2, lsr #24
>> - bx lr
>> + pacbti_epilogue
>> .cfi_endproc
>> + .fnend
>> .size strcmp, . - .Lstrcmp_start_addr
>> diff --git a/newlib/libc/machine/arm/strcmp-armv7m.S b/newlib/libc/machine/arm/strcmp-armv7m.S
>> index cdb4912df..fe1519f4d 100644
>> --- a/newlib/libc/machine/arm/strcmp-armv7m.S
>> +++ b/newlib/libc/machine/arm/strcmp-armv7m.S
>> @@ -29,6 +29,8 @@
>> /* Very similar to the generic code, but uses Thumb2 as implemented
>> in ARMv7-M. */
>> +#include "pacbti.h"
>> +
>> /* Parameters and result. */
>> #define src1 r0
>> #define src2 r1
>> @@ -44,8 +46,10 @@
>> .thumb
>> .syntax unified
>> def_fn strcmp
>> + .fnstart
>> .cfi_sections .debug_frame
>> .cfi_startproc
>> + pacbti_prologue
>> eor tmp1, src1, src2
>> tst tmp1, #3
>> /* Strings not at same byte offset from a word boundary. */
>> @@ -106,7 +110,7 @@ def_fn strcmp
>> lsrs result, result, #24
>> subs result, result, data2
>> #endif
>> - bx lr
>> + pacbti_epilogue
>> #if 0
>> @@ -214,12 +218,18 @@ def_fn strcmp
>> cmpcs data1, data2
>> beq .Lstrcmp_unaligned
>> sub result, data1, data2
>> - bx lr
>> + pacbti_epilogue
>> 2:
>> stmfd sp!, {r5}
>> - .cfi_def_cfa_offset 4
>> + .save {r5}
>> + .cfi_adjust_cfa_offset 4
>> +#if __HAVE_PAC_LEAF
>> + .cfi_offset 5, -8 /* Account for ip register already on stack. */
>> +#else
>> .cfi_offset 5, -4
>> +#endif /* __HAVE_PAC_LEAF */
>> +
>> ldr data1, [src1], #4
>> and tmp2, src2, #3
>> @@ -353,10 +363,17 @@ def_fn strcmp
>> .Lstrcmp_done_equal:
>> mov result, #0
>> .cfi_remember_state
>> +#if __HAVE_PAC_LEAF
>> + pop {r5, ip}
>> + .cfi_restore 5
>> + .cfi_restore 143
>> + aut ip, lr, sp
>> +#else
>> ldmfd sp!, {r5}
>> .cfi_restore 5
>> +#endif /* __HAVE_PAC_LEAF */
>
> I think we could define a macro for this as well, something like
> pop_reg_and_pac r5
This sounds good, but a survey through relevant function epilogues
reveals a series of different `pop' scenarios. On the one hand, we have
pop {r5, ip}, as we have here but on another we also have, for example,
pop {r4,r5,r6,r7,ip}.
Do we wish to provide a short-hand notation for one pop operation but
not the other via macros? My initial understanding is that assembler
macros don't provide us with the necessary flexibility to handle all
relevant cases.
I think the use of a macro here would greatly enhance code readability
and by extension maintainability, but at first sight it feels inelegant
to provide a solution applicable only to a single case of a more general
problem.
Any wisdom here would be appreciated.
V.
> While you're doing that, please change the single-register ldmfd into 'pop'.
>
>> .cfi_def_cfa_offset 0
>> - bx lr
>> + bx lr
>
> stray change to the BX instruction.
>
>> .Lstrcmp_tail:
>> .cfi_restore_state
>> @@ -370,9 +387,18 @@ def_fn strcmp
>> S2LOEQ data2, data2, #8
>> beq .Lstrcmp_tail
>> sub result, r2, result
>> +#if __HAVE_PAC_LEAF
>> + pop {r5, ip}
>> + .cfi_restore 5
>> + .cfi_restore 143
>> + .cfi_def_cfa_offset 0
>> + aut ip, lr, sp
>> +#else
>> ldmfd sp!, {r5}
>> .cfi_restore 5
>> .cfi_def_cfa_offset 0
>> - bx lr
>> +#endif /* __HAVE_PAC_LEAF */
>
> Same as above
>
>> + bx lr
>
> tab between mnemonic and operands.
>
>> .cfi_endproc
>> + .fnend
>> .size strcmp, . - strcmp
>
> R.
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