Re: [PATCH 4.19 208/338] ARM: ftrace: avoid redundant loads or clobbering IP

[Ard Biesheuvel]

On Thu, 14 Apr 2022 at 15:23, Greg Kroah-Hartman
<gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
>
> From: Ard Biesheuvel <ardb@xxxxxxxxxx>
>
> [ Upstream commit d11967870815b5ab89843980e35aab616c97c463 ]
>

NAK. Please don't backport these patches to -stable, I thought I had
been clear on this.


> Tweak the ftrace return paths to avoid redundant loads of SP, as well as
> unnecessary clobbering of IP.
>
> This also fixes the inconsistency of using MOV to perform a function
> return, which is sub-optimal on recent micro-architectures but more
> importantly, does not perform an interworking return, unlike compiler
> generated function returns in Thumb2 builds.
>
> Let's fix this by popping PC from the stack like most ordinary code
> does.
>
> Signed-off-by: Ard Biesheuvel <ardb@xxxxxxxxxx>
> Reviewed-by: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>
> Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
> ---
>  arch/arm/kernel/entry-ftrace.S | 51 +++++++++++++++-------------------
>  1 file changed, 22 insertions(+), 29 deletions(-)
>
> diff --git a/arch/arm/kernel/entry-ftrace.S b/arch/arm/kernel/entry-ftrace.S
> index 1acf4d05e94c..393c342ecd51 100644
> --- a/arch/arm/kernel/entry-ftrace.S
> +++ b/arch/arm/kernel/entry-ftrace.S
> @@ -41,10 +41,7 @@
>   * mcount can be thought of as a function called in the middle of a subroutine
>   * call.  As such, it needs to be transparent for both the caller and the
>   * callee: the original lr needs to be restored when leaving mcount, and no
> - * registers should be clobbered.  (In the __gnu_mcount_nc implementation, we
> - * clobber the ip register.  This is OK because the ARM calling convention
> - * allows it to be clobbered in subroutines and doesn't use it to hold
> - * parameters.)
> + * registers should be clobbered.
>   *
>   * When using dynamic ftrace, we patch out the mcount call by a "mov r0, r0"
>   * for the mcount case, and a "pop {lr}" for the __gnu_mcount_nc case (see
> @@ -96,26 +93,25 @@
>
>  .macro __ftrace_regs_caller
>
> -       sub     sp, sp, #8      @ space for PC and CPSR OLD_R0,
> +       str     lr, [sp, #-8]!  @ store LR as PC and make space for CPSR/OLD_R0,
>                                 @ OLD_R0 will overwrite previous LR
>
> -       add     ip, sp, #12     @ move in IP the value of SP as it was
> -                               @ before the push {lr} of the mcount mechanism
> +       ldr     lr, [sp, #8]    @ get previous LR
>
> -       str     lr, [sp, #0]    @ store LR instead of PC
> +       str     r0, [sp, #8]    @ write r0 as OLD_R0 over previous LR
>
> -       ldr     lr, [sp, #8]    @ get previous LR
> +       str     lr, [sp, #-4]!  @ store previous LR as LR
>
> -       str     r0, [sp, #8]    @ write r0 as OLD_R0 over previous LR
> +       add     lr, sp, #16     @ move in LR the value of SP as it was
> +                               @ before the push {lr} of the mcount mechanism
>
> -       stmdb   sp!, {ip, lr}
> -       stmdb   sp!, {r0-r11, lr}
> +       push    {r0-r11, ip, lr}
>
>         @ stack content at this point:
>         @ 0  4          48   52       56            60   64    68       72
> -       @ R0 | R1 | ... | LR | SP + 4 | previous LR | LR | PSR | OLD_R0 |
> +       @ R0 | R1 | ... | IP | SP + 4 | previous LR | LR | PSR | OLD_R0 |
>
> -       mov r3, sp                              @ struct pt_regs*
> +       mov     r3, sp                          @ struct pt_regs*
>
>         ldr r2, =function_trace_op
>         ldr r2, [r2]                            @ pointer to the current
> @@ -138,11 +134,9 @@ ftrace_graph_regs_call:
>  #endif
>
>         @ pop saved regs
> -       ldmia   sp!, {r0-r12}                   @ restore r0 through r12
> -       ldr     ip, [sp, #8]                    @ restore PC
> -       ldr     lr, [sp, #4]                    @ restore LR
> -       ldr     sp, [sp, #0]                    @ restore SP
> -       mov     pc, ip                          @ return
> +       pop     {r0-r11, ip, lr}                @ restore r0 through r12
> +       ldr     lr, [sp], #4                    @ restore LR
> +       ldr     pc, [sp], #12
>  .endm
>
>  #ifdef CONFIG_FUNCTION_GRAPH_TRACER
> @@ -158,11 +152,9 @@ ftrace_graph_regs_call:
>         bl      prepare_ftrace_return
>
>         @ pop registers saved in ftrace_regs_caller
> -       ldmia   sp!, {r0-r12}                   @ restore r0 through r12
> -       ldr     ip, [sp, #8]                    @ restore PC
> -       ldr     lr, [sp, #4]                    @ restore LR
> -       ldr     sp, [sp, #0]                    @ restore SP
> -       mov     pc, ip                          @ return
> +       pop     {r0-r11, ip, lr}                @ restore r0 through r12
> +       ldr     lr, [sp], #4                    @ restore LR
> +       ldr     pc, [sp], #12
>
>  .endm
>  #endif
> @@ -273,16 +265,17 @@ ENDPROC(ftrace_graph_caller_old)
>  .endm
>
>  .macro mcount_exit
> -       ldmia   sp!, {r0-r3, ip, lr}
> -       ret     ip
> +       ldmia   sp!, {r0-r3}
> +       ldr     lr, [sp, #4]
> +       ldr     pc, [sp], #8
>  .endm
>
>  ENTRY(__gnu_mcount_nc)
>  UNWIND(.fnstart)
>  #ifdef CONFIG_DYNAMIC_FTRACE
> -       mov     ip, lr
> -       ldmia   sp!, {lr}
> -       ret     ip
> +       push    {lr}
> +       ldr     lr, [sp, #4]
> +       ldr     pc, [sp], #8
>  #else
>         __mcount
>  #endif
> --
> 2.34.1
>
>
>