From ad1c2f39fda0acb51b5a93604c5e1a703b849a7d Mon Sep 17 00:00:00 2001 From: Ard Biesheuvel Date: Wed, 8 Dec 2021 08:42:03 +0100 Subject: [PATCH] ARM: ftrace: use ADD not POP to counter PUSH at entry The compiler emitted hook used for ftrace consists of a PUSH {LR} to preserve the link register, followed by a branch-and-link (BL) to __gnu_mount_nc. Dynamic ftrace patches away the latter to turn the combined sequence into a NOP, using a POP {LR} instruction. This is not necessary, since the link register does not get clobbered in this case, and simply adding #4 to the stack pointer is sufficient, and avoids a memory access that may take a few cycles to resolve depending on the micro-architecture. Signed-off-by: Ard Biesheuvel Reviewed-by: Nick Desaulniers Reviewed-by: Linus Walleij Reviewed-by: Steven Rostedt (Google) --- arch/arm/kernel/entry-ftrace.S | 2 +- arch/arm/kernel/ftrace.c | 15 +++++++++++++-- 2 files changed, 14 insertions(+), 3 deletions(-) diff --git a/arch/arm/kernel/entry-ftrace.S b/arch/arm/kernel/entry-ftrace.S index f4886fb..dca12a0 100644 --- a/arch/arm/kernel/entry-ftrace.S +++ b/arch/arm/kernel/entry-ftrace.S @@ -27,7 +27,7 @@ * allows it to be clobbered in subroutines and doesn't use it to hold * parameters.) * - * When using dynamic ftrace, we patch out the mcount call by a "pop {lr}" + * When using dynamic ftrace, we patch out the mcount call by a "add sp, #4" * instead of the __gnu_mcount_nc call (see arch/arm/kernel/ftrace.c). */ diff --git a/arch/arm/kernel/ftrace.c b/arch/arm/kernel/ftrace.c index a006585..811cadf 100644 --- a/arch/arm/kernel/ftrace.c +++ b/arch/arm/kernel/ftrace.c @@ -24,10 +24,21 @@ #include #include +/* + * The compiler emitted profiling hook consists of + * + * PUSH {LR} + * BL __gnu_mcount_nc + * + * To turn this combined sequence into a NOP, we need to restore the value of + * SP before the PUSH. Let's use an ADD rather than a POP into LR, as LR is not + * modified anyway, and reloading LR from memory is highly likely to be less + * efficient. + */ #ifdef CONFIG_THUMB2_KERNEL -#define NOP 0xf85deb04 /* pop.w {lr} */ +#define NOP 0xf10d0d04 /* add.w sp, sp, #4 */ #else -#define NOP 0xe8bd4000 /* pop {lr} */ +#define NOP 0xe28dd004 /* add sp, sp, #4 */ #endif #ifdef CONFIG_DYNAMIC_FTRACE -- 2.7.4