[PATCH v2 09/12] ARM: kprobes: treat R7 as the frame pointer register in Thumb2 builds

[Ard Biesheuvel]

Thumb2 code uses R7 as the frame pointer rather than R11, because the
opcodes to access it are generally shorter.

This means that there are cases where we cannot simply add it to the
clobber list of an asm() block, but need to preserve/restore it
explicitly, or the compiler may complain in some cases (e.g., Clang
builds with ftrace enabled).

Since R11 is not special in that case, clobber it instead, and use it to
preserve/restore the value of R7.

Signed-off-by: Ard Biesheuvel <ardb at kernel.org>
---
 arch/arm/include/asm/opcodes.h           | 14 ++++++++
 arch/arm/probes/kprobes/actions-common.c |  6 ++--
 arch/arm/probes/kprobes/actions-thumb.c  | 38 ++++++++++++++++----
 3 files changed, 48 insertions(+), 10 deletions(-)

diff --git a/arch/arm/include/asm/opcodes.h b/arch/arm/include/asm/opcodes.h
index 38e3eabff5c3..9a6362408ea0 100644
--- a/arch/arm/include/asm/opcodes.h
+++ b/arch/arm/include/asm/opcodes.h
@@ -230,4 +230,18 @@ extern __u32 __opcode_to_mem_thumb32(__u32);
 	".short " __stringify(first) ", " __stringify(second) "\n\t"
 #endif
 
+/*
+ * Which register to preserve and which register can be clobbered in inline asm
+ * that needs to be compatible with code that emits frame pointers.
+ */
+#ifdef CONFIG_THUMB2_KERNEL
+#define FPREG_PRESERVE	"r7"
+#define FPREG_CLOBBER	"r11"
+#define FPREG_PRESERVE_R7
+#else
+#define FPREG_PRESERVE	"fp"
+#define FPREG_CLOBBER	"r7"
+#define FPREG_PRESERVE_R11
+#endif
+
 #endif /* __ASM_ARM_OPCODES_H */
diff --git a/arch/arm/probes/kprobes/actions-common.c b/arch/arm/probes/kprobes/actions-common.c
index 836aebe596cd..f0efe16e2fdb 100644
--- a/arch/arm/probes/kprobes/actions-common.c
+++ b/arch/arm/probes/kprobes/actions-common.c
@@ -84,7 +84,7 @@ emulate_generic_r0_12_noflags(probes_opcode_t insn,
 	register void *rfn asm("lr") = asi->insn_fn;
 
 	__asm__ __volatile__ (
-		"stmdb	sp!, {%[regs], r11}	\n\t"
+		"stmdb	sp!, {%[regs], " FPREG_PRESERVE "}\n\t"
 		"ldmia	%[regs], {r0-r12}	\n\t"
 #if __LINUX_ARM_ARCH__ >= 6
 		"blx	%[fn]			\n\t"
@@ -96,10 +96,10 @@ emulate_generic_r0_12_noflags(probes_opcode_t insn,
 #endif
 		"ldr	lr, [sp], #4		\n\t" /* lr = regs */
 		"stmia	lr, {r0-r12}		\n\t"
-		"ldr	r11, [sp], #4		\n\t"
+		"ldr	" FPREG_PRESERVE ", [sp], #4\n\t"
 		: [regs] "=r" (rregs), [fn] "=r" (rfn)
 		: "0" (rregs), "1" (rfn)
-		: "r0", "r2", "r3", "r4", "r5", "r6", "r7",
+		: "r0", "r2", "r3", "r4", "r5", "r6", FPREG_CLOBBER,
 		  "r8", "r9", "r10", "r12", "memory", "cc"
 		);
 }
diff --git a/arch/arm/probes/kprobes/actions-thumb.c b/arch/arm/probes/kprobes/actions-thumb.c
index 7884fcb81c26..873757326533 100644
--- a/arch/arm/probes/kprobes/actions-thumb.c
+++ b/arch/arm/probes/kprobes/actions-thumb.c
@@ -447,14 +447,20 @@ t16_emulate_loregs(probes_opcode_t insn,
 
 	__asm__ __volatile__ (
 		"msr	cpsr_fs, %[oldcpsr]	\n\t"
+#ifdef FPREG_PRESERVE_R7
+		"mov	fp, r7			\n\t"
+#endif
 		"ldmia	%[regs], {r0-r7}	\n\t"
 		"blx	%[fn]			\n\t"
 		"stmia	%[regs], {r0-r7}	\n\t"
+#ifdef FPREG_PRESERVE_R7
+		"mov	r7, fp			\n\t"
+#endif
 		"mrs	%[newcpsr], cpsr	\n\t"
 		: [newcpsr] "=r" (newcpsr)
 		: [oldcpsr] "r" (oldcpsr), [regs] "r" (regs),
 		  [fn] "r" (asi->insn_fn)
-		: "r0", "r1", "r2", "r3", "r4", "r5", "r6", "r7",
+		: "r0", "r1", "r2", "r3", "r4", "r5", "r6", FPREG_CLOBBER,
 		  "lr", "memory", "cc"
 		);
 
@@ -524,15 +530,21 @@ t16_emulate_push(probes_opcode_t insn,
 		struct arch_probes_insn *asi, struct pt_regs *regs)
 {
 	__asm__ __volatile__ (
+#ifdef FPREG_PRESERVE_R7
+		"mov	fp, r7			\n\t"
+#endif
 		"ldr	r9, [%[regs], #13*4]	\n\t"
 		"ldr	r8, [%[regs], #14*4]	\n\t"
 		"ldmia	%[regs], {r0-r7}	\n\t"
 		"blx	%[fn]			\n\t"
 		"str	r9, [%[regs], #13*4]	\n\t"
+#ifdef FPREG_PRESERVE_R7
+		"mov	r7, fp			\n\t"
+#endif
 		:
 		: [regs] "r" (regs), [fn] "r" (asi->insn_fn)
-		: "r0", "r1", "r2", "r3", "r4", "r5", "r6", "r7", "r8", "r9",
-		  "lr", "memory", "cc"
+		: "r0", "r1", "r2", "r3", "r4", "r5", "r6", "r8", "r9",
+		  FPREG_CLOBBER, "lr", "memory", "cc"
 		);
 }
 
@@ -558,15 +570,21 @@ t16_emulate_pop_nopc(probes_opcode_t insn,
 		struct arch_probes_insn *asi, struct pt_regs *regs)
 {
 	__asm__ __volatile__ (
+#ifdef FPREG_PRESERVE_R7
+		"mov	fp, r7			\n\t"
+#endif
 		"ldr	r9, [%[regs], #13*4]	\n\t"
 		"ldmia	%[regs], {r0-r7}	\n\t"
 		"blx	%[fn]			\n\t"
 		"stmia	%[regs], {r0-r7}	\n\t"
 		"str	r9, [%[regs], #13*4]	\n\t"
+#ifdef FPREG_PRESERVE_R7
+		"mov	r7, fp			\n\t"
+#endif
 		:
 		: [regs] "r" (regs), [fn] "r" (asi->insn_fn)
-		: "r0", "r1", "r2", "r3", "r4", "r5", "r6", "r7", "r9",
-		  "lr", "memory", "cc"
+		: "r0", "r1", "r2", "r3", "r4", "r5", "r6", "r9",
+		  FPREG_CLOBBER, "lr", "memory", "cc"
 		);
 }
 
@@ -577,15 +595,21 @@ t16_emulate_pop_pc(probes_opcode_t insn,
 	register unsigned long pc asm("r8");
 
 	__asm__ __volatile__ (
+#ifdef FPREG_PRESERVE_R7
+		"mov	fp, r7			\n\t"
+#endif
 		"ldr	r9, [%[regs], #13*4]	\n\t"
 		"ldmia	%[regs], {r0-r7}	\n\t"
 		"blx	%[fn]			\n\t"
 		"stmia	%[regs], {r0-r7}	\n\t"
 		"str	r9, [%[regs], #13*4]	\n\t"
+#ifdef FPREG_PRESERVE_R7
+		"mov	r7, fp			\n\t"
+#endif
 		: "=r" (pc)
 		: [regs] "r" (regs), [fn] "r" (asi->insn_fn)
-		: "r0", "r1", "r2", "r3", "r4", "r5", "r6", "r7", "r9",
-		  "lr", "memory", "cc"
+		: "r0", "r1", "r2", "r3", "r4", "r5", "r6", "r9",
+		  FPREG_CLOBBER, "lr", "memory", "cc"
 		);
 
 	bx_write_pc(pc, regs);
-- 
2.30.2