[PATCH] Avoid using kprobe in the arm64 kretprobe trampoline

[William Cohen]

The aarch64 exception handling does not allow nesting of debug exceptions.
Using a kprobe in the kretprobe trampoline can cause problems if any of
the functions used by the kretprobe handler are instrumented with kprobes.
This problem was observed with the systemtap functioncallcount.stp
test when it instrumented the kfree function.  The kretprobe handler
under some conditions can all kfree.

Instead of using a kprobe to trigger an exception to save the processor state
and calling the kretprobe handler from the kprobei, the trampoline can
directly save the register state and call the kretprobe handler.  This
avoids the possible nested debug exceptions and reduces the overhead of
handling a kretprobe.  This code for the arm64 was based on the x86 code.

Signed-off-by: William Cohen <wcohen@xxxxxxxxxx>
---
 arch/arm64/kernel/kprobes-arm64.h | 43 +++++++++++++++++++++++++++++++++
 arch/arm64/kernel/kprobes.c       | 51 +++++++++++++++------------------------
 2 files changed, 63 insertions(+), 31 deletions(-)

diff --git a/arch/arm64/kernel/kprobes-arm64.h b/arch/arm64/kernel/kprobes-arm64.h
index ff8a55f..46dab24 100644
--- a/arch/arm64/kernel/kprobes-arm64.h
+++ b/arch/arm64/kernel/kprobes-arm64.h
@@ -27,4 +27,47 @@ extern kprobes_pstate_check_t * const kprobe_condition_checks[16];
 enum kprobe_insn __kprobes
 arm_kprobe_decode_insn(kprobe_opcode_t insn, struct arch_specific_insn *asi);
 
+#define SAVE_REGS_STRING\
+	"	stp x0, x1, [sp, #16 * 0]\n"	\
+	"	stp x2, x3, [sp, #16 * 1]\n"	\
+	"	stp x4, x5, [sp, #16 * 2]\n"	\
+	"	stp x6, x7, [sp, #16 * 3]\n"	\
+	"	stp x8, x9, [sp, #16 * 4]\n"	\
+	"	stp x10, x11, [sp, #16 * 5]\n"	\
+	"	stp x12, x13, [sp, #16 * 6]\n"	\
+	"	stp x14, x15, [sp, #16 * 7]\n"	\
+	"	stp x16, x17, [sp, #16 * 8]\n"	\
+	"	stp x18, x19, [sp, #16 * 9]\n"	\
+	"	stp x20, x21, [sp, #16 * 10]\n"	\
+	"	stp x22, x23, [sp, #16 * 11]\n"	\
+	"	stp x24, x25, [sp, #16 * 12]\n"	\
+	"	stp x26, x27, [sp, #16 * 13]\n"	\
+	"	stp x28, x29, [sp, #16 * 14]\n"	\
+	"	str x30,   [sp, #16 * 15]\n"    \
+	"	mrs x0, nzcv\n"			\
+	"	str x0, [sp, #8 * 33]\n"
+	
+
+#define RESTORE_REGS_STRING\
+	"	ldr x0, [sp, #8 * 33]\n"	\
+	"	msr nzcv, x0\n"			\
+	"	ldp x0, x1, [sp, #16 * 0]\n"	\
+	"	ldp x2, x3, [sp, #16 * 1]\n"	\
+	"	ldp x4, x5, [sp, #16 * 2]\n"	\
+	"	ldp x6, x7, [sp, #16 * 3]\n"	\
+	"	ldp x8, x9, [sp, #16 * 4]\n"	\
+	"	ldp x10, x11, [sp, #16 * 5]\n"	\
+	"	ldp x12, x13, [sp, #16 * 6]\n"	\
+	"	ldp x14, x15, [sp, #16 * 7]\n"	\
+	"	ldp x16, x17, [sp, #16 * 8]\n"	\
+	"	ldp x18, x19, [sp, #16 * 9]\n"	\
+	"	ldp x20, x21, [sp, #16 * 10]\n"	\
+	"	ldp x22, x23, [sp, #16 * 11]\n"	\
+	"	ldp x24, x25, [sp, #16 * 12]\n"	\
+	"	ldp x26, x27, [sp, #16 * 13]\n"	\
+	"	ldp x28, x29, [sp, #16 * 14]\n"	\
+	"	ldr x30,   [sp, #16 * 15]\n"	\
+
+
+
 #endif /* _ARM_KERNEL_KPROBES_ARM64_H */
diff --git a/arch/arm64/kernel/kprobes.c b/arch/arm64/kernel/kprobes.c
index d2aa4bc..af55e37 100644
--- a/arch/arm64/kernel/kprobes.c
+++ b/arch/arm64/kernel/kprobes.c
@@ -565,32 +565,27 @@ int __kprobes longjmp_break_handler(struct kprobe *p, struct pt_regs *regs)
 }
 
 /*
- * Kretprobes: kernel return probes handling
- *
- * AArch64 mode does not support popping the PC value from the
- * stack like on ARM 32-bit (ldmia {..,pc}), so atleast one
- * register need to be used to achieve branching/return.
- * It means return probes cannot return back to the original
- * return address directly without modifying the register context.
- *
- * So like other architectures, we prepare a global routine
- * with NOPs, which serve as trampoline address that hack away the
- * function return, with the exact register context.
- * Placing a kprobe on trampoline routine entry will trap again to
- * execute return probe handlers and restore original return address
- * in ELR_EL1, this way saved pt_regs still hold the original
- * register values to be carried back to the caller.
+ * When a retprobed function returns, this code saves registers and
+ * calls trampoline_handler() runs, which calls the kretprobe's handler.
  */
-static void __used kretprobe_trampoline_holder(void)
+static void __used __kprobes kretprobe_trampoline_holder(void)
 {
 	asm volatile (".global kretprobe_trampoline\n"
 			"kretprobe_trampoline:\n"
-			"NOP\n\t"
-			"NOP\n\t");
+		        "sub sp, sp, %0\n"
+			SAVE_REGS_STRING
+			"mov x0, sp\n"
+			"bl trampoline_probe_handler\n"
+			/* Replace trampoline address in lr with actual
+			   orig_ret_addr return address. */
+			"str x0, [sp, #16 * 15]\n"
+			RESTORE_REGS_STRING
+		        "add sp, sp, %0\n"
+			"ret\n"
+		      : : "I"(sizeof(struct pt_regs)) : "memory");
 }
 
-static int __kprobes
-trampoline_probe_handler(struct kprobe *p, struct pt_regs *regs)
+static void __kprobes __used *trampoline_probe_handler(struct pt_regs *regs)
 {
 	struct kretprobe_instance *ri = NULL;
 	struct hlist_head *head, empty_rp;
@@ -651,7 +646,7 @@ trampoline_probe_handler(struct kprobe *p, struct pt_regs *regs)
 	}
 
 	/* return 1 so that post handlers not called */
-	return 1;
+	return (void *) orig_ret_addr;
 }
 
 void __kprobes arch_prepare_kretprobe(struct kretprobe_instance *ri,
@@ -663,18 +658,12 @@ void __kprobes arch_prepare_kretprobe(struct kretprobe_instance *ri,
 	regs->regs[30] = (long)&kretprobe_trampoline;
 }
 
-static struct kprobe trampoline = {
-	.addr = (kprobe_opcode_t *) &kretprobe_trampoline,
-	.pre_handler = trampoline_probe_handler
-};
-
-int __kprobes arch_trampoline_kprobe(struct kprobe *p)
+int __init arch_init_kprobes(void)
 {
-	return p->addr == (kprobe_opcode_t *) &kretprobe_trampoline;
+	return 0;
 }
 
-int __init arch_init_kprobes(void)
+int arch_trampoline_kprobe(struct kprobe *p)
 {
-	/* register trampoline for kret probe */
-	return register_kprobe(&trampoline);
+	return 0;
 }
-- 
1.8.3.1


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