arm64: Kprobes with single stepping support

Add support for basic kernel probes(kprobes) and jump probes
(jprobes) for ARM64.

Kprobes utilizes software breakpoint and single step debug
exceptions supported on ARM v8.

A software breakpoint is placed at the probe address to trap the
kernel execution into the kprobe handler.

ARM v8 supports enabling single stepping before the break exception
return (ERET), with next PC in exception return address (ELR_EL1). The
kprobe handler prepares an executable memory slot for out-of-line
execution with a copy of the original instruction being probed, and
enables single stepping. The PC is set to the out-of-line slot address
before the ERET. With this scheme, the instruction is executed with the
exact same register context except for the PC (and DAIF) registers.

Debug mask (PSTATE.D) is enabled only when single stepping a recursive
kprobe, e.g.: during kprobes reenter so that probed instruction can be
single stepped within the kprobe handler -exception- context.
The recursion depth of kprobe is always 2, i.e. upon probe re-entry,
any further re-entry is prevented by not calling handlers and the case
counted as a missed kprobe).

Single stepping from the x-o-l slot has a drawback for PC-relative accesses
like branching and symbolic literals access as the offset from the new PC
(slot address) may not be ensured to fit in the immediate value of
the opcode. Such instructions need simulation, so reject
probing them.

Instructions generating exceptions or cpu mode change are rejected
for probing.

Exclusive load/store instructions are rejected too.  Additionally, the
code is checked to see if it is inside an exclusive load/store sequence
(code from Pratyush).

System instructions are mostly enabled for stepping, except MSR/MRS
accesses to "DAIF" flags in PSTATE, which are not safe for
probing.

This also changes arch/arm64/include/asm/ptrace.h to use
include/asm-generic/ptrace.h.

Thanks to Steve Capper and Pratyush Anand for several suggested
Changes.

Signed-off-by: Sandeepa Prabhu <sandeepa.s.prabhu@gmail.com>
Signed-off-by: David A. Long <dave.long@linaro.org>
Signed-off-by: Pratyush Anand <panand@redhat.com>
Acked-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

1 files changed, 12 insertions, 2 deletions

diff --git a/arch/arm64/include/asm/ptrace.h b/arch/arm64/include/asm/ptrace.h
index 941e12d235be..baf938135986 100644
--- a/arch/arm64/include/asm/ptrace.h
+++ b/arch/arm64/include/asm/ptrace.h
@@ -147,9 +147,12 @@ struct pt_regs {
 #define fast_interrupts_enabled(regs) \
 	(!((regs)->pstate & PSR_F_BIT))
 
-#define user_stack_pointer(regs) \
+#define GET_USP(regs) \
 	(!compat_user_mode(regs) ? (regs)->sp : (regs)->compat_sp)
 
+#define SET_USP(ptregs, value) \
+	(!compat_user_mode(regs) ? ((regs)->sp = value) : ((regs)->compat_sp = value))
+
 extern int regs_query_register_offset(const char *name);
 extern const char *regs_query_register_name(unsigned int offset);
 extern unsigned long regs_get_kernel_stack_nth(struct pt_regs *regs,
@@ -206,8 +209,15 @@ static inline unsigned long regs_return_value(struct pt_regs *regs)
 struct task_struct;
 int valid_user_regs(struct user_pt_regs *regs, struct task_struct *task);
 
-#define instruction_pointer(regs)	((unsigned long)(regs)->pc)
+#define GET_IP(regs)		((unsigned long)(regs)->pc)
+#define SET_IP(regs, value)	((regs)->pc = ((u64) (value)))
+
+#define GET_FP(ptregs)		((unsigned long)(ptregs)->regs[29])
+#define SET_FP(ptregs, value)	((ptregs)->regs[29] = ((u64) (value)))
+
+#include <asm-generic/ptrace.h>
 
+#undef profile_pc
 extern unsigned long profile_pc(struct pt_regs *regs);
 
 #endif /* __ASSEMBLY__ */