ARM: 6668/1: ptrace: remove single-step emulation code

PTRACE_SINGLESTEP is a ptrace request designed to offer single-stepping
support to userspace when the underlying architecture has hardware
support for this operation.

On ARM, we set arch_has_single_step() to 1 and attempt to emulate hardware
single-stepping by disassembling the current instruction to determine the
next pc and placing a software breakpoint on that location.

Unfortunately this has the following problems:

1.) Only a subset of ARMv7 instructions are supported
2.) Thumb-2 is unsupported
3.) The code is not SMP safe

We could try to fix this code, but it turns out that because of the above
issues it is rarely used in practice.  GDB, for example, uses PTRACE_POKETEXT
and PTRACE_PEEKTEXT to manage breakpoints itself and does not require any
kernel assistance.

This patch removes the single-step emulation code from ptrace meaning that
the PTRACE_SINGLESTEP request will return -EIO on ARM. Portable code must
check the return value from a ptrace call and handle the failure gracefully.

Acked-by: Nicolas Pitre <nicolas.pitre@linaro.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

3 files changed, 1 insertions, 14 deletions

diff --git a/arch/arm/include/asm/processor.h b/arch/arm/include/asm/processor.h
index 67357baaeeeb..b439b41aeac1 100644
--- a/arch/arm/include/asm/processor.h
+++ b/arch/arm/include/asm/processor.h
@@ -29,19 +29,7 @@
 #define STACK_TOP_MAX   TASK_SIZE
 #endif
 
-union debug_insn {
-        u32     arm;
-        u16     thumb;
-};
-
-struct debug_entry {
-        u32                     address;
-        union debug_insn        insn;
-};
-
 struct debug_info {
-        int                     nsaved;
-        struct debug_entry      bp[2];
 #ifdef CONFIG_HAVE_HW_BREAKPOINT
         struct perf_event       *hbp[ARM_MAX_HBP_SLOTS];
 #endif
diff --git a/arch/arm/include/asm/ptrace.h b/arch/arm/include/asm/ptrace.h
index 783d50f32618..a8ff22b2a391 100644
--- a/arch/arm/include/asm/ptrace.h
+++ b/arch/arm/include/asm/ptrace.h
@@ -130,8 +130,6 @@ struct pt_regs {
 
 #ifdef __KERNEL__
 
-#define arch_has_single_step()  (1)
-
 #define user_mode(regs) \
         (((regs)->ARM_cpsr & 0xf) == 0)
 
diff --git a/arch/arm/include/asm/traps.h b/arch/arm/include/asm/traps.h
index 1b960d5ef6a5..f90756dc16dc 100644
--- a/arch/arm/include/asm/traps.h
+++ b/arch/arm/include/asm/traps.h
@@ -45,6 +45,7 @@ static inline int in_exception_text(unsigned long ptr)
 
 extern void __init early_trap_init(void);
 extern void dump_backtrace_entry(unsigned long where, unsigned long from, unsigned long frame);
+extern void ptrace_break(struct task_struct *tsk, struct pt_regs *regs);
 
 extern void *vectors_page;