[ARM] 3200/1: Singlestep over ARM BX and BLX instructions using ptrac…

…e fix

Patch from Nikola Valerjev

Single stepping an application using ptrace() fails over ARM instructions BX and BLX.

Steps to reproduce:

Compile and link the following files

main.c
-----
void foo();
int main() {
    foo();
    return 0;
}

foo.s
-----
	.text
	.globl foo
foo:
	BX LR

Using ptrace() functionality, run to main(), and start singlestepping.
Singlestep over \"BX LR\" instruction won\'t transfer the control back
to main, but run the code to completion.

This problems seems to be in the function get_branch_address() in
arch/arm/kernel/ptrace.c. The function doesn\'t seem to recognize BX
and BLX instructions as branches. BX and BLX instructions can be used
to convert from ARM to Thumb mode if the target address has the low
bit set. However, they are also perfectly legal in the ARM only mode.
Although other things in the kernel seem to indicate that only ARM
mode is accepted (and not Thumb), many compilers will generate BX
and BLX instructions even when generating ARM only code.

Signed-off-by: Nikola Valerjev <nikola@ghs.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

arch/arm/kernel/ptrace.c

@@ -242,6 +242,15 @@ get_branch_address(struct task_struct *child, unsigned long pc, unsigned long in
 		 */
 		long aluop1, aluop2, ccbit;
 
+	        if ((insn & 0x0fffffd0) == 0x012fff10) {
+		        /*
+			 * bx or blx
+			 */
+			alt = get_user_reg(child, insn & 15);
+			break;
+		}
+
+
 		if ((insn & 0xf000) != 0xf000)
 			break;