Subject: [PATCH 01/22] ARM: add mechanism for late code patching

From: [email-masked] (Cyril Chemparathy)
Date: Tue, 7 Aug 2012 18:52:53 -0400

Hi Nicolas,

On 8/4/2012 1:38 AM, Nicolas Pitre wrote:
[...]
>> extern unsigned __patch_table_begin, __patch_table_end;
>
> You could use "exttern void __patch_table_begin" so those symbols don't
> get any type that could be misused by mistake, while you still can take
> their addresses.
>

Looks like we'll have to stick with a non-void type here.  The compiler 
throws a warning when we try to take the address of a void.

[...]
> Did you verify with some test program that your patching routines do
> produce the same opcodes as the assembled equivalent for all possible
> shift values?  Especially for Thumb2 code which isn't as trivial to get
> right as the ARM one.
>

We've refactored the patching code into separate functions as:

static int do_patch_imm8_arm(u32 insn, u32 imm, u32 *ninsn);
static int do_patch_imm8_thumb(u32 insn, u32 imm, u32 *ninsn);

With this, the following test code has been used to verify the generated 
instruction encoding:

u32 arm_check[] = {
	0xe2810041, 0xe2810082, 0xe2810f41, 0xe2810f82, 0xe2810e41,
	0xe2810e82, 0xe2810d41, 0xe2810d82, 0xe2810c41, 0xe2810c82,
	0xe2810b41, 0xe2810b82, 0xe2810a41, 0xe2810a82, 0xe2810941,
	0xe2810982, 0xe2810841, 0xe2810882, 0xe2810741, 0xe2810782,
	0xe2810641, 0xe2810682, 0xe2810541, 0xe2810582, 0xe2810441,
};

u32 thumb_check[] = {
	0xf1010081, 0xf5017081, 0xf5017001, 0xf5016081, 0xf5016001,
	0xf5015081, 0xf5015001, 0xf5014081, 0xf5014001, 0xf5013081,
	0xf5013001, 0xf5012081, 0xf5012001, 0xf5011081, 0xf5011001,
	0xf5010081, 0xf5010001, 0xf1017081, 0xf1017001, 0xf1016081,
	0xf1016001, 0xf1015081, 0xf1015001, 0xf1014081, 0xf1014001,
};

int do_test(void)
{
	int i, ret;
	u32 ninsn, insn;
	
	insn = arm_check[0];
	for (i = 0; i < ARRAY_SIZE(arm_check); i++) {
		ret = do_patch_imm8_arm(insn, 0x41 << i, &ninsn);
		if (ret < 0)
			pr_err("patch failed at shift %d\n", i);
		if (ninsn != arm_check[i])
			pr_err("mismatch at %d, expect %x, got %x\n",
			       i, arm_check[i], ninsn);
	}

	insn = thumb_check[0];
	for (i = 0; i < ARRAY_SIZE(thumb_check); i++) {
		ret = do_patch_imm8_thumb(insn, 0x81 << i, &ninsn);
		if (ret < 0)
			pr_err("patch failed at shift %d\n", i);
		if (ninsn != thumb_check[i])
			pr_err("mismatch@%d, expect %x, got %x\n",
			       i, thumb_check[i], ninsn);
	}
}

Any ideas on improving these tests?

-- 
Thanks
- Cyril