static void __init_or_module __nospec_revert()

in kernel/nospec-branch.c [99:161]

• 52 lines of code
• 21 McCabe index (conditional complexity)

static void __init_or_module __nospec_revert(s32 *start, s32 *end)
{
	enum { BRCL_EXPOLINE, BRASL_EXPOLINE } type;
	static const u8 branch[] = { 0x47, 0x00, 0x07, 0x00 };
	u8 *instr, *thunk, *br;
	u8 insnbuf[6];
	s32 *epo;

	/* Second part of the instruction replace is always a nop */
	for (epo = start; epo < end; epo++) {
		instr = (u8 *) epo + *epo;
		if (instr[0] == 0xc0 && (instr[1] & 0x0f) == 0x04)
			type = BRCL_EXPOLINE;	/* brcl instruction */
		else if (instr[0] == 0xc0 && (instr[1] & 0x0f) == 0x05)
			type = BRASL_EXPOLINE;	/* brasl instruction */
		else
			continue;
		thunk = instr + (*(int *)(instr + 2)) * 2;
		if (thunk[0] == 0xc6 && thunk[1] == 0x00)
			/* exrl %r0,<target-br> */
			br = thunk + (*(int *)(thunk + 2)) * 2;
		else if (thunk[0] == 0xc0 && (thunk[1] & 0x0f) == 0x00 &&
			 thunk[6] == 0x44 && thunk[7] == 0x00 &&
			 (thunk[8] & 0x0f) == 0x00 && thunk[9] == 0x00 &&
			 (thunk[1] & 0xf0) == (thunk[8] & 0xf0))
			/* larl %rx,<target br> + ex %r0,0(%rx) */
			br = thunk + (*(int *)(thunk + 2)) * 2;
		else
			continue;
		/* Check for unconditional branch 0x07f? or 0x47f???? */
		if ((br[0] & 0xbf) != 0x07 || (br[1] & 0xf0) != 0xf0)
			continue;

		memcpy(insnbuf + 2, branch, sizeof(branch));
		switch (type) {
		case BRCL_EXPOLINE:
			insnbuf[0] = br[0];
			insnbuf[1] = (instr[1] & 0xf0) | (br[1] & 0x0f);
			if (br[0] == 0x47) {
				/* brcl to b, replace with bc + nopr */
				insnbuf[2] = br[2];
				insnbuf[3] = br[3];
			} else {
				/* brcl to br, replace with bcr + nop */
			}
			break;
		case BRASL_EXPOLINE:
			insnbuf[1] = (instr[1] & 0xf0) | (br[1] & 0x0f);
			if (br[0] == 0x47) {
				/* brasl to b, replace with bas + nopr */
				insnbuf[0] = 0x4d;
				insnbuf[2] = br[2];
				insnbuf[3] = br[3];
			} else {
				/* brasl to br, replace with basr + nop */
				insnbuf[0] = 0x0d;
			}
			break;
		}

		s390_kernel_write(instr, insnbuf, 6);
	}
}