Commit d2408e04 authored by Borislav Petkov (AMD)'s avatar Borislav Petkov (AMD)
Browse files

x86/alternative: Optimize returns patching 



Instead of decoding each instruction in the return sites range only to
realize that that return site is a jump to the default return thunk
which is needed - X86_FEATURE_RETHUNK is enabled - lift that check
before the loop and get rid of that loop overhead.

Add comments about what gets patched, while at it.

Signed-off-by: default avatarBorislav Petkov (AMD) <bp@alien8.de>
Acked-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20230512120952.7924-1-bp@alien8.de
parent b6c881b2

arch/x86/kernel/alternative.c

+10 −3
Original line number Diff line number Diff line
@@ -693,13 +693,12 @@ static int patch_return(void *addr, struct insn *insn, u8 *bytes) 
{

	int i = 0;



	/* Patch the custom return thunks... */

	if (cpu_feature_enabled(X86_FEATURE_RETHUNK)) {

		if (x86_return_thunk == __x86_return_thunk)

			return -1;



		i = JMP32_INSN_SIZE;

		__text_gen_insn(bytes, JMP32_INSN_OPCODE, addr, x86_return_thunk, i);

	} else {

		/* ... or patch them out if not needed. */

		bytes[i++] = RET_INSN_OPCODE;

	}
@@ -712,6 +711,14 @@ void __init_or_module noinline apply_returns(s32 *start, s32 *end) 
{

	s32 *s;



	/*

	 * Do not patch out the default return thunks if those needed are the

	 * ones generated by the compiler.

	 */

	if (cpu_feature_enabled(X86_FEATURE_RETHUNK) &&

	    (x86_return_thunk == __x86_return_thunk))

		return;



	for (s = start; s < end; s++) {

		void *dest = NULL, *addr = (void *)s + *s;

		struct insn insn;