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x86/alternative: Optimize returns patching

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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: Borislav Petkov (AMD) <bp@alien8.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20230512120952.7924-1-bp@alien8.de
This commit is contained in:
Borislav Petkov (AMD) 2023-05-12 14:05:11 +02:00
parent b6c881b248
commit d2408e043e
1 changed files with 10 additions and 3 deletions
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13
arch/x86/kernel/alternative.c
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@ -693,13 +693,12 @@ static int patch_return(void *addr, struct insn *insn, u8 *bytes)
{ {
int i = 0; int i = 0;
/* Patch the custom return thunks... */
if (cpu_feature_enabled(X86_FEATURE_RETHUNK)) { if (cpu_feature_enabled(X86_FEATURE_RETHUNK)) {
if (x86_return_thunk == __x86_return_thunk)
return -1;
i = JMP32_INSN_SIZE; i = JMP32_INSN_SIZE;
__text_gen_insn(bytes, JMP32_INSN_OPCODE, addr, x86_return_thunk, i); __text_gen_insn(bytes, JMP32_INSN_OPCODE, addr, x86_return_thunk, i);
} else { } else {
/* ... or patch them out if not needed. */
bytes[i++] = RET_INSN_OPCODE; bytes[i++] = RET_INSN_OPCODE;
} }
@ -712,6 +711,14 @@ void __init_or_module noinline apply_returns(s32 *start, s32 *end)
{ {
s32 *s; 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++) { for (s = start; s < end; s++) {
void *dest = NULL, *addr = (void *)s + *s; void *dest = NULL, *addr = (void *)s + *s;
struct insn insn; struct insn insn;