Re: [PATCH v5 3/5] x86/alternative: add indirect call patching

[Borislav Petkov]

On Wed, Nov 29, 2023 at 02:33:30PM +0100, Juergen Gross wrote:
> In order to prepare replacing of paravirt patching with alternative
> patching, add the capability to replace an indirect call with a direct
> one to alternative patching.
> 
> This is done via a new flag ALT_FLAG_CALL as the target of the call
> instruction needs to be evaluated using the value of the location
> addressed by the indirect call. For convenience add a macro for a
> default call instruction. In case it is being used without the new
> flag being set, it will result in a BUG() when being executed. As in
> most cases the feature used will be X86_FEATURE_ALWAYS add another
> macro ALT_CALL_ALWAYS usable for the flags parameter of the ALTERNATIVE
> macros.
> 
> For a complete replacement handle the special cases of calling a nop
> function and an indirect call of NULL the same way as paravirt does.
> 
> Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>

That SOB chain basically says that you're PeterZ and you're sending this
patch. :)

Are you trying to say that he co-developed it or suggested it or
Originally-by?

Documentation/process/submitting-patches.rst has it all.

Also, what I did ontop of this is below as we must dump the full flags
now with the debug output since you're adding more flags than ALT_NOT.

Also, the naked magic numbers need explanation.

Please include into your next submission.

Thx.

diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index e7e53b9e801b..888205234f15 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -395,6 +395,9 @@ noinstr void BUG_func(void)
 }
 EXPORT_SYMBOL_GPL(BUG_func);
 
+#define CALL_RIP_REL_PREFIX	0xff
+#define CALL_RIP_REL_MODRM	0x15
+
 /*
  * Rewrite the "call BUG_func" replacement to point to the target of the
  * indirect pv_ops call "call *disp(%ip)".
@@ -409,11 +412,14 @@ static int alt_replace_call(u8 *instr, u8 *insn_buff, struct alt_instr *a)
 		BUG();
 	}
 
-	if (a->instrlen != 6 || instr[0] != 0xff || instr[1] != 0x15) {
+	if (a->instrlen != 6 ||
+	    instr[0] != CALL_RIP_REL_PREFIX ||
+	    instr[1] != CALL_RIP_REL_MODRM) {
 		pr_err("ALT_FLAG_DIRECT_CALL set for unrecognized indirect call\n");
 		BUG();
 	}
 
+	/* Skip CALL_RIP_REL_PREFIX and CALL_RIP_REL_MODRM */
 	disp = *(s32 *)(instr + 2);
 #ifdef CONFIG_X86_64
 	/* ff 15 00 00 00 00   call   *0x0(%rip) */
@@ -493,12 +499,11 @@ void __init_or_module noinline apply_alternatives(struct alt_instr *start,
 			continue;
 		}
 
-		DPRINTK(ALT, "feat: %s%d*32+%d, old: (%pS (%px) len: %d), repl: (%px, len: %d)",
-			(a->flags & ALT_FLAG_NOT) ? "!" : "",
+		DPRINTK(ALT, "feat: %d32+%d, old: (%pS (%px) len: %d), repl: (%px, len: %d) flags: 0x%x",
 			a->cpuid >> 5,
 			a->cpuid & 0x1f,
 			instr, instr, a->instrlen,
-			replacement, a->replacementlen);
+			replacement, a->replacementlen, a->flags);
 
 		memcpy(insn_buff, replacement, a->replacementlen);
 		insn_buff_sz = a->replacementlen;

-- 
Regards/Gruss,
    Boris.

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