Re: [RFC PATCH 2/3] x86/alternative: add indirect call patching

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Jun 08, 2023 at 04:03:32PM +0200, 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@suse.com>

Since I wrecked all that with 270a69c4485d ("x86/alternative: Support
relocations in alternatives") I wrote you a replacement :-)

---
Index: linux-2.6/arch/x86/include/asm/alternative.h
===================================================================
--- linux-2.6.orig/arch/x86/include/asm/alternative.h
+++ linux-2.6/arch/x86/include/asm/alternative.h
@@ -10,6 +10,9 @@
 
 #define ALT_FLAG_NOT		(1 << 0)
 #define ALT_NOT(feature)	((ALT_FLAG_NOT << ALT_FLAGS_SHIFT) | (feature))
+#define ALT_FLAG_CALL		(1 << 1)
+#define ALT_CALL(feature)	((ALT_FLAG_CALL << ALT_FLAGS_SHIFT) | (feature))
+#define ALT_CALL_ALWAYS		ALT_CALL(X86_FEATURE_ALWAYS)
 
 #ifndef __ASSEMBLY__
 
@@ -150,6 +153,8 @@ static inline int alternatives_text_rese
 }
 #endif	/* CONFIG_SMP */
 
+#define ALT_CALL_INSTR		"call x86_BUG"
+
 #define b_replacement(num)	"664"#num
 #define e_replacement(num)	"665"#num
 
Index: linux-2.6/arch/x86/kernel/alternative.c
===================================================================
--- linux-2.6.orig/arch/x86/kernel/alternative.c
+++ linux-2.6/arch/x86/kernel/alternative.c
@@ -396,6 +396,40 @@ noinstr void x86_BUG(void)
 EXPORT_SYMBOL_GPL(x86_BUG);
 
 /*
+ * Rewrite the "call x86_BUG" replacement to point to the target of the
+ * indirect pv_ops call "call *disp(%ip)".
+ */
+static int alt_replace_call(u8 *instr, u8 *insn_buff, struct alt_instr *a)
+{
+	void *target, *bug = &x86_BUG;
+
+	if (a->replacementlen != 5 || insn_buff[0] != CALL_INSN_OPCODE) {
+		pr_err("Alternative: ALT_FLAG_CALL set for a non-call replacement instruction\n");
+		pr_err("  Ignoring the flag for the instruction at %pS (%px)\n", instr, instr);
+		return 5;
+	}
+
+	if (a->instrlen != 6 || instr[0] != 0xff || instr[1] != 0x15) {
+		pr_err("Alternative: ALT_FLAG_CALL set for unrecognized indirect call\n");
+		pr_err("  Not replacing the instruction at %pS (%px)\n", instr, instr);
+		return -1;
+	}
+
+	/* ff 15 00 00 00 00   call   *0x0(%rip) */
+	target = *(void **)(instr + a->instrlen + *(s32 *)(instr + 2));
+	if (!target)
+		target = bug;
+
+	/* (x86_BUG - .) + (target - x86_BUG) := target - . */
+	*(s32 *)(insn_buff + 1) += target - bug;
+
+	if (target == &x86_nop)
+		return 0;
+
+	return 5;
+}
+
+/*
  * Replace instructions with better alternatives for this CPU type. This runs
  * before SMP is initialized to avoid SMP problems with self modifying code.
  * This implies that asymmetric systems where APs have less capabilities than
@@ -451,6 +485,12 @@ void __init_or_module noinline apply_alt
 		memcpy(insn_buff, replacement, a->replacementlen);
 		insn_buff_sz = a->replacementlen;
 
+		if (a->flags & ALT_FLAG_CALL) {
+			insn_buff_sz = alt_replace_call(instr, insn_buff, a);
+			if (insn_buff_sz < 0)
+				continue;
+		}
+
 		for (; insn_buff_sz < a->instrlen; insn_buff_sz++)
 			insn_buff[insn_buff_sz] = 0x90;