Re: [PATCH v4 2/3] x86/alternative: Use helper functions for patching alternatives

[Borislav Petkov]

On Wed, Nov 19, 2025 at 05:04:19PM +0100, Juergen Gross wrote:
> Tidy up apply_alternatives() by moving the main patching action of a
> single alternative instance into 3 helper functions:
> 
> - analyze_patch_site() for selection whether patching should occur or
>   not and to handle nested alternatives.
> 
> - prep_patch_site() for applying any needed relocations and issuing
>   debug prints for the site.
> 
> - patch_site() doing the real patching action, including optimization
>   of any padding NOPs.
> 
> In prep_patch_site() use __apply_relocation() instead of
> text_poke_apply_relocation(), as the NOP optimization is now done
> in patch_site() for all cases.
> 
> Suggested-by: Borislav Petkov <bp@xxxxxxxxx>
> Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
> ---
> V3:
> - new patch
> V4:
> - further split coding in more helpers (Borislav Petkov)
> ---
>  arch/x86/kernel/alternative.c | 140 +++++++++++++++++++++-------------
>  1 file changed, 85 insertions(+), 55 deletions(-)

Better...

Some additional changes ontop:

- put the comment over the loop where it belongs

- name variables into something more descriptive than just a single letter.

With that I think it is starting to look better/more readable.

diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index 49a9eed24fdd..66868ecdebd2 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -594,25 +594,27 @@ struct patch_site {
 };
 
 static void __init_or_module analyze_patch_site(struct patch_site *ps,
-						struct alt_instr *p, struct alt_instr *end)
+						struct alt_instr *start,
+						struct alt_instr *end)
 {
 	struct alt_instr *r;
 
+	ps->instr = instr_va(start);
+	ps->len   = start->instrlen;
+
 	/*
 	 * In case of nested ALTERNATIVE()s the outer alternative might add
 	 * more padding. To ensure consistent patching find the max padding for
 	 * all alt_instr entries for this site (nested alternatives result in
 	 * consecutive entries).
 	 */
-	ps->instr = instr_va(p);
-	ps->len = p->instrlen;
-	for (r = p+1; r < end && instr_va(r) == ps->instr; r++) {
+	for (r = start+1; r < end && instr_va(r) == ps->instr; r++) {
 		ps->len = max(ps->len, r->instrlen);
-		p->instrlen = r->instrlen = ps->len;
+		start->instrlen = r->instrlen = ps->len;
 	}
 
 	BUG_ON(ps->len > sizeof(ps->buff));
-	BUG_ON(p->cpuid >= (NCAPINTS + NBUGINTS) * 32);
+	BUG_ON(start->cpuid >= (NCAPINTS + NBUGINTS) * 32);
 
 	/*
 	 * Patch if either:
@@ -620,43 +622,43 @@ static void __init_or_module analyze_patch_site(struct patch_site *ps,
 	 * - feature not present but ALT_FLAG_NOT is set to mean,
 	 *   patch if feature is *NOT* present.
 	 */
-	if (!boot_cpu_has(p->cpuid) == !(p->flags & ALT_FLAG_NOT))
+	if (!boot_cpu_has(start->cpuid) == !(start->flags & ALT_FLAG_NOT))
 		ps->alt = NULL;
 	else
-		ps->alt = p;
+		ps->alt = start;
 }
 
 static void __init_or_module prep_patch_site(struct patch_site *ps)
 {
-	struct alt_instr *p = ps->alt;
+	struct alt_instr *alt = ps->alt;
 	u8 buff_sz;
 	u8 *repl;
 
-	if (!p) {
+	if (!alt) {
 		/* Nothing to patch, use original instruction. */
 		memcpy(ps->buff, ps->instr, ps->len);
 		return;
 	}
 
-	repl = (u8 *)&p->repl_offset + p->repl_offset;
+	repl = (u8 *)&alt->repl_offset + alt->repl_offset;
 	DPRINTK(ALT, "feat: %d*32+%d, old: (%pS (%px) len: %d), repl: (%px, len: %d) flags: 0x%x",
-		p->cpuid >> 5, p->cpuid & 0x1f,
+		alt->cpuid >> 5, alt->cpuid & 0x1f,
 		ps->instr, ps->instr, ps->len,
-		repl, p->replacementlen, p->flags);
+		repl, alt->replacementlen, alt->flags);
 
-	memcpy(ps->buff, repl, p->replacementlen);
-	buff_sz = p->replacementlen;
+	memcpy(ps->buff, repl, alt->replacementlen);
+	buff_sz = alt->replacementlen;
 
-	if (p->flags & ALT_FLAG_DIRECT_CALL)
-		buff_sz = alt_replace_call(ps->instr, ps->buff, p);
+	if (alt->flags & ALT_FLAG_DIRECT_CALL)
+		buff_sz = alt_replace_call(ps->instr, ps->buff, alt);
 
 	for (; buff_sz < ps->len; buff_sz++)
 		ps->buff[buff_sz] = 0x90;
 
-	__apply_relocation(ps->buff, ps->instr, ps->len, repl, p->replacementlen);
+	__apply_relocation(ps->buff, ps->instr, ps->len, repl, alt->replacementlen);
 
 	DUMP_BYTES(ALT, ps->instr, ps->len, "%px:   old_insn: ", ps->instr);
-	DUMP_BYTES(ALT, repl, p->replacementlen, "%px:   rpl_insn: ", repl);
+	DUMP_BYTES(ALT, repl, alt->replacementlen, "%px:   rpl_insn: ", repl);
 	DUMP_BYTES(ALT, ps->buff, ps->len, "%px: final_insn: ", ps->instr);
 }
 

-- 
Regards/Gruss,
    Boris.

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