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Message-ID: <20251119160420.22160-3-jgross@suse.com>
Date: Wed, 19 Nov 2025 17:04:19 +0100
From: Juergen Gross <jgross@...e.com>
To: linux-kernel@...r.kernel.org,
x86@...nel.org
Cc: Juergen Gross <jgross@...e.com>,
Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>,
Borislav Petkov <bp@...en8.de>,
Dave Hansen <dave.hansen@...ux.intel.com>,
"H. Peter Anvin" <hpa@...or.com>
Subject: [PATCH v4 2/3] x86/alternative: Use helper functions for patching alternatives
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@...en8.de>
Signed-off-by: Juergen Gross <jgross@...e.com>
---
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(-)
diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index 4f3ea50e41e8..afcc681ff3bd 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -604,6 +604,86 @@ static inline u8 * instr_va(struct alt_instr *i)
return (u8 *)&i->instr_offset + i->instr_offset;
}
+struct patch_site {
+ u8 *instr;
+ struct alt_instr *alt;
+ u8 buff[MAX_PATCH_LEN];
+ u8 len;
+};
+
+static void __init_or_module analyze_patch_site(struct patch_site *ps,
+ struct alt_instr *p, struct alt_instr *end)
+{
+ struct alt_instr *r;
+
+ /*
+ * 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++) {
+ ps->len = max(ps->len, r->instrlen);
+ p->instrlen = r->instrlen = ps->len;
+ }
+
+ BUG_ON(ps->len > sizeof(ps->buff));
+ BUG_ON(p->cpuid >= (NCAPINTS + NBUGINTS) * 32);
+
+ /*
+ * Patch if either:
+ * - feature is present
+ * - 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))
+ ps->alt = NULL;
+ else
+ ps->alt = p;
+}
+
+static void __init_or_module prep_patch_site(struct patch_site *ps)
+{
+ struct alt_instr *p = ps->alt;
+ u8 buff_sz;
+ u8 *repl;
+
+ if (!p) {
+ /* Nothing to patch, use original instruction. */
+ memcpy(ps->buff, ps->instr, ps->len);
+ return;
+ }
+
+ repl = (u8 *)&p->repl_offset + p->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,
+ ps->instr, ps->instr, ps->len,
+ repl, p->replacementlen, p->flags);
+
+ memcpy(ps->buff, repl, p->replacementlen);
+ buff_sz = p->replacementlen;
+
+ if (p->flags & ALT_FLAG_DIRECT_CALL)
+ buff_sz = alt_replace_call(ps->instr, ps->buff, p);
+
+ for (; buff_sz < ps->len; buff_sz++)
+ ps->buff[buff_sz] = 0x90;
+
+ __apply_relocation(ps->buff, ps->instr, ps->len, repl, p->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, ps->buff, ps->len, "%px: final_insn: ", ps->instr);
+}
+
+static void __init_or_module patch_site(struct patch_site *ps)
+{
+ optimize_nops(ps->instr, ps->buff, ps->len);
+ text_poke_early(ps->instr, ps->buff, ps->len);
+}
+
/*
* Replace instructions with better alternatives for this CPU type. This runs
* before SMP is initialized to avoid SMP problems with self modifying code.
@@ -617,9 +697,7 @@ static inline u8 * instr_va(struct alt_instr *i)
void __init_or_module noinline apply_alternatives(struct alt_instr *start,
struct alt_instr *end)
{
- u8 insn_buff[MAX_PATCH_LEN];
- u8 *instr, *replacement;
- struct alt_instr *a, *b;
+ struct alt_instr *a;
DPRINTK(ALT, "alt table %px, -> %px", start, end);
@@ -643,59 +721,11 @@ void __init_or_module noinline apply_alternatives(struct alt_instr *start,
* order.
*/
for (a = start; a < end; a++) {
- unsigned int insn_buff_sz = 0;
-
- /*
- * 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).
- */
- for (b = a+1; b < end && instr_va(b) == instr_va(a); b++) {
- u8 len = max(a->instrlen, b->instrlen);
- a->instrlen = b->instrlen = len;
- }
-
- instr = instr_va(a);
- replacement = (u8 *)&a->repl_offset + a->repl_offset;
- BUG_ON(a->instrlen > sizeof(insn_buff));
- BUG_ON(a->cpuid >= (NCAPINTS + NBUGINTS) * 32);
-
- /*
- * Patch if either:
- * - feature is present
- * - feature not present but ALT_FLAG_NOT is set to mean,
- * patch if feature is *NOT* present.
- */
- if (!boot_cpu_has(a->cpuid) == !(a->flags & ALT_FLAG_NOT)) {
- memcpy(insn_buff, instr, a->instrlen);
- optimize_nops(instr, insn_buff, a->instrlen);
- text_poke_early(instr, insn_buff, a->instrlen);
- continue;
- }
-
- DPRINTK(ALT, "feat: %d*32+%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, a->flags);
-
- memcpy(insn_buff, replacement, a->replacementlen);
- insn_buff_sz = a->replacementlen;
-
- if (a->flags & ALT_FLAG_DIRECT_CALL)
- insn_buff_sz = alt_replace_call(instr, insn_buff, a);
-
- for (; insn_buff_sz < a->instrlen; insn_buff_sz++)
- insn_buff[insn_buff_sz] = 0x90;
-
- text_poke_apply_relocation(insn_buff, instr, a->instrlen, replacement, a->replacementlen);
-
- DUMP_BYTES(ALT, instr, a->instrlen, "%px: old_insn: ", instr);
- DUMP_BYTES(ALT, replacement, a->replacementlen, "%px: rpl_insn: ", replacement);
- DUMP_BYTES(ALT, insn_buff, insn_buff_sz, "%px: final_insn: ", instr);
+ struct patch_site ps;
- text_poke_early(instr, insn_buff, insn_buff_sz);
+ analyze_patch_site(&ps, a, end);
+ prep_patch_site(&ps);
+ patch_site(&ps);
}
kasan_enable_current();
--
2.51.0
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