Instead of patching a single location potentially multiple times in
case of nested ALTERNATIVE()s, do the patching only after having
evaluated all alt_instr instances for that location.
This has multiple advantages:
- In case of replacing an indirect with a direct call using the
ALT_FLAG_DIRECT_CALL flag, there is no longer the need to have that
instance before any other instances at the same location (the
original instruction is needed for finding the target of the direct
call).
This issue has been hit when trying to do paravirt patching similar
to the following:
ALTERNATIVE_2(PARAVIRT_CALL, // indirect call
instr, feature, // native instruction
ALT_CALL_INSTR, X86_FEATURE_XENPV) // Xen function
In case "feature" was true, "instr" replaced the indirect call. Under
Xen PV the patching to have a direct call failed, as the original
indirect call was no longer there to find the call target.
- In case of nested ALTERNATIVE()s there is no intermediate replacement
visible. This avoids any problems in case e.g. an interrupt is
happening between the single instances and the patched location is
used during handling the interrupt.
Signed-off-by: Juergen Gross <jgross@suse.com>
---
V2:
- complete rework (Boris Petkov)
V3:
- rebase to added patch 2
---
arch/x86/kernel/alternative.c | 42 +++++++++++++++++------------------
1 file changed, 21 insertions(+), 21 deletions(-)
diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index afcc681ff3bd..c1032840df4e 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -611,37 +611,36 @@ struct patch_site {
u8 len;
};
-static void __init_or_module analyze_patch_site(struct patch_site *ps,
- struct alt_instr *p, struct alt_instr *end)
+static struct alt_instr * __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).
+ * Find the last alt_instr eligible for patching at the site.
*/
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;
+ ps->alt = NULL;
+ ps->len = 0;
+ for (; p < end && instr_va(p) == ps->instr; p++) {
+ ps->len = max(ps->len, p->instrlen);
+
+ 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 = p;
}
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;
+ return p;
}
static void __init_or_module prep_patch_site(struct patch_site *ps)
@@ -720,10 +719,11 @@ void __init_or_module noinline apply_alternatives(struct alt_instr *start,
* So be careful if you want to change the scan order to any other
* order.
*/
- for (a = start; a < end; a++) {
+ a = start;
+ while (a < end) {
struct patch_site ps;
- analyze_patch_site(&ps, a, end);
+ a = analyze_patch_site(&ps, a, end);
prep_patch_site(&ps);
patch_site(&ps);
}
--
2.51.0On Wed, Nov 19, 2025 at 05:04:20PM +0100, Juergen Gross wrote:
> Instead of patching a single location potentially multiple times in
> case of nested ALTERNATIVE()s, do the patching only after having
> evaluated all alt_instr instances for that location.
>
> This has multiple advantages:
>
> - In case of replacing an indirect with a direct call using the
> ALT_FLAG_DIRECT_CALL flag, there is no longer the need to have that
> instance before any other instances at the same location (the
> original instruction is needed for finding the target of the direct
> call).
> This issue has been hit when trying to do paravirt patching similar
> to the following:
> ALTERNATIVE_2(PARAVIRT_CALL, // indirect call
> instr, feature, // native instruction
> ALT_CALL_INSTR, X86_FEATURE_XENPV) // Xen function
> In case "feature" was true, "instr" replaced the indirect call. Under
> Xen PV the patching to have a direct call failed, as the original
> indirect call was no longer there to find the call target.
>
> - In case of nested ALTERNATIVE()s there is no intermediate replacement
> visible. This avoids any problems in case e.g. an interrupt is
> happening between the single instances and the patched location is
> used during handling the interrupt.
>
> Signed-off-by: Juergen Gross <jgross@suse.com>
> ---
> V2:
> - complete rework (Boris Petkov)
> V3:
> - rebase to added patch 2
> ---
> arch/x86/kernel/alternative.c | 42 +++++++++++++++++------------------
> 1 file changed, 21 insertions(+), 21 deletions(-)
That one would then look like this as I've moved the struct patch_site ps at
the beginning of the loop, where it belongs.
Other than that, yap, looks ok, and it seems to work.
Thx.
---
diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index 66868ecdebd2..6250c192bb59 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -593,39 +593,39 @@ struct patch_site {
u8 len;
};
-static void __init_or_module analyze_patch_site(struct patch_site *ps,
- struct alt_instr *start,
- struct alt_instr *end)
+static struct alt_instr * __init_or_module analyze_patch_site(struct patch_site *ps,
+ struct alt_instr *start,
+ struct alt_instr *end)
{
- struct alt_instr *r;
+ struct alt_instr *p = start;
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).
+ * Find the last alt_instr eligible for patching at the site.
*/
- for (r = start+1; r < end && instr_va(r) == ps->instr; r++) {
- ps->len = max(ps->len, r->instrlen);
- start->instrlen = r->instrlen = ps->len;
+ for (; p < end && instr_va(p) == ps->instr; p++) {
+ ps->len = max(ps->len, p->instrlen);
+
+ 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 = p;
}
BUG_ON(ps->len > sizeof(ps->buff));
- BUG_ON(start->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(start->cpuid) == !(start->flags & ALT_FLAG_NOT))
- ps->alt = NULL;
- else
- ps->alt = start;
+ return p;
}
static void __init_or_module prep_patch_site(struct patch_site *ps)
@@ -704,10 +704,14 @@ void __init_or_module noinline apply_alternatives(struct alt_instr *start,
* So be careful if you want to change the scan order to any other
* order.
*/
- for (a = start; a < end; a++) {
- struct patch_site ps;
-
- analyze_patch_site(&ps, a, end);
+ a = start;
+ while (a < end) {
+ struct patch_site ps = {
+ .alt = NULL,
+ .len = 0
+ };
+
+ a = analyze_patch_site(&ps, a, end);
prep_patch_site(&ps);
patch_site(&ps);
}
--
Regards/Gruss,
Boris.
https://people.kernel.org/tglx/notes-about-netiquette
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