alternatives is used both at boot time, and when loading livepatch payloads.
While for the former it makes sense to panic, it's not useful for the later, as
for livepatches it's possible to fail to load the livepatch if alternatives
cannot be resolved and continue operating normally.

Relax the BUGs in _apply_alternatives() to instead return an error code.  The
caller will figure out whether the failures are fatal and panic.

Print an error message to provide some user-readable information about what
went wrong.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
---
Changes since v2:
 - Improve log messages.

Changes since v1:
 - Unconditionally return from _apply_alternative() and let the caller panic
   if required.
 - Remove test, as next patch imposes restrictions that break the test.
---
 xen/arch/x86/alternative.c             | 46 ++++++++++++++++++++------
 xen/arch/x86/include/asm/alternative.h |  2 +-
 xen/common/livepatch.c                 | 10 +++++-
 3 files changed, 46 insertions(+), 12 deletions(-)

diff --git a/xen/arch/x86/alternative.c b/xen/arch/x86/alternative.c
index 7824053c9d33..990b7c600932 100644
--- a/xen/arch/x86/alternative.c
+++ b/xen/arch/x86/alternative.c
@@ -175,9 +175,9 @@ extern void *const __initdata_cf_clobber_end[];
  * invocation, such that no CALLs/JMPs to NULL pointers will be left
  * around. See also the further comment below.
  */
-static void init_or_livepatch _apply_alternatives(struct alt_instr *start,
-                                                  struct alt_instr *end,
-                                                  bool force)
+static int init_or_livepatch _apply_alternatives(struct alt_instr *start,
+                                                 struct alt_instr *end,
+                                                 bool force)
 {
     struct alt_instr *a, *base;
 
@@ -198,9 +198,29 @@ static void init_or_livepatch _apply_alternatives(struct alt_instr *start,
         uint8_t buf[MAX_PATCH_LEN];
         unsigned int total_len = a->orig_len + a->pad_len;
 
-        BUG_ON(a->repl_len > total_len);
-        BUG_ON(total_len > sizeof(buf));
-        BUG_ON(a->cpuid >= NCAPINTS * 32);
+        if ( a->repl_len > total_len )
+        {
+            printk(XENLOG_ERR
+                   "alt for %ps, replacement size %#x larger than origin %#x\n",
+                    ALT_ORIG_PTR(a), a->repl_len, total_len);
+            return -ENOSPC;
+        }
+
+        if ( total_len > sizeof(buf) )
+        {
+            printk(XENLOG_ERR
+                   "alt for %ps, origin size %#x bigger than buffer %#zx\n",
+                   ALT_ORIG_PTR(a), total_len, sizeof(buf));
+            return -ENOSPC;
+        }
+
+        if ( a->cpuid >= NCAPINTS * 32 )
+        {
+             printk(XENLOG_ERR
+                   "alt for %ps, feature %#x outside of featureset range %#x\n",
+                   ALT_ORIG_PTR(a), a->cpuid, NCAPINTS * 32);
+            return -ERANGE;
+        }
 
         /*
          * Detect sequences of alt_instr's patching the same origin site, and
@@ -356,12 +376,14 @@ static void init_or_livepatch _apply_alternatives(struct alt_instr *start,
 
         printk("altcall: Optimised away %u endbr64 instructions\n", clobbered);
     }
+
+    return 0;
 }
 
 #ifdef CONFIG_LIVEPATCH
-void apply_alternatives(struct alt_instr *start, struct alt_instr *end)
+int apply_alternatives(struct alt_instr *start, struct alt_instr *end)
 {
-    _apply_alternatives(start, end, true);
+    return _apply_alternatives(start, end, true);
 }
 #endif
 
@@ -383,6 +405,8 @@ static int __init cf_check nmi_apply_alternatives(
      */
     if ( !(alt_done & alt_todo) )
     {
+        int rc;
+
         /*
          * Relax perms on .text to be RWX, so we can modify them.
          *
@@ -394,8 +418,10 @@ static int __init cf_check nmi_apply_alternatives(
                                  PAGE_HYPERVISOR_RWX);
         flush_local(FLUSH_TLB_GLOBAL);
 
-        _apply_alternatives(__alt_instructions, __alt_instructions_end,
-                            alt_done);
+        rc = _apply_alternatives(__alt_instructions, __alt_instructions_end,
+                                 alt_done);
+        if ( rc )
+            panic("Unable to apply alternatives: %d\n", rc);
 
         /*
          * Reinstate perms on .text to be RX.  This also cleans out the dirty
diff --git a/xen/arch/x86/include/asm/alternative.h b/xen/arch/x86/include/asm/alternative.h
index a86eadfaecbd..69555d781ef9 100644
--- a/xen/arch/x86/include/asm/alternative.h
+++ b/xen/arch/x86/include/asm/alternative.h
@@ -24,7 +24,7 @@ struct __packed alt_instr {
 
 extern void add_nops(void *insns, unsigned int len);
 /* Similar to alternative_instructions except it can be run with IRQs enabled. */
-extern void apply_alternatives(struct alt_instr *start, struct alt_instr *end);
+extern int apply_alternatives(struct alt_instr *start, struct alt_instr *end);
 extern void alternative_instructions(void);
 extern void alternative_branches(void);
 
diff --git a/xen/common/livepatch.c b/xen/common/livepatch.c
index f7db4be96e66..6793cb60d1e2 100644
--- a/xen/common/livepatch.c
+++ b/xen/common/livepatch.c
@@ -888,7 +888,15 @@ static int prepare_payload(struct payload *payload,
                 return -EINVAL;
             }
         }
-        apply_alternatives(start, end);
+
+        rc = apply_alternatives(start, end);
+        if ( rc )
+        {
+            printk(XENLOG_ERR LIVEPATCH "%s applying alternatives failed: %d\n",
+                   elf->name, rc);
+            return rc;
+        }
+
     alt_done:;
 #else
         printk(XENLOG_ERR LIVEPATCH "%s: We don't support alternative patching\n",
-- 
2.46.0

On 26/09/2024 11:14 am, Roger Pau Monne wrote:
> diff --git a/xen/arch/x86/alternative.c b/xen/arch/x86/alternative.c
> index 7824053c9d33..990b7c600932 100644
> --- a/xen/arch/x86/alternative.c
> +++ b/xen/arch/x86/alternative.c
> @@ -198,9 +198,29 @@ static void init_or_livepatch _apply_alternatives(struct alt_instr *start,
>          uint8_t buf[MAX_PATCH_LEN];
>          unsigned int total_len = a->orig_len + a->pad_len;
>  
> -        BUG_ON(a->repl_len > total_len);
> -        BUG_ON(total_len > sizeof(buf));
> -        BUG_ON(a->cpuid >= NCAPINTS * 32);
> +        if ( a->repl_len > total_len )
> +        {
> +            printk(XENLOG_ERR
> +                   "alt for %ps, replacement size %#x larger than origin %#x\n",

"Alt" I think.  It's both an abbreviation, and the start of the sentence
here.

Can fix on commit.

Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>

On Thu, Sep 26, 2024 at 12:09:05PM +0100, Andrew Cooper wrote:
> On 26/09/2024 11:14 am, Roger Pau Monne wrote:
> > diff --git a/xen/arch/x86/alternative.c b/xen/arch/x86/alternative.c
> > index 7824053c9d33..990b7c600932 100644
> > --- a/xen/arch/x86/alternative.c
> > +++ b/xen/arch/x86/alternative.c
> > @@ -198,9 +198,29 @@ static void init_or_livepatch _apply_alternatives(struct alt_instr *start,
> >          uint8_t buf[MAX_PATCH_LEN];
> >          unsigned int total_len = a->orig_len + a->pad_len;
> >  
> > -        BUG_ON(a->repl_len > total_len);
> > -        BUG_ON(total_len > sizeof(buf));
> > -        BUG_ON(a->cpuid >= NCAPINTS * 32);
> > +        if ( a->repl_len > total_len )
> > +        {
> > +            printk(XENLOG_ERR
> > +                   "alt for %ps, replacement size %#x larger than origin %#x\n",
> 
> "Alt" I think.  It's both an abbreviation, and the start of the sentence
> here.

I'm always unsure whether to use uppercase at the beginning of log
messages, because those are preceded by the (XEN) prefix.

> Can fix on commit.
> 
> Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>

Thanks, Roger.