xen/livepatch: Support new altcall scheme

The new altcall scheme uses an .alt_call_sites section.  Wire this up in very
much the same way as the .altinstructions section, although there is less
sanity checking necessary.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>

xen/arch/x86/alternative.c

@@ -479,6 +479,12 @@ int apply_alternatives(struct alt_instr *start, struct alt_instr *end)
 {
     return _apply_alternatives(start, end, true);
 }
+
+int livepatch_apply_alt_calls(const struct alt_call *start,
+                              const struct alt_call *end)
+{
+    return apply_alt_calls(start, end);
+}
 #endif
 
 #define ALT_INSNS (1U << 0)

xen/common/livepatch.c

@@ -905,6 +905,64 @@ static int prepare_payload(struct payload *payload,
 #endif
     }
 
+    sec = livepatch_elf_sec_by_name(elf, ".alt_call_sites");
+    if ( sec )
+    {
+#ifdef CONFIG_ALTERNATIVE_CALL
+        const struct alt_call *a, *start, *end;
+
+        if ( !section_ok(elf, sec, sizeof(*a)) )
+            return -EINVAL;
+
+        /* Tolerate an empty .alt_call_sites section... */
+        if ( sec->sec->sh_size == 0 )
+            goto alt_call_done;
+
+        /* ... but otherwise, there needs to be something to alter... */
+        if ( payload->text_size == 0 )
+        {
+            printk(XENLOG_ERR LIVEPATCH "%s Alternative calls provided, but no .text\n",
+                   elf->name);
+            return -EINVAL;
+        }
+
+        start = sec->addr;
+        end = sec->addr + sec->sec->sh_size;
+
+        for ( a = start; a < end; a++ )
+        {
+            const void *orig = ALT_CALL_PTR(a);
+            size_t len = ALT_CALL_LEN(a);
+
+            /* orig must be fully within .text. */
+            if ( orig       < payload->text_addr ||
+                 len        > payload->text_size ||
+                 orig + len > payload->text_addr + payload->text_size )
+            {
+                printk(XENLOG_ERR LIVEPATCH
+                       "%s: Alternative call %p+%#zx outside payload text %p+%#zx\n",
+                       elf->name, orig, len,
+                       payload->text_addr, payload->text_size);
+                return -EINVAL;
+            }
+        }
+
+        rc = livepatch_apply_alt_calls(start, end);
+        if ( rc )
+        {
+            printk(XENLOG_ERR LIVEPATCH "%s: Applying alternative calls failed: %d\n",
+                   elf->name, rc);
+            return rc;
+        }
+
+    alt_call_done:;
+#else /* CONFIG_ALTERNATIVE_CALL */
+        printk(XENLOG_ERR LIVEPATCH "%s: Alternative calls not supported\n",
+               elf->name);
+        return -EOPNOTSUPP;
+#endif /* !CONFIG_ALTERNATIVE_CALL */
+    }
+
     sec = livepatch_elf_sec_by_name(elf, ".ex_table");
     if ( sec )
     {

xen/include/xen/alternative-call.h

@@ -17,8 +17,8 @@
  *   generation requirements are to emit a function pointer call at build
  *   time, and stash enough metadata to simplify the call at boot once the
  *   implementation has been resolved.
- * - Implement boot_apply_alt_calls() to convert the function pointer calls
- *   into direct calls on boot.
+ * - Implement {boot,livepatch}_apply_alt_calls() to convert the function
+ *   pointer calls into direct calls on boot/livepatch.
  * - Select ALTERNATIVE_CALL in Kconfig.
  *
  * To use:
@@ -65,6 +65,10 @@
  */
 void boot_apply_alt_calls(void);
 
+/* As per boot_apply_alt_calls() but for a livepatch. */
+int livepatch_apply_alt_calls(const struct alt_call *start,
+                              const struct alt_call *end);
+
 #else /* CONFIG_ALTERNATIVE_CALL */
 
 #define alternative_call(func, args...)  (func)(args)