x86/alternatives: build time check feature is in range

Ensure at build time the feature(s) used for the alternative blocks are in range of the featureset. No functional change intended, as all current usages are correct. Signed-off-by: Roger Pau Monné <[email protected]> Reviewed-by: Andrew Cooper <[email protected]>
xen-project · Sep 26, 2024 · a184ac7 · a184ac7
1 parent aa5a06d
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diff --git a/xen/arch/x86/include/asm/alternative-asm.h b/xen/arch/x86/include/asm/alternative-asm.h
@@ -12,6 +12,9 @@
  * instruction. See apply_alternatives().
  */
 .macro altinstruction_entry orig, repl, feature, orig_len, repl_len, pad_len
+    .if \feature >= NCAPINTS * 32
+        .error "alternative feature outside of featureset range"
+    .endif
     .long \orig - .
     .long \repl - .
     .word \feature

diff --git a/xen/arch/x86/include/asm/alternative.h b/xen/arch/x86/include/asm/alternative.h
@@ -7,6 +7,7 @@
 #include <xen/lib.h>
 #include <xen/stringify.h>
 #include <asm/asm-macros.h>
+#include <asm/cpufeatureset.h>
 
 struct __packed alt_instr {
     int32_t  orig_offset;   /* original instruction */
@@ -59,6 +60,9 @@ extern void alternative_branches(void);
                     alt_repl_len(n2)) "-" alt_orig_len)
 
 #define ALTINSTR_ENTRY(feature, num)                                    \
+        " .if " STR(feature) " >= " STR(NCAPINTS * 32) "\n"             \
+        " .error \"alternative feature outside of featureset range\"\n" \
+        " .endif\n"                                                     \
         " .long .LXEN%=_orig_s - .\n"             /* label           */ \
         " .long " alt_repl_s(num)" - .\n"         /* new instruction */ \
         " .word " __stringify(feature) "\n"       /* feature bit     */ \