x86/extable: hide use of negative offset from array start

In COVERAGE=y but DEBUG=n builds (observed by randconfig testing) gcc12
takes issue with the subtraction of 1 from __stop___pre_ex_table[],
considering this an out of bounds access. Not being able to know that
the symbol actually marks the end of an array, the compiler is kind of
right with this diagnosis. Move the subtraction into the function.

Reported-by: Anthony Perard <anthony.perard@citrix.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>

xen/arch/x86/extable.c

@@ -64,9 +64,10 @@ void __init sort_exception_tables(void)
 
 static unsigned long
 search_one_extable(const struct exception_table_entry *first,
-                   const struct exception_table_entry *last,
+                   const struct exception_table_entry *end,
                    unsigned long value)
 {
+    const struct exception_table_entry *last = end - 1;
     const struct exception_table_entry *mid;
     long diff;
 
@@ -91,7 +92,7 @@ search_exception_table(const struct cpu_user_regs *regs)
     unsigned long stub = this_cpu(stubs.addr);
 
     if ( region && region->ex )
-        return search_one_extable(region->ex, region->ex_end - 1, regs->rip);
+        return search_one_extable(region->ex, region->ex_end, regs->rip);
 
     if ( regs->rip >= stub + STUB_BUF_SIZE / 2 &&
          regs->rip < stub + STUB_BUF_SIZE &&
@@ -102,7 +103,7 @@ search_exception_table(const struct cpu_user_regs *regs)
 
         region = find_text_region(retptr);
         retptr = region && region->ex
-                 ? search_one_extable(region->ex, region->ex_end - 1, retptr)
+                 ? search_one_extable(region->ex, region->ex_end, retptr)
                  : 0;
         if ( retptr )
         {
@@ -198,7 +199,7 @@ search_pre_exception_table(struct cpu_user_regs *regs)
 {
     unsigned long addr = regs->rip;
     unsigned long fixup = search_one_extable(
-        __start___pre_ex_table, __stop___pre_ex_table-1, addr);
+        __start___pre_ex_table, __stop___pre_ex_table, addr);
     if ( fixup )
     {
         dprintk(XENLOG_INFO, "Pre-exception: %p -> %p\n", _p(addr), _p(fixup));