[Xen-devel] [PATCH] x86/EFI + Live Patch: avoid symbol address truncation

["Jan Beulich" <JBeulich@xxxxxxxx>]

ld associates __init_end, placed outside of any section by the linker
script, with the following section, resulting in a huge (wrapped, as it
would be negative) section relative offset. COFF symbol tables store
section relative addresses, and hence the above leads to assembler
truncation warnings when all symbols get included in the symbol table
(for Live Patching code). To overcome this, move __init_end past both
ALIGN() directives. The consuming code (init_done()) is fine with such
an adjustment (the distinction really would only be relevant for the
loop claring the pages, and I think it's acceptable to clear a few
more on - for now - EFI). This effectively results in the
(__init_begin,__init_end) and (__2M_init_start,__2M_init_end) pairs to
become identical, with their different names only serving documentation
purposes now.

Note that moving __init_end and __2M_init_end into .init is not a good
idea, as that would significantly grow xen.efi binary size.

While inspecting symbol table and ld behavior I also noticed that
__2M_text_start gets put at address zero in the EFI case, which hasn't
caused problems solely because we don't actually reference that symbol.
Correct the setting of the initial address, and comment out said symbol
for the time being, as with the initial address correction it would in
turn cause an assembler truncation warning similar to the one mentioned
above.

While checking init_done() for correctness with the above changes I
noticed that code can easily be folded there, at once correcting the
logged amount of memory which has got freed for the 2M-alignment case
(i.e. EFI right now).

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/arch/x86/setup.c
+++ b/xen/arch/x86/setup.c
@@ -515,6 +515,7 @@ static inline bool_t using_2M_mapping(vo
 static void noinline init_done(void)
 {
     void *va;
+    unsigned long start, end;
 
     system_state = SYS_STATE_active;
 
@@ -530,18 +531,18 @@ static void noinline init_done(void)
     /* Destroy Xen's mappings, and reuse the pages. */
     if ( using_2M_mapping() )
     {
-        destroy_xen_mappings((unsigned long)&__2M_init_start,
-                             (unsigned long)&__2M_init_end);
-        init_xenheap_pages(__pa(__2M_init_start), __pa(__2M_init_end));
+        start = (unsigned long)&__2M_init_start,
+        end   = (unsigned long)&__2M_init_end;
     }
     else
     {
-        destroy_xen_mappings((unsigned long)&__init_begin,
-                             (unsigned long)&__init_end);
-        init_xenheap_pages(__pa(__init_begin), __pa(__init_end));
+        start = (unsigned long)&__init_begin;
+        end   = (unsigned long)&__init_end;
     }
 
-    printk("Freed %ldkB init memory.\n", (long)(__init_end-__init_begin)>>10);
+    destroy_xen_mappings(start, end);
+    init_xenheap_pages(__pa(start), __pa(end));
+    printk("Freed %ldkB init memory\n", (end - start) >> 10);
 
     startup_cpu_idle_loop();
 }
--- a/xen/arch/x86/xen.lds.S
+++ b/xen/arch/x86/xen.lds.S
@@ -40,9 +40,20 @@ SECTIONS
 #if !defined(EFI)
   . = __XEN_VIRT_START;
   __image_base__ = .;
+#else
+  . = __image_base__;
 #endif
 
+#if 0
+/*
+ * We don't really use this symbol anywhere, and the way it would get defined
+ * here would result in it having a negative (wrapped to huge positive)
+ * offset relative to the .text section. That, in turn, causes an assembler
+ * truncation warning when including all symbols in the symbol table for Live
+ * Patching code.
+ */
   __2M_text_start = .;         /* Start of 2M superpages, mapped RX. */
+#endif
 
   . = __XEN_VIRT_START + MB(1);
   _start = .;
@@ -194,14 +205,13 @@ SECTIONS
        *(.ctors)
        __ctors_end = .;
   } :text
-  . = ALIGN(PAGE_SIZE);
-  __init_end = .;
 
 #ifdef EFI
   . = ALIGN(MB(2));
 #else
   . = ALIGN(PAGE_SIZE);
 #endif
+  __init_end = .;
   __2M_init_end = .;
 
   __2M_rwdata_start = .;       /* Start of 2M superpages, mapped RW. */
@@ -296,7 +306,6 @@ ASSERT(__image_base__ > XEN_VIRT_START |
 ASSERT(kexec_reloc_size - kexec_reloc <= PAGE_SIZE, "kexec_reloc is too large")
 #endif
 
-ASSERT(IS_ALIGNED(__2M_text_start,   MB(2)), "__2M_text_start misaligned")
 #ifdef EFI
 ASSERT(IS_ALIGNED(__2M_text_end,     MB(2)), "__2M_text_end misaligned")
 ASSERT(IS_ALIGNED(__2M_rodata_start, MB(2)), "__2M_rodata_start misaligned")

Attachment: x86-EFI-symbol-truncation.patch
Description: Text document

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