[Xen-changelog] [xen stable-4.7] x86/EFI + Live Patch: avoid symbol address truncation

[patchbot@xxxxxxx]

commit d37c2b9db312f42ac40164c55201c4aca527beae
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Fri Aug 5 13:37:46 2016 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Fri Aug 5 13:37:46 2016 +0200

    x86/EFI + Live Patch: avoid symbol address truncation
    
    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>
    Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    master commit: 72e4a1b40a8b62ad3abf14869f78e7ffa5c680a0
    master date: 2016-06-29 16:38:50 +0200
---
 xen/arch/x86/setup.c   | 15 ++++++++-------
 xen/arch/x86/xen.lds.S | 15 ++++++++++++---
 2 files changed, 20 insertions(+), 10 deletions(-)

diff --git a/xen/arch/x86/setup.c b/xen/arch/x86/setup.c
index de682e7..3459156 100644
--- a/xen/arch/x86/setup.c
+++ b/xen/arch/x86/setup.c
@@ -515,6 +515,7 @@ static inline bool_t using_2M_mapping(void)
 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 %lukB init memory\n", (end - start) >> 10);
 
     startup_cpu_idle_loop();
 }
diff --git a/xen/arch/x86/xen.lds.S b/xen/arch/x86/xen.lds.S
index b14bcd2..c948a59 100644
--- 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 = .;
@@ -195,14 +206,13 @@ SECTIONS
        *(.xsm_initcall.init)
        __xsm_initcall_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. */
@@ -297,7 +307,6 @@ 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")
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.7

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