[Xen-devel] [PATCH] x86/link: Don't merge .init.text and .init.data

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

c/s 1308f0170c merged .init.text and .init.data, because EFI might properly
write-protect r/o sections.

However, this change makes xen-syms unusable for disassembly analysis.  In
particular, searching for indirect branches as part of the SP2/Spectre
mitigation series.

Revert the relevent bits of 1308f0170c and instead modify the EFI relocation
code to disable CR0.WP, which is how we deal with relocations in r/o mappings
elsewhere.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
---
 xen/arch/x86/efi/efi-boot.h | 12 ++++++++++++
 xen/arch/x86/efi/mkreloc.c  |  5 -----
 xen/arch/x86/xen.lds.S      |  7 +++----
 3 files changed, 15 insertions(+), 9 deletions(-)

diff --git a/xen/arch/x86/efi/efi-boot.h b/xen/arch/x86/efi/efi-boot.h
index d30f688..0b5f218 100644
--- a/xen/arch/x86/efi/efi-boot.h
+++ b/xen/arch/x86/efi/efi-boot.h
@@ -47,8 +47,17 @@ extern const struct pe_base_relocs {
 
 static void __init efi_arch_relocate_image(unsigned long delta)
 {
+    unsigned long cr0;
     const struct pe_base_relocs *base_relocs;
 
+    /*
+     * Conditionally disable CR0.WP in case there are relocations present in
+     * read-only mappings.
+     */
+    cr0 = read_cr0();
+    if ( cr0 & X86_CR0_WP )
+        write_cr0(cr0 & ~X86_CR0_WP);
+
     for ( base_relocs = __base_relocs_start; base_relocs < __base_relocs_end; )
     {
         unsigned int i = 0, n;
@@ -96,6 +105,9 @@ static void __init efi_arch_relocate_image(unsigned long 
delta)
         }
         base_relocs = (const void *)(base_relocs->entries + i + (i & 1));
     }
+
+    if ( cr0 & X86_CR0_WP )
+        write_cr0(cr0);
 }
 
 extern const s32 __trampoline_rel_start[], __trampoline_rel_stop[];
diff --git a/xen/arch/x86/efi/mkreloc.c b/xen/arch/x86/efi/mkreloc.c
index 1aca796..509fd83 100644
--- a/xen/arch/x86/efi/mkreloc.c
+++ b/xen/arch/x86/efi/mkreloc.c
@@ -267,11 +267,6 @@ static void diff_sections(const unsigned char *ptr1, const 
unsigned char *ptr2,
             exit(3);
         }
 
-        if ( !(sec->flags & COFF_SECTION_WRITEABLE) )
-            fprintf(stderr,
-                    "Warning: relocation to r/o section %.8s:%08" PRIxFAST32 
"\n",
-                    sec->name, i);
-
         printf("\t.word (%u << 12) | 0x%03" PRIxFAST32 "\n",
                reloc, sec->rva + i - disp - rva);
         reloc_size += 2;
diff --git a/xen/arch/x86/xen.lds.S b/xen/arch/x86/xen.lds.S
index d5e8821..6a7bbb8 100644
--- a/xen/arch/x86/xen.lds.S
+++ b/xen/arch/x86/xen.lds.S
@@ -159,7 +159,7 @@ SECTIONS
   __2M_init_start = .;         /* Start of 2M superpages, mapped RWX (boot 
only). */
   . = ALIGN(PAGE_SIZE);             /* Init code and data */
   __init_begin = .;
-  .init : {
+  .init.text : {
        _sinittext = .;
        *(.init.text)
        /*
@@ -169,9 +169,8 @@ SECTIONS
         */
        *(.altinstr_replacement)
        _einittext = .;
-
-       . = ALIGN(SMP_CACHE_BYTES);
-
+  } :text
+  .init.data : {
        *(.init.rodata)
        *(.init.rodata.rel)
        *(.init.rodata.str*)
-- 
2.1.4


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