[Xen-devel] [RFC v3 17/27] xen: Adapt assembly for PIE support

[Thomas Garnier <thgarnie@xxxxxxxxxx>]

Change the assembly code to use the new _ASM_GET_PTR macro which get a
symbol reference while being PIE compatible. Adapt the relocation tool
to ignore 32-bit Xen code.

Position Independent Executable (PIE) support will allow to extended the
KASLR randomization range below the -2G memory limit.

Signed-off-by: Thomas Garnier <thgarnie@xxxxxxxxxx>
---
 arch/x86/tools/relocs.c | 16 +++++++++++++++-
 arch/x86/xen/xen-head.S |  9 +++++----
 arch/x86/xen/xen-pvh.S  | 13 +++++++++----
 3 files changed, 29 insertions(+), 9 deletions(-)

diff --git a/arch/x86/tools/relocs.c b/arch/x86/tools/relocs.c
index 5d3eb2760198..bc032ad88b22 100644
--- a/arch/x86/tools/relocs.c
+++ b/arch/x86/tools/relocs.c
@@ -831,6 +831,16 @@ static int is_percpu_sym(ElfW(Sym) *sym, const char 
*symname)
                strncmp(symname, "init_per_cpu_", 13);
 }
 
+/*
+ * Check if the 32-bit relocation is within the xenpvh 32-bit code.
+ * If so, ignores it.
+ */
+static int is_in_xenpvh_assembly(ElfW(Addr) offset)
+{
+       ElfW(Sym) *sym = sym_lookup("pvh_start_xen");
+       return sym && (offset >= sym->st_value) &&
+               (offset < (sym->st_value + sym->st_size));
+}
 
 static int do_reloc64(struct section *sec, Elf_Rel *rel, ElfW(Sym) *sym,
                      const char *symname)
@@ -892,8 +902,12 @@ static int do_reloc64(struct section *sec, Elf_Rel *rel, 
ElfW(Sym) *sym,
                 * the relocations are processed.
                 * Make sure that the offset will fit.
                 */
-               if (r_type != R_X86_64_64 && (int32_t)offset != (int64_t)offset)
+               if (r_type != R_X86_64_64 &&
+                   (int32_t)offset != (int64_t)offset) {
+                       if (is_in_xenpvh_assembly(offset))
+                               break;
                        die("Relocation offset doesn't fit in 32 bits\n");
+               }
 
                if (r_type == R_X86_64_64)
                        add_reloc(&relocs64, offset);
diff --git a/arch/x86/xen/xen-head.S b/arch/x86/xen/xen-head.S
index 124941d09b2b..e5b7b9566191 100644
--- a/arch/x86/xen/xen-head.S
+++ b/arch/x86/xen/xen-head.S
@@ -25,14 +25,15 @@ ENTRY(startup_xen)
 
        /* Clear .bss */
        xor %eax,%eax
-       mov $__bss_start, %_ASM_DI
-       mov $__bss_stop, %_ASM_CX
+       _ASM_GET_PTR(__bss_start, %_ASM_DI)
+       _ASM_GET_PTR(__bss_stop, %_ASM_CX)
        sub %_ASM_DI, %_ASM_CX
        shr $__ASM_SEL(2, 3), %_ASM_CX
        rep __ASM_SIZE(stos)
 
-       mov %_ASM_SI, xen_start_info
-       mov $init_thread_union+THREAD_SIZE, %_ASM_SP
+       _ASM_GET_PTR(xen_start_info, %_ASM_AX)
+       mov %_ASM_SI, (%_ASM_AX)
+       _ASM_GET_PTR(init_thread_union+THREAD_SIZE, %_ASM_SP)
 
        jmp xen_start_kernel
 END(startup_xen)
diff --git a/arch/x86/xen/xen-pvh.S b/arch/x86/xen/xen-pvh.S
index e1a5fbeae08d..43e234c7c2de 100644
--- a/arch/x86/xen/xen-pvh.S
+++ b/arch/x86/xen/xen-pvh.S
@@ -101,8 +101,8 @@ ENTRY(pvh_start_xen)
        call xen_prepare_pvh
 
        /* startup_64 expects boot_params in %rsi. */
-       mov $_pa(pvh_bootparams), %rsi
-       mov $_pa(startup_64), %rax
+       movabs $_pa(pvh_bootparams), %rsi
+       movabs $_pa(startup_64), %rax
        jmp *%rax
 
 #else /* CONFIG_X86_64 */
@@ -137,10 +137,15 @@ END(pvh_start_xen)
 
        .section ".init.data","aw"
        .balign 8
+       /*
+        * Use a quad for _pa(gdt_start) because PIE does not understand a
+        * long is enough. The resulting value will still be in the lower long
+        * part.
+        */
 gdt:
        .word gdt_end - gdt_start
-       .long _pa(gdt_start)
-       .word 0
+       .quad _pa(gdt_start)
+       .balign 8
 gdt_start:
        .quad 0x0000000000000000            /* NULL descriptor */
        .quad 0x0000000000000000            /* reserved */
-- 
2.14.2.920.gcf0c67979c-goog


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