[Xen-devel] [PATCH] [VERY RFC] Clang: Issues with .data.rel.ro relocations

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

Clang-3.8 generates several .data.rel.ro sections when compiling Xen.

c/s eb2952b4 "x86: move alternative.c data fully into .init.*" cited "While at
it also drop the non-local section names from SPECIAL_DATA_SECTIONS - they
can't be safely converted." without any further information, and google isn't
overly helpful.

One solution to fix the compilation is:

diff --git a/xen/Rules.mk b/xen/Rules.mk
index f29491e..b4f13f0 100644
--- a/xen/Rules.mk
+++ b/xen/Rules.mk
@@ -177,7 +177,7 @@ SPECIAL_DATA_SECTIONS := rodata $(foreach a,1 2 4 8 16, \
 $(filter %.init.o,$(obj-y) $(obj-bin-y) $(extra-y)): %.init.o: %.o Makefile
        $(OBJDUMP) -h $< | sed -n '/[0-9]/{s,00*,0,g;p;}' | while read idx name 
sz rest; do \
                case "$$name" in \
-               .*.local) ;; \
+               .*.local|.data.rel.ro) ;; \
                .text|.text.*|.data|.data.*|.bss) \
                        test $$sz != 0 || continue; \
                        echo "Error: size of $<:$$name is 0x$$sz" >&2; \

but this goes against the statement in c/s eb2952b4.

The alternative is in the body of this patch, which shuffles the data such
that Clang doesn't create problematic relocations.

Given no obvious guidence on why .data.rel.ro relocations are unsafe, I can't
judge which is the correct approach to take.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
---
 xen/arch/x86/alternative.c |  4 ++--
 xen/common/efi/boot.c      | 58 +++++++++++++++-------------------------------
 2 files changed, 21 insertions(+), 41 deletions(-)

diff --git a/xen/arch/x86/alternative.c b/xen/arch/x86/alternative.c
index 46ac0fd..02b5e92 100644
--- a/xen/arch/x86/alternative.c
+++ b/xen/arch/x86/alternative.c
@@ -38,7 +38,7 @@ static const unsigned char k8nops[] __initconst = {
     K8_NOP7,
     K8_NOP8
 };
-static const unsigned char * const k8_nops[ASM_NOP_MAX+1] = {
+static const unsigned char * const k8_nops[ASM_NOP_MAX+1] __initconst = {
     NULL,
     k8nops,
     k8nops + 1,
@@ -62,7 +62,7 @@ static const unsigned char p6nops[] __initconst = {
     P6_NOP7,
     P6_NOP8
 };
-static const unsigned char * const p6_nops[ASM_NOP_MAX+1] = {
+static const unsigned char * const p6_nops[ASM_NOP_MAX+1] __initconst = {
     NULL,
     p6nops,
     p6nops + 1,
diff --git a/xen/common/efi/boot.c b/xen/common/efi/boot.c
index 53c7452..64ed433 100644
--- a/xen/common/efi/boot.c
+++ b/xen/common/efi/boot.c
@@ -241,53 +241,33 @@ static void __init noreturn blexit(const CHAR16 *str)
 /* generic routine for printing error messages */
 static void __init PrintErrMesg(const CHAR16 *mesg, EFI_STATUS ErrCode)
 {
+    static __initconst CHAR16* ErrCodeToStr[] = {
+        [EFI_NOT_FOUND & ~EFI_ERROR_MASK]           = L"Not found",
+        [EFI_NO_MEDIA & ~EFI_ERROR_MASK]            = L"The device has no 
media",
+        [EFI_MEDIA_CHANGED & ~EFI_ERROR_MASK]       = L"Media changed",
+        [EFI_DEVICE_ERROR & ~EFI_ERROR_MASK]        = L"Device error",
+        [EFI_VOLUME_CORRUPTED & ~EFI_ERROR_MASK]    = L"Volume corrupted",
+        [EFI_ACCESS_DENIED & ~EFI_ERROR_MASK]       = L"Access denied",
+        [EFI_OUT_OF_RESOURCES & ~EFI_ERROR_MASK]    = L"Out of resources",
+        [EFI_VOLUME_FULL & ~EFI_ERROR_MASK]         = L"Volume is full",
+        [EFI_SECURITY_VIOLATION & ~EFI_ERROR_MASK]  = L"Security violation",
+        [EFI_CRC_ERROR & ~EFI_ERROR_MASK]           = L"CRC error",
+        [EFI_COMPROMISED_DATA & ~EFI_ERROR_MASK]    = L"Compromised data",
+        [EFI_BUFFER_TOO_SMALL & ~EFI_ERROR_MASK]    = L"Buffer too small",
+    };
+    EFI_STATUS ErrIdx = ErrCode & ~EFI_ERROR_MASK;
+
     StdOut = StdErr;
     PrintErr((CHAR16 *)mesg);
     PrintErr(L": ");
 
-    switch (ErrCode)
+    if( (ErrIdx < ARRAY_SIZE(ErrCodeToStr)) && ErrCodeToStr[ErrIdx] )
+        mesg = ErrCodeToStr[ErrIdx];
+    else
     {
-    case EFI_NOT_FOUND:
-        mesg = L"Not found";
-        break;
-    case EFI_NO_MEDIA:
-        mesg = L"The device has no media";
-        break;
-    case EFI_MEDIA_CHANGED:
-        mesg = L"Media changed";
-        break;
-    case EFI_DEVICE_ERROR:
-        mesg = L"Device error";
-        break;
-    case EFI_VOLUME_CORRUPTED:
-        mesg = L"Volume corrupted";
-        break;
-    case EFI_ACCESS_DENIED:
-        mesg = L"Access denied";
-        break;
-    case EFI_OUT_OF_RESOURCES:
-        mesg = L"Out of resources";
-        break;
-    case EFI_VOLUME_FULL:
-        mesg = L"Volume is full";
-        break;
-    case EFI_SECURITY_VIOLATION:
-        mesg = L"Security violation";
-        break;
-    case EFI_CRC_ERROR:
-        mesg = L"CRC error";
-        break;
-    case EFI_COMPROMISED_DATA:
-        mesg = L"Compromised data";
-        break;
-    case EFI_BUFFER_TOO_SMALL:
-        mesg = L"Buffer too small";
-        break;
-    default:
         PrintErr(L"ErrCode: ");
         DisplayUint(ErrCode, 0);
         mesg = NULL;
-        break;
     }
     blexit(mesg);
 }
-- 
2.1.4


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