[PATCH v2] Avoid crash calling PrintErrMesg from efi_multiboot2

[Frediano Ziglio <frediano.ziglio@cloud.com>]

Although code is compiled with -fpic option data is not position
independent. This causes data pointer to become invalid if
code is not relocated properly which is what happens for
efi_multiboot2 which is called by multiboot entry code.

Code tested adding
   PrintErrMesg(L"Test message", EFI_BUFFER_TOO_SMALL);
in efi_multiboot2 before calling efi_arch_edd (this function
can potentially call PrintErrMesg).

Before the patch (XenServer installation on Qemu, xen replaced
with vanilla xen.gz):
  Booting `XenServer (Serial)'Booting `XenServer (Serial)'
  Test message: !!!! X64 Exception Type - 0E(#PF - Page-Fault)  CPU Apic ID - 00000000 !!!!
  ExceptionData - 0000000000000000  I:0 R:0 U:0 W:0 P:0 PK:0 SS:0 SGX:0
  RIP  - 000000007DC29E46, CS  - 0000000000000038, RFLAGS - 0000000000210246
  RAX  - 0000000000000000, RCX - 0000000000000050, RDX - 0000000000000000
  RBX  - 000000007DAB4558, RSP - 000000007EFA1200, RBP - 0000000000000000
  RSI  - FFFF82D040467A88, RDI - 0000000000000000
  R8   - 000000007EFA1238, R9  - 000000007EFA1230, R10 - 0000000000000000
  R11  - 000000007CF42665, R12 - FFFF82D040467A88, R13 - 000000007EFA1228
  R14  - 000000007EFA1225, R15 - 000000007DAB45A8
  DS   - 0000000000000030, ES  - 0000000000000030, FS  - 0000000000000030
  GS   - 0000000000000030, SS  - 0000000000000030
  CR0  - 0000000080010033, CR2 - FFFF82D040467A88, CR3 - 000000007EC01000
  CR4  - 0000000000000668, CR8 - 0000000000000000
  DR0  - 0000000000000000, DR1 - 0000000000000000, DR2 - 0000000000000000
  DR3  - 0000000000000000, DR6 - 00000000FFFF0FF0, DR7 - 0000000000000400
  GDTR - 000000007E9E2000 0000000000000047, LDTR - 0000000000000000
  IDTR - 000000007E4E5018 0000000000000FFF,   TR - 0000000000000000
  FXSAVE_STATE - 000000007EFA0E60
  !!!! Find image based on IP(0x7DC29E46) (No PDB)  (ImageBase=000000007DC28000, EntryPoint=000000007DC2B917) !!!!

After the patch:
  Booting `XenServer (Serial)'Booting `XenServer (Serial)'
  Test message: Buffer too small
  BdsDxe: loading Boot0000 "UiApp" from Fv(7CB8BDC9-F8EB-4F34-AAEA-3EE4AF6516A1)/FvFile(462CAA21-7614-4503-836E-8AB6F4662331)
  BdsDxe: starting Boot0000 "UiApp" from Fv(7CB8BDC9-F8EB-4F34-AAEA-3EE4AF6516A1)/FvFile(462CAA21-7614-4503-836E-8AB6F4662331)

Fixes: 00d5d5ce23e6 ("work around Clang generating .data.rel.ro section for init-only files")
Signed-off-by: Frediano Ziglio <frediano.ziglio@cloud.com>
---
 xen/common/efi/boot.c | 45 +++++++++++++++++++++++++++++--------------
 1 file changed, 31 insertions(+), 14 deletions(-)
---
Changes since v1:
- added "Fixes:" tag;
- fixed cast style change.

diff --git a/xen/common/efi/boot.c b/xen/common/efi/boot.c
index efbec00af9..bbcd9263e1 100644
--- a/xen/common/efi/boot.c
+++ b/xen/common/efi/boot.c
@@ -287,19 +287,36 @@ static bool __init match_guid(const EFI_GUID *guid1, const EFI_GUID *guid2)
 /* generic routine for printing error messages */
 static void __init PrintErrMesg(const CHAR16 *mesg, EFI_STATUS ErrCode)
 {
-    static const CHAR16* const ErrCodeToStr[] __initconstrel = {
-        [~EFI_ERROR_MASK & EFI_NOT_FOUND]           = L"Not found",
-        [~EFI_ERROR_MASK & EFI_NO_MEDIA]            = L"The device has no media",
-        [~EFI_ERROR_MASK & EFI_MEDIA_CHANGED]       = L"Media changed",
-        [~EFI_ERROR_MASK & EFI_DEVICE_ERROR]        = L"Device error",
-        [~EFI_ERROR_MASK & EFI_VOLUME_CORRUPTED]    = L"Volume corrupted",
-        [~EFI_ERROR_MASK & EFI_ACCESS_DENIED]       = L"Access denied",
-        [~EFI_ERROR_MASK & EFI_OUT_OF_RESOURCES]    = L"Out of resources",
-        [~EFI_ERROR_MASK & EFI_VOLUME_FULL]         = L"Volume is full",
-        [~EFI_ERROR_MASK & EFI_SECURITY_VIOLATION]  = L"Security violation",
-        [~EFI_ERROR_MASK & EFI_CRC_ERROR]           = L"CRC error",
-        [~EFI_ERROR_MASK & EFI_COMPROMISED_DATA]    = L"Compromised data",
-        [~EFI_ERROR_MASK & EFI_BUFFER_TOO_SMALL]    = L"Buffer too small",
+#define ERROR_MESSAGE_LIST \
+    ERROR_MESSAGE(EFI_NOT_FOUND, "Not found") \
+    ERROR_MESSAGE(EFI_NO_MEDIA, "The device has no media") \
+    ERROR_MESSAGE(EFI_MEDIA_CHANGED, "Media changed") \
+    ERROR_MESSAGE(EFI_DEVICE_ERROR, "Device error") \
+    ERROR_MESSAGE(EFI_VOLUME_CORRUPTED, "Volume corrupted") \
+    ERROR_MESSAGE(EFI_ACCESS_DENIED, "Access denied") \
+    ERROR_MESSAGE(EFI_OUT_OF_RESOURCES, "Out of resources") \
+    ERROR_MESSAGE(EFI_VOLUME_FULL, "Volume is full") \
+    ERROR_MESSAGE(EFI_SECURITY_VIOLATION, "Security violation") \
+    ERROR_MESSAGE(EFI_CRC_ERROR, "CRC error") \
+    ERROR_MESSAGE(EFI_COMPROMISED_DATA, "Compromised data") \
+    ERROR_MESSAGE(EFI_BUFFER_TOO_SMALL, "Buffer too small")
+
+    static const struct ErrorStrings {
+        CHAR16 start;
+#undef ERROR_MESSAGE
+#define ERROR_MESSAGE(code, str) CHAR16 msg_ ## code[sizeof(str)];
+        ERROR_MESSAGE_LIST
+    } ErrorStrings __initconst = {
+        0
+#undef ERROR_MESSAGE
+#define ERROR_MESSAGE(code, str) , L ## str
+        ERROR_MESSAGE_LIST
+    };
+    static const uint16_t ErrCodeToStr[] __initconst = {
+#undef ERROR_MESSAGE
+#define ERROR_MESSAGE(code, str) \
+        [~EFI_ERROR_MASK & code] = offsetof(struct ErrorStrings, msg_ ## code),
+        ERROR_MESSAGE_LIST
     };
     EFI_STATUS ErrIdx = ErrCode & ~EFI_ERROR_MASK;
 
@@ -308,7 +325,7 @@ static void __init PrintErrMesg(const CHAR16 *mesg, EFI_STATUS ErrCode)
     PrintErr(L": ");
 
     if( (ErrIdx < ARRAY_SIZE(ErrCodeToStr)) && ErrCodeToStr[ErrIdx] )
-        mesg = ErrCodeToStr[ErrIdx];
+        mesg = (const CHAR16 *)((const void *)&ErrorStrings + ErrCodeToStr[ErrIdx]);
     else
     {
         PrintErr(L"ErrCode: ");
-- 
2.46.0