[PATCH v2 2/3] x86/mce: Add new return value to get_user() for machine check

[Tony Luck <tony.luck@intel.com>]

When an exception occurs during any of the get_user() functions
fixup_exception() passes the trap number of the exception in regs->ax
to the fixup code.

Check for X86_TRAP_MC and return -ENXIO ("No such device or address")
so that callers can take action to avoid repeating an access to an
address that has an uncorrectable error.

Signed-off-by: Tony Luck <tony.luck@intel.com>
---
 arch/x86/lib/getuser.S | 8 +++++++-
 arch/x86/mm/extable.c  | 1 +
 2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/arch/x86/lib/getuser.S b/arch/x86/lib/getuser.S
index fa1bc2104b32..c49a449fced6 100644
--- a/arch/x86/lib/getuser.S
+++ b/arch/x86/lib/getuser.S
@@ -17,7 +17,7 @@
  *
  * Inputs:	%[r|e]ax contains the address.
  *
- * Outputs:	%[r|e]ax is error code (0 or -EFAULT)
+ * Outputs:	%[r|e]ax is error code (0 or -EFAULT or -ENXIO)
  *		%[r|e]dx contains zero-extended value
  *		%ecx contains the high half for 32-bit __get_user_8
  *
@@ -34,6 +34,7 @@
 #include <asm/asm.h>
 #include <asm/smap.h>
 #include <asm/export.h>
+#include <asm/trapnr.h>
 
 #define ASM_BARRIER_NOSPEC ALTERNATIVE "", "lfence", X86_FEATURE_LFENCE_RDTSC
 
@@ -168,8 +169,13 @@ SYM_CODE_START_LOCAL(.Lbad_get_user_clac)
 	ASM_CLAC
 bad_get_user:
 	xor %edx,%edx
+	cmpl $X86_TRAP_MC,%eax
+	je mce_get_user
 	mov $(-EFAULT),%_ASM_AX
 	ret
+mce_get_user:
+	mov $(-ENXIO),%_ASM_AX
+	ret
 SYM_CODE_END(.Lbad_get_user_clac)
 
 #ifdef CONFIG_X86_32
diff --git a/arch/x86/mm/extable.c b/arch/x86/mm/extable.c
index b93d6cd08a7f..ac4fcb820c40 100644
--- a/arch/x86/mm/extable.c
+++ b/arch/x86/mm/extable.c
@@ -77,6 +77,7 @@ __visible bool ex_handler_uaccess(const struct exception_table_entry *fixup,
 {
 	WARN_ONCE(trapnr == X86_TRAP_GP, "General protection fault in user access. Non-canonical address?");
 	regs->ip = ex_fixup_addr(fixup);
+	regs->ax = trapnr;
 	return true;
 }
 EXPORT_SYMBOL(ex_handler_uaccess);
-- 
2.21.1