x86: prioritize the FPU traps for the error code

In the case of multiple FPU errors, prioritize the error codes,
instead of returning __SI_FAULT, which ends up pushing a 0 as the
error code to userspace, a POSIX violation.

For i386, we will simply return if there are no errors at all; for
x86-64 this is probably a "can't happen" (and the code should be
unified), but for this patch, return __SI_FAULT|SI_KERNEL if this ever
happens.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>

1 files changed, 15 insertions, 19 deletions

diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index 04d242ab0161..c320c29255c2 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -664,7 +664,7 @@ void math_error(void __user *ip)
 {
         struct task_struct *task;
         siginfo_t info;
-        unsigned short cwd, swd;
+        unsigned short cwd, swd, err;
 
         /*
          * Save the info for the exception handler and clear the error.
@@ -675,7 +675,6 @@ void math_error(void __user *ip)
         task->thread.error_code = 0;
         info.si_signo = SIGFPE;
         info.si_errno = 0;
-        info.si_code = __SI_FAULT;
         info.si_addr = ip;
         /*
          * (~cwd & swd) will mask out exceptions that are not set to unmasked
@@ -689,34 +688,31 @@ void math_error(void __user *ip)
          */
         cwd = get_fpu_cwd(task);
         swd = get_fpu_swd(task);
-        switch (swd & ~cwd & 0x3f) {
-        case 0x000: /* No unmasked exception */
-#ifdef CONFIG_X86_32
+
+        err = swd & ~cwd & 0x3f;
+
+#if CONFIG_X86_32
+        if (!err)
                 return;
 #endif
-        default: /* Multiple exceptions */
-                break;
-        case 0x001: /* Invalid Op */
+
+        if (err & 0x001) {      /* Invalid op */
                 /*
                  * swd & 0x240 == 0x040: Stack Underflow
                  * swd & 0x240 == 0x240: Stack Overflow
                  * User must clear the SF bit (0x40) if set
                  */
                 info.si_code = FPE_FLTINV;
-                break;
-        case 0x002: /* Denormalize */
-        case 0x010: /* Underflow */
-                info.si_code = FPE_FLTUND;
-                break;
-        case 0x004: /* Zero Divide */
+        } else if (err & 0x004) { /* Divide by Zero */
                 info.si_code = FPE_FLTDIV;
-                break;
-        case 0x008: /* Overflow */
+        } else if (err & 0x008) { /* Overflow */
                 info.si_code = FPE_FLTOVF;
-                break;
-        case 0x020: /* Precision */
+        } else if (err & 0x012) { /* Denormal, Underflow */
+                info.si_code = FPE_FLTUND;
+        } else if (err & 0x020) { /* Precision */
                 info.si_code = FPE_FLTRES;
-                break;
+        } else {
+                info.si_code = __SI_FAULT|SI_KERNEL; /* WTF? */
         }
         force_sig_info(SIGFPE, &info, task);
 }